Magic-angle twisted graphene systems, including bilayer (MATBG) and trilayer (MATTG) structures, constitute a highly tunable platform for exploring strongly correlated electronic phenomena and unconventional superconductivity. Despite extensive studies, the local electronic structure, symmetry-breaking transitions, and their interplay with superconductivity remain elusive. In this thesis, we employ high-resolution scanning tunneling microscopy and spectroscopy, to investigate the evolution, and hierarchy of correlated phases in twisted multilayer graphene as functions of doping, temperature, magnetic field, and twist angle.
\r\n\r\nIn twisted bilayer graphene, we map the evolution of flat electronic bands and detect filling-dependent band flattening, which drives cascades of symmetry-breaking transitions and the emergence of correlated gaps. Correlated gaps that occur at high magnetic fields are identified as Chern insulators, driven by interaction induced degeneracy breaking. In twisted trilayer graphene, we identify a sequence of correlated gaps at the Fermi level, including a robust outer gap associated with intervalley coherence and a more fragile inner gap linked to superconductivity. Atomic-scale reconstruction reveals Kekul\u00e9 reconstruction indicative of inter-valley coherence, which coexists with moir\u00e9-scale translation symmetry breaking.
\r\n\r\nOur results demonstrate that superconductivity in twisted multilayer graphene emerges from a hierarchy of correlated states, starting from cascade physics, to formation of Kondo resonance, flavor symmetry breaking to superconductivity. Our findings provide an insightful microscopic framework that is relevant to many moir\\'e systems and offer guiding principles for engineering correlated and topological states in designer quantum materials.
", "doi": "10.7907/2rk8-2q20", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18780", "collection": "thesis", "collection_id": "18780", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06032026-175718616", "type": "thesis", "title": "Measurement-Altered Quantum Criticality", "author": [ { "family_name": "Liu", "given_name": "Yue", "orcid": "0000-0002-5965-0644", "clpid": "Liu Yue-Physics" } ], "thesis_advisor": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" } ], "thesis_committee": [ { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Measurements are usually introduced as probes of quantum systems, but they can also act as a powerful, intrinsically nonunitary control knob for manipulating quantum states. This thesis develops the theory, experimental realization, and detection of measurement-altered quantum criticality in one-dimension: the phenomenon where local measurements on critical wave functions reshape universal long-distance correlations, entanglement, and other scaling behaviors. We show that measurements generate defect or boundary perturbations in the underlying conformal field theory, producing new measurement-induced fixed points and new universal observables that are often invisible to the pristine theory.
\r\n\r\nIn transverse-field Ising chains, we design measurement-altered criticality protocols in which correlated ancillas are entangled with the critical system and then projectively measured. This setting shows that measurement basis, entangling gate, measurement outcome, and ancilla correlations can qualitatively change correlations in the system and induce outcome-dependent order-parameter condensation. We also develop nonstandard probes --- including higher moments and symmetry-resolved averages --- that retain nontrivial signatures of the post-measurement ensemble.
\r\n\r\nThese ideas are then connected to quantum information by studying imperfect teleportation of critical many-body wavefunctions. Teleportation errors can be reinterpreted as effective weak measurements acting on an otherwise faithfully teleported critical state. This perspective yields a classification of protocols in which imperfections either preserve universal correlations and entanglement scaling, continuously deform them, or destroy long-range entanglement while leaving altered power-law correlations behind. Measurement-altered criticality therefore becomes not only a fundamental phenomenon, but also a tool for understanding and optimizing noisy quantum-information protocols.
\r\n\r\nWe also broaden the framework to measurement-induced boundary physics. In a gapless parent of the one-dimensional cluster state, a single round of measurements can generate boundary conformal field theories with distinct universal properties. Rotating the measurement basis drives transitions between these boundary fixed points, producing measurement-induced boundary transitions that have no analogue in the descendant gapped cluster state. Extensions to tricritical Ising and three-state Potts criticality show that such transitions arise generally in 1+1D conformal field theories.
\r\n\r\nFinally, the thesis addresses experimental access. We propose practical protocols for observing measurement-altered criticality in Rydberg atom arrays tuned to Ising and tricritical-Ising transitions, where projective measurements of selected atoms can controllably modify critical correlations with experimentally favorable post-selection probabilities.
\r\n\r\nWe also develop a post-selection-free framework for extracting nonlinear observables from raw measurement records using statistical learning. By recasting higher moments of conditioned observables as supervised learning objectives, the method replaces exponentially costly sector-by-sector postselection with regression whose sample complexity is controlled by decoder capacity. In critical Ising chains, linear, logistic, and convolutional decoders recover measurement-altered scaling and correlations from simulated unpostselected data.
\r\n\r\nTogether, these results establish measurements as a practical and universal route to engineering, diagnosing, and exploiting new forms of quantum critical behavior.
", "doi": "10.7907/t6m9-8h16", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:18735", "collection": "thesis", "collection_id": "18735", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012026-120746276", "type": "thesis", "title": "Scaling Neutral Atom Tweezer Arrays to 6,100 Qubits", "author": [ { "family_name": "Manetsch", "given_name": "Hannah Jean", "orcid": "0009-0002-3805-3389", "clpid": "Manetsch-Hannah-Jean" } ], "thesis_advisor": [ { "family_name": "Endres", "given_name": "Manuel A.", "orcid": "0000-0002-4461-224X", "clpid": "Endres-M" } ], "thesis_committee": [ { "family_name": "Faraon", "given_name": "Andrei", "orcid": "0000-0002-8141-391X", "clpid": "Faraon-A" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Minnich", "given_name": "Austin J.", "orcid": "0000-0002-9671-9540", "clpid": "Minnich-A-J" }, { "family_name": "Endres", "given_name": "Manuel A.", "orcid": "0000-0002-4461-224X", "clpid": "Endres-M" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Optical tweezer arrays of neutral atoms have been demonstrated in recent years to be a useful platform for quantum simulation, computation, and metrology. Realizing the full potential of these applications requires increasing qubit numbers while simultaneously maintaining high-fidelity control, which has proved a major challenge across quantum platforms. To date, quantum science experiments have typically been operated with at most hundreds of qubits.
\r\n\r\nIn this thesis, we review the relevant physics of the optical tweezer array architecture and present the technical advances involved in scaling the platform to 6,100 atomic qubits. In tandem with scalability, we demonstrate robust vacuum design, state-of-the-art coherence times, and high single-qubit gate fidelities. To capitalize on the intrinsic connectivity of the architecture at large scale, we investigate long-distance coherent atom transport and briefly describe progress towards two-qubit gate implementation. Finally, we discuss the prospects for scaling the optical tweezer array platform further by an order of magnitude.
", "doi": "10.7907/y7kk-1e96", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17729", "collection": "thesis", "collection_id": "17729", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:10232025-192458502", "primary_object_url": { "basename": "ADThesisResubmit20251023.pdf", "content": "final", "filesize": 73758354, "license": "other", "mime_type": "application/pdf", "url": "/17729/9/ADThesisResubmit20251023.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Resolving and Mathematizing Energetic Gradients That\r\nFacilitate Cytoskeletal Self-Assembly", "author": [ { "family_name": "Duarte", "given_name": "Ana Isabel", "orcid": "0000-0003-3726-3018", "clpid": "Duarte-Ana-Isabel" } ], "thesis_advisor": [ { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" } ], "thesis_committee": [ { "family_name": "Thomson", "given_name": "Matthew", "orcid": "0000-0003-1021-1234", "clpid": "Thomson-M-W" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Patterson", "given_name": "Ryan B.", "orcid": "0000-0002-5787-9517", "clpid": "Patterson-R-B" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "In the thesis that follows, I describe three interconnected stories. These threads strive to paint a unified picture of the energetic and mechanical assembly of motor proteins and microtubules into structures that resemble mitotic spindles, the complex molecular machines that segregate chromosomes during cell division. In the work described, we introduce a new method for direct measurement of ATP molecules in space and time, building upon the field\u2019s excitement towards witnessing gradients in isolated processes. We additionally write mathematical models exploring the physics of building and maintaining gradients in non-equilibrium steady states. And, in the spirit of comprehensively understanding our system, we explore the material properties of dynamic network formation.", "doi": "10.7907/1ptv-0r61", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17824", "collection": "thesis", "collection_id": "17824", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01162026-000012922", "primary_object_url": { "basename": "Yao_Jia_2026.pdf", "content": "final", "filesize": 17035128, "license": "other", "mime_type": "application/pdf", "url": "/17824/1/Yao_Jia_2026.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "First-Principles Simulation of Nonequilibrium Coupled Electron\u2013Phonon Dynamics: Algorithms, Acceleration, and Coherent Phenomena", "author": [ { "family_name": "Yao", "given_name": "Jia (Kelly)", "orcid": "0000-0002-3250-6132", "clpid": "Yao-Jia-Kelly" } ], "thesis_advisor": [ { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-Scott-K" }, { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" }, { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Studying nonequilibrium charge and heat transport is central to the design and control of modern electronic, optoelectronic, and energy materials. On ultrafast timescales, these processes, such as carrier relaxation, lattice heating, and lattice-driven changes of material properties are governed by coupled electron-phonon dynamics. While first-principles methods based on density functional theory enable quantitative predictions of electron-phonon and phonon-phonon (ph-ph) \\mbox{interactions}, their extension to real-time, fully coupled nonequilibrium simulations remains computationally challenging, particularly for complex materials and long simulation times.
\r\n\r\nThis thesis develops and advances a comprehensive first-principles framework for simulating nonequilibrium coupled electron-phonon dynamics within the real-time Boltzmann transport equation (rt-BTE). Several algorithmic and computational strategies are introduced to overcome the prohibitive cost of such simulations. First, adaptive and multirate time-integration schemes are developed to efficiently resolve disparate electronic and phononic timescales, enabling accurate simulations of coupled dynamics in complex materials with anharmonic ph-ph interactions. Second, dynamic mode decomposition is applied to extrapolate long-time behavior from short-time simulations, providing efficient access to steady-state and transient transport regimes. Third, GPU parallelization and algorithm optimization are implemented to accelerate the evaluation of collision integrals, yielding substantial performance improvements on modern high-performance computing architectures. Fourth, tensor-learning and compression techniques are introduced to reduce the computational and memory costs associated with ph-ph interactions, further enabling simulations previously inaccessible due to system size or interaction complexity.
\r\n\r\nBeyond algorithmic acceleration, this work extends the theoretical description of nonequilibrium lattice dynamics from the rt-BTE by introducing a framework accounting for coherently driven phonon dynamics, bridging the gap between incoherent transport and coherent ultrafast phenomena observed in modern pump-probe experiments. Together, these developments expand the efficiency and scope of first-principles simulations of nonequilibrium electron-phonon dynamics, providing new tools for studying transport, relaxation, and coherent control in quantum materials.
", "doi": "10.7907/pw5b-ge31", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17699", "collection": "thesis", "collection_id": "17699", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09242025-004457327", "primary_object_url": { "basename": "S_Davis_Dissertation-submission.pdf", "content": "final", "filesize": 73936635, "license": "other", "mime_type": "application/pdf", "url": "/17699/1/S_Davis_Dissertation-submission.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Teleportation from Quantum Networks to Traversable Wormholes: the Physics and Technology of Entanglement", "author": [ { "family_name": "Davis", "given_name": "Samantha Isabel", "orcid": "0000-0001-9994-8165", "clpid": "Davis-Samantha-Isabel" } ], "thesis_advisor": [ { "family_name": "Spiropulu", "given_name": "Maria", "orcid": "0000-0001-8172-7081", "clpid": "Spiropulu-M" } ], "thesis_committee": [ { "family_name": "Hutzler", "given_name": "Nicholas R.", "orcid": "0000-0002-5203-3635", "clpid": "Hutzler-N-R" }, { "family_name": "Spiropulu", "given_name": "Maria", "orcid": "0000-0001-8172-7081", "clpid": "Spiropulu-M" }, { "family_name": "Endres", "given_name": "Manuel A.", "orcid": "0000-0002-4461-224X", "clpid": "Endres-M" }, { "family_name": "Faraon", "given_name": "Andrei", "orcid": "0000-0002-8141-391X", "clpid": "Faraon-A" }, { "family_name": "Kitaev", "given_name": "Alexei", "orcid": "0000-0002-5777-642X", "clpid": "Kitaev-A" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "This thesis presents developments in quantum information technologies and their applications to both quantum networks and fundamental physics. It is organized into three parts. Part I focuses on the design and implementation of state-of-the-art sources and detectors for quantum networks. Key contributions include the development of photon-number-resolving superconducting nanowire detectors and their application to heralded single-photon generation and photon-number discrimination; a high-rate multiplexed entangled photon-pair source for quantum key distribution; and on-chip balanced homodyne detectors for the detection of squeezed light. I describe how phased arrays can facilitate wireless quantum communications by introducing the concept of ``quantum phased arrays'' and present the first large-scale optoelectronic phased array receiver on a chip capable of interfacing with nonclassical light, with first demonstrations of coherent imaging and beamforming of squeezed states of light. Part II details the construction of quantum network testbeds at Caltech and Fermilab, designed to realize scalable architectures for the quantum internet. These systems demonstrate high-fidelity quantum teleportation over 45 km of optical fiber and entanglement swapping with time-bin qubits. The experiments are supported by the development of theoretical models that guide system optimization. I also present demonstrations of entanglement distribution at Caltech and remote sites at Fermi and Argonne National Labs with picosecond-level clock synchronization, representing milestones toward the deployment of quantum networking infrastructure across national laboratories. Part III investigates how quantum networks can be used to probe fundamental questions in physics. I report the first experimental generation of GHZ states with time-bin qubits, towards the deployment of multipartite entanglement distribution in real-word networks for tests of quantum mechanics and distributed sensing. Finally, I present the first experimental realization of a traversable wormhole teleportation protocol implemented on a quantum processor, a step in the program of quantum gravity in the lab. I conclude with an outlook and discuss future directions of this work.", "doi": "10.7907/v1zm-yz68", "publication_date": "2026", "thesis_type": "phd", "thesis_year": "2026" }, { "id": "thesis:17150", "collection": "thesis", "collection_id": "17150", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04112025-180844471", "primary_object_url": { "basename": "Christopher_Yang_Thesis-2.pdf", "content": "final", "filesize": 20240940, "license": "other", "mime_type": "application/pdf", "url": "/17150/1/Christopher_Yang_Thesis-2.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Dynamical Control of Many-Body Interactions in Driven Quantum Matter", "author": [ { "family_name": "Yang", "given_name": "Christopher Kai-Chen", "orcid": "0000-0002-9462-9074", "clpid": "Yang-Christopher-Kai-Chen" } ], "thesis_advisor": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Strongly driven Floquet systems have emerged as promising platforms for exotic non-equilibrium physics, but their instability to heating motivates practical questions about how Floquet engineering can be useful. Although drive-induced heating is often attributed to interactions, this thesis adopts a different perspective, identifying regimes where dissipative many-body dynamics can stabilize Floquet physics and define remarkable new drive-tunable properties. This principle enables highly tunable many-body steady states with minimal heating, leading to a novel regime where drive control over single-particle Floquet states can extend to many-body interactions. Our theoretical and experimental results in Parts II and III center around two themes. The first theme focuses on discovering controllable and stable many-body Floquet states. The second explores further into what the future holds--envisioning the prospects for unconventional Floquet physics with nontraditional driving fields and three-dimensional materials.
\r\n\r\nPart II of this thesis leverages kinematic constraints on low-dimensional many-body scattering as new principles for tuning and stabilizing Floquet phases. First, we predict that a circularly polarized laser can drive slow electrons of moir\u00e9 systems into a subsonic regime where they decouple from the intrinsic 2D acoustic phonons of the system. This \"slow-electron regime\" enables optical control over the steady-state occupation of topological Floquet states and the resulting anomalous Hall conductivity. Second, we present experimental transport signatures of steady Floquet physics in graphene irradiated by a continuous-wave laser. Our experiment, performed at 3-4 K lattice temperatures with lasers off-resonant to optical phonons, creates electron-phonon scattering bottlenecks that stabilize persistent low-temperature phases with light-induced longitudinal transport characteristics. The long-lived many-body phase represents the first experimental signatures of steady Floquet physics in a metallic solid.
\r\n\r\nPart III presents emerging opportunities for many-body Floquet engineering beyond traditional optically-driven, low-dimensional materials. We first explore beyond-optical driving fields, revealing the emergence of quantized charge transport in 1D systems driven by coherent phonons. Incoherent phonons relax electrons into a topological spatiotemporal Floquet state with quantized group velocity set by the coherent phonon, realizing topological charge pumping in a highly non-adiabatic setting. Finally, we address the topological effects of time-periodic drives beyond low-dimensional systems, revealing that THz-frequency, circularly polarized light can induce topological chiral plasmons in Weyl semimetals with band anisotropy, broken time-reversal symmetry, and broken inversion symmetry.
\r\n\r\nThe theoretical and experimental work in this thesis represent key progress towards realizing persistent Floquet physics for diverse applications in quantum device engineering.
", "doi": "10.7907/fh52-tw61", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:17305", "collection": "thesis", "collection_id": "17305", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05302025-053900946", "primary_object_url": { "basename": "Duxing_Hao_2025_final_full.pdf", "content": "final", "filesize": 21708307, "license": "cc_by_nc_sa", "mime_type": "application/pdf", "url": "/17305/10/Duxing_Hao_2025_final_full.pdf", "version": "v8.0.0" }, "type": "thesis", "title": "Novel Electronic and Optoelectronic Interactions in Two-Dimensional Materials", "author": [ { "family_name": "Hao", "given_name": "Duxing", "orcid": "0000-0002-8907-9776", "clpid": "Hao-Duxing" } ], "thesis_advisor": [ { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" }, { "family_name": "Patrick", "given_name": "Lee A.", "orcid": "0000-0001-7809-8157", "clpid": "Lee-Patrick-A" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Two-dimensional (2D) materials host a rich set of emerging physical phenomena such as superconductivity, ferroelectricity, quantum magnetism, and circular dichroism. Moreover, these phenomena are highly tunable by crystalline composition variations and crystalline structural phase modifications and are sensitive to external conditions such as temperature, magnetic field and optical excitation, substrate and gate tuning. Therefore, 2D material-based devices are highly desirable for modern electronic and optoelectronic devices applications. In this thesis, we employed a fully scalable approach to synthesize materials and fabricate 2D material-based devices such as those based on graphene and 1H-Molybdenum disulfide (1H-MoS2), and explore their electronic and optoelectronic properties in cryogenic conditions under various excitation sources, such as external magnetic field and structured light.
\r\n\r\nIn the first part of the thesis (Chapters 2 and 3), we provide experimental details for achieving nanoscale strain engineering of monolayer (ML)-graphene and demonstrate that periodic patterns of nanoscale strain distributions in ML-graphene can lead to local giant pseudomagnetic fields as well as global modifications to the electronic properties of ML-graphene, including strain-induced valley Hall and anomalous Hall effects in the absence of external magnetic fields, nonlocal valley-polarized currents and evidence of quantum valley Hall effect under external magnetic field. These findings suggest new approaches towards developing emerging quantum states with tunable electronic correlation based on graphene straintronics.
\r\n\r\nThe second part of the thesis (Chapters 4 and 5) focus more on the semiconducting monolayer transition metal dichalcogenides (ML-TMDs), whose broken inversion symmetry and strong spin-orbit coupling result in spin-valley lock-in effects so that the valley degeneracy may be lifted by external magnetic fields, potentially leading to real-space structural transformation.
\r\n\r\nIn Chapter 4, we report magnetic field (B)-induced giant electric hysteretic responses to back-gate voltages in ML-MoS\u2082 field-effect transistors (FETs) on SiO\u2082/Si at temperatures < 20 K. The observed hysteresis increases with |B| up to 12 T and is tunable by varying the temperature. Raman spectroscopic and scanning tunneling microscopic studies reveal significant lattice expansion with increasing |B| at 4.2 K, and this lattice expansion becomes asymmetric in ML-MoS\u2082 FETs on rigid SiO\u2082/Si substrates, leading to out-of-plane mirror symmetry breaking and the emergence of a tunable out-of-plane ferroelectric-like polar order. This broken symmetry-induced polarization in ML-MoS\u2082 shows typical ferroelectric butterfly hysteresis in piezo-response force microscopy, adding ML-MoS\u2082 to the single-layer material family that exhibit out-of-plane polar order-induced ferroelectricity, which is promising for such technological applications as cryo-temperature ultracompact non-volatile memories, memtransistors, and ultrasensitive magnetic field sensors. Moreover, the polar effect induced by asymmetric lattice expansion may be further generalized to other ML-TMDs and achieved by nanoscale strain engineering of the substrate without magnetic fields.
\r\n\r\nIn Chapter 5, we further demonstrate the design and application of a novel instrument that integrates scanning spectroscopic photocurrent measurements with structured light of controlled spin and orbital angular momentum. For structured photons with wavelengths between 500 nm to 700 nm, this instrument can perform spatially resolved photocurrent measurements of 2D materials or thin crystals under magnetic fields up to \u00b114 Tesla, at temperatures from 300 K down to 3 K, with either spin angular momentum (SAM) \u2113\u0127 or orbital angular momentum (OAM) \u00b1 \u2113\u0127 (where \u2113 = 1, 2, 3\u2026 is the topological charge), and over a (35x25) \u00b5m\u00b2 area with ~ 1 \u00b5m spatial resolution. These capabilities of the instrument are exemplified by magneto-photocurrent spectroscopic measurements of monolayer 2H-MoS\u2082 field-effect transistors, which not only reveal the excitonic spectra but also demonstrate monotonically increasing photocurrents with increasing |\u2113| as well as excitonic Zeeman splitting and an enhanced Land\u00e9 g-factor due to the enhanced formation of intervalley dark excitons under magnetic field. These studies thus demonstrate the versatility of the scanning photocurrent spectrometry for investigating excitonic physics, optical selection rules, and optoelectronic responses of novel quantum materials and engineered quantum devices to structured light.
\r\n\r\nFinally, we summarize the research accomplishments of this thesis work in Chapter 6 and discuss the outlook for new research directions associated with these 2D quantum materials.
", "doi": "10.7907/wegr-tg72", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16642", "collection": "thesis", "collection_id": "16642", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08152024-222635476", "primary_object_url": { "basename": "Yiran_Zhang_2024_thesis.pdf", "content": "final", "filesize": 106668206, "license": "other", "mime_type": "application/pdf", "url": "/16642/1/Yiran_Zhang_2024_thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Spin-Orbit Enhanced Superconductivity in Graphene Heterostructures", "author": [ { "family_name": "Zhang", "given_name": "Yiran", "orcid": "0000-0002-8477-0074", "clpid": "Zhang-Yiran" } ], "thesis_advisor": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Flat electronic bands in moire and crystalline graphene multilayers showcase emergent correlated phenomena including correlated insulators, superconductivity, topological orders, etc. This thesis focuses on the electrical transport characterization of superconductivity in moire and crystalline graphene, with the proximity of a layer of tungsten diselenide (WSe\u2082) that induces spin-orbit coupling (SOC). The interplay between spontaneous symmetry-breaking and explicit spin-orbit interactions emerges various unconventional superconducting pairing.
\r\n\r\nIn the case of moire graphene multilayers, superconductivity in twisted bilayer graphene persists much far away from the magic angle at which electronic correlations dominate. At the lowest twist angle 0.79\u00b0, superconductivity appears despite the absence of any insulating states. By changing the moire twist angle, the ratio between Coulomb interactions and kinetic energy is reduced, and we thus established a hierarchy of various symmetry-breaking orders. Importantly, superconductivity is tightly related to the half-filling symmetry-breaking reconstructions. We further generalize the twisted moire graphene to trilayer, quadrilayer and pentalayer cases. Characterizations around their respective magic angle show that superconductivity is more prominent in filling phase space when the number of layers is increased.
\r\n\r\nWe then investigated the effect of SOC on correlated phases in crystalline Bernal-stacked bilayer graphene. Surprisingly, placing monolayer WSe\u2082 on bilayer graphene promotes Cooper pairing to an extraordinary degree: field-induced superconductivity is stabilized at zero magnetic field, exhibits an order of magnitude enhancement in critical temperature and occurs over a density range that is wider by a factor of eight. The superconductivity descends from a broken-symmetry parent state with two out of the four spin-valley flavors being predominantly populated. Moreover, the superconductivity arises only for perpendicular electric fields that push hole wavefunctions toward WSe\u2082, indicating that proximity-induced Ising spin-orbit coupling plays a key role in stabilizing the pairing.
\r\n\r\nThe last part of the thesis focuses on a new degree of freedom: interfacial twisting between graphene and WSe\u2082. We experimentally demonstrate the \"moireless\" tuning of superconductivity in Bernal bilayer graphene proximitized by WSe\u2082. The precise alignment between the two materials systematically controls the strength of the induced Ising SOC, profoundly altering the phase diagram. As Ising SOC is increased, superconductivity onsets at a higher displacement field and features a higher critical temperature, reaching up to 0.5K. Within the main superconducting dome and in the strong Ising SOC limit, we find an unusual phase transition characterized by a nematic redistribution of holes among trigonally warped Fermi pockets and enhanced resilience to in-plane magnetic fields. Moreover, we identify two additional superconducting regions, one of which descends from an inter-valley coherent normal state and exhibits a Pauli-limit violation ratio exceeding 40, among the highest for all known superconductors.
", "doi": "10.7907/nfyx-3565", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16340", "collection": "thesis", "collection_id": "16340", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03232024-174719340", "primary_object_url": { "basename": "thesis_v4_proofread_correction.pdf", "content": "final", "filesize": 9930907, "license": "other", "mime_type": "application/pdf", "url": "/16340/2/thesis_v4_proofread_correction.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Electron-Phonon Interactions and Charge Transport in Organic Crystals and Transition Metal Oxides from First-Principles Calculations", "author": [ { "family_name": "Chang", "given_name": "Benjamin K.", "orcid": "0000-0003-1304-9324", "clpid": "Chang-Benjamin-K" } ], "thesis_advisor": [ { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" } ], "thesis_committee": [ { "family_name": "Minnich", "given_name": "Austin J.", "orcid": "0000-0002-9671-9540", "clpid": "Minnich-A-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Electron-phonon (e-ph) interactions play a critical role in determining material properties, such as charge and heat transport, optical response, and superconductivity. Recent advances in first-principles calculations based on density functional theory (DFT) enable quantitatively predictive studies of e-ph interactions and charge transport in a wide range of simple semiconductors and metals. However, certain technologically important materials, such as organic crystals and transition metal oxides (TMOs), remain less explored. Organic molecular crystals, known for their versatile electronic and mechanical properties, typically require high charge carrier mobility for practical applications. Yet accurately predicting the mobility and engineering approaches to improve it are challenging in organic crystals, because of their complex crystal structures with large unit cells and various charge transport regimes induced by e-ph interactions. Similarly, TMOs, both conventional and strongly correlated, are materials with broad applications and unique physics. A notable example are copper oxides (cuprate) superconductors, which are central to the study of high-temperature superconductivity and other exotic physical phenomena. Extensive experimental studies, particularly using photoemission techniques, have been employed to indirectly probe the e-ph interactions in TMOs. Nevertheless, many results are not fully understood, and calculations of e-ph coupling in TMOs are still scarce. This is mainly due to the strong correlation induced by d- and f-electrons posing a significant challenge to modeling.
\r\n\r\nThis thesis aims to develop state-of-the-art first-principles calculations to accurately describe e-ph interactions and the associated physical properties in organic crystals and TMOs. We focus on three research topics. First, we investigate the high-mobility bandlike transport regime in organic crystals. Using the formalism of the Boltzmann transport equation with electronic collisions computed from first principles, we study the mobility and its temperature dependence in benzene, anthracene, tetracene, pentacene, and biphenyl. Our results are in excellent agreement with experiments in all cases, and our pentacene calculation (72 atoms per unit cell) sets the record for the largest first-principles e-ph calculation to date. We find that the mobility is mainly regulated by e-ph scattering from low-frequency intermolecular phonons. Our analysis evidences the effectiveness of strain-based engineering to improve the mobility of organic crystals. Second, we propose a computational approach to study the intermediate polaronic transport regime in organic crystals. This method combines a finite-temperature cumulant-expansion approach for calculating electron spectral functions with the Kubo formula to compute the electronic conductivity and mobility. We show calculations of electron mobility in a naphthalene crystal in excellent agreement with experiments, and find that polaron effects, encoded in the satellites of the spectral functions, are induced by strong e-ph coupling of intramolecular hydrogen-atom vibrations. In the third and final topic, we study quantitatively the e-ph interactions in cuprate superconducting materials. Using the framework of Hubbard-corrected DFT, we focus on the prototypical parent (undoped) cuprate compound La2CuO4, which becomes superconducting upon doping. We show the first quantitative evidence of strong Fr\u00f6hlich-type e-ph interactions between holes and oxygen atomic vibrations, as well as polaron effects in hole spectral functions. Our findings explain a range of observations in photoemission experiments on both undoped and doped cuprates, suggesting the strong e-ph coupling is an intrinsic feature of the parent compounds rather than being induced by doping. The computational workflow presented in this work can be easily extended to a broad class of strongly-correlated oxides and insulators more generally. In summary, this thesis pushes the boundaries of first-principles calculations of e-ph interactions and transport, paving the way for a microscopic understanding of materials with large and complex unit cells, strong electronic correlations, and strong e-ph interactions.
", "doi": "10.7907/6zkq-2p13", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16386", "collection": "thesis", "collection_id": "16386", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05162024-213746585", "primary_object_url": { "basename": "Aike Liu thesis.pdf", "content": "final", "filesize": 3049077, "license": "other", "mime_type": "application/pdf", "url": "/16386/3/Aike Liu thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Bootstrapping the Gross-Neveu-Yukawa Archipelago and Skydiving Algorithm", "author": [ { "family_name": "Liu", "given_name": "Aike", "orcid": "0009-0009-8509-4635", "clpid": "Liu-Aike" } ], "thesis_advisor": [ { "family_name": "Simmons-Duffin", "given_name": "David", "orcid": "0000-0002-2937-9515", "clpid": "Simmons-Duffin-D" } ], "thesis_committee": [ { "family_name": "Ooguri", "given_name": "Hirosi", "orcid": "0000-0001-6021-3778", "clpid": "Ooguri-H" }, { "family_name": "Kapustin", "given_name": "Anton N.", "orcid": "0000-0003-3903-5158", "clpid": "Kapustin-A" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Simmons-Duffin", "given_name": "David", "orcid": "0000-0002-2937-9515", "clpid": "Simmons-Duffin-D" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The goal of the conformal bootstrap is to solve conformal field theories (CFTs) by imposing physical constraints including symmetries and unitarity. It has been a powerful tool to rigorously constrain CFT data, especially for strongly-coupled theories where traditional perturbative methods fail. Based solely on unitarity, symmetry, and assumptions about the spectrum of scaling dimensions, the bootstrap method has produced stringent bounds on critical exponents of several universality classes describing real-world statistical and quantum phase transitions.
\r\n\r\nThe numerical bootstrap method combines the physical constraints with convex optimization. Specifically, the physics problems are converted into semidefinite programs and solved numerically. Such methods have led to precise and rigorous predictions on critical exponents of condensed-matter systems, such as 3d Ising models and the $O(N)$ models. In my first research project, we perform a bootstrap analysis of a mixed system of four-point functions of bosonic and fermionic operators in parity-preserving 3d CFTs with O(N) global symmetry. Our results provide rigorous bounds on scaling dimensions and OPE coefficients of the O(N) symmetric Gross-Neveu-Yukawa (GNY) fixed-points, constraining these theories to live in isolated islands in the space of CFT data. We delivered the bounds on the critical points with N = 1, 2, 4, and 8, which have applications to phase transitions in condensed matter systems. We were also able to demonstrate the existence of the supercurrent when supersymmetry emerges at N=1 without prior assumptions of the symmetry.
\r\n\r\nOn the other hand, as we progress towards larger systems to study and to obtain more precise bounds on various CFTs, the limits on computational resources cannot be overlooked. To tackle the numerical challenges and improve efficiency, my second research project studies families of semidefinite programs (SDPs) that depend nonlinearly on a small number of \u201cexternal\u201d parameters. Such families appear universally in numerical bootstrap computations. The traditional method for finding an optimal point in parameter space works by first solving an SDP with fixed external parameters, then moving to a new point in parameter space and repeating the process. Instead, we unify solving the SDP and moving in parameter space in a single algorithm that we call \u201cskydiving\u201d. We test skydiving on some representative problems in the conformal bootstrap, finding significant speedups compared to traditional methods.
", "doi": "10.7907/qxe6-3s08", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:16460", "collection": "thesis", "collection_id": "16460", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06012024-155841744", "primary_object_url": { "basename": "Caltech_Thesis_Chen_Li_2023.pdf", "content": "final", "filesize": 20241306, "license": "other", "mime_type": "application/pdf", "url": "/16460/1/Caltech_Thesis_Chen_Li_2023.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Ultrafast Optical Studies of Pressure-Tuned Spin-Orbit Materials", "author": [ { "family_name": "Li", "given_name": "Chen", "orcid": "0000-0001-6750-5925", "clpid": "Li-Chen" } ], "thesis_advisor": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Rosenbaum", "given_name": "Thomas F.", "orcid": "0009-0008-6152-666X", "clpid": "Rosenbaum-T-F" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The advent of quantum materials has provided researchers with a remarkable opportunity to delve into the intricate interplay among various degrees of freedom, encompassing charge, orbital, spin, and lattice dynamics. Transition metal compounds, possessing distinct characteristics, exemplify the captivating competition between interactions arising from different degrees of freedom, each with comparable strength. These interactions encompass the on-site Coulomb interaction, kinetic hopping, spin-orbit coupling (SOC), crystal electric field splitting, and Hunds exchange coupling. In correlated electron systems of this nature, the intricate interplay of these complex interactions gives rise to a plethora of exotic phenomena, rendering the understanding of each variable a daunting task. Hence, it becomes imperative to explore their responses to external stimuli, and in this regard, hydrostatic pressure emerges as a versatile tool capable of tuning the strength of competing interactions and shifting the delicate balance between coexisting and competing ground states. This engenders a rich diversity of quantum phases and holds the potential to decouple these intertwined variables in phase transitions, thus unveiling the distinctive roles played by each constituent.
\r\n\r\nIn Chapter I, a comprehensive discussion on pressure-induced phase transitions will ensue, encompassing phenomena such as insulator-metal transitions, spin-crossover transitions, structural transformations, and the fervent search for elusive quantum spin liquid and topological superconductive states. Chapter II shall delve into the experimental techniques that have been extensively employed throughout my research endeavors. This will encompass a synergistic combination of a high-pressure environment and cutting-edge ultrafast optical probing techniques, including optical second harmonic generation (SHG), harnessed by the high peak power of femtosecond lasers, as well as time-resolved reflectivity, capitalizing on the exceedingly short time duration of laser pulses. Moreover, a wide-field microscopy approach based on the magneto-optical Kerr effect shall be expounded upon, enabling direct observations of intricate domain structures. In subsequent chapters, three projects shall be elucidated, encompassing Weyl semimetals, with a specific focus on TaAs in Chapter III, Co3Sn2S2 in Chapter V, and an investigation into the spin-orbit-coupled Mott insulator Sr2IrO4 in Chapter IV.
\r\n\r\nThe transition metal monopnictide family of Weyl semimetals recently has been shown to exhibit anomalously strong second-order optical nonlinearity, which is theoretically attributed to a highly asymmetric polarization distribution induced by their polar structure. We experimentally test this hypothesis by measuring optical SHG from TaAs across a pressure-tuned polar to non-polar structural phase transition. Despite the high-pressure structure remaining non-centrosymmetric, the SHG yield is reduced by more than 60% by 20 GPa as compared to the ambient pressure value. By examining the pressure dependence of distinct groups of SHG susceptibility tensor elements, we find that the yield is primarily controlled by a single element that governs the response along the polar axis. Our results confirm a connection between the polar axis and the giant optical nonlinearity of Weyl semimetals and demonstrate pressure as a means to tune this effect in situ.
\r\n\r\nSr2IrO4 stands as an archetypal SOC-mediated Mott insulator, where the electronic and magnetic structures are highly sensitive to the intricacies of the crystallographic structure, particularly the rotation and tilting of the IrO6 cages. External pressure serves as a direct means to manipulate these characteristics. Under high pressure, fascinating phenomena have emerged, including the persistence of the insulating state up to an extreme pressure of 185 GPa, a sequence of magnetic transitions culminating in a quantum paramagnetic phase around 20 GPa. However, a dearth of information exists concerning the low-energy electronic band structure. To address this gap, we conducted time-resolved reflectivity measurements under pressures up to 14 GPa. Within the low-pressure range below 10 GPa, anomalies in the temperature-dependent reflectivity transients exhibit a trend akin to the Neel temperature. Yet, as pressure increases further, the temperature associated with these anomalies rises and deviates from the monotonically decreasing magnetic ordering temperature, thereby unveiling a mysterious underlying mechanism governing the relaxation dynamics.
\r\n\r\nIn addition to the breaking of inversion symmetry, Weyl topology can also arise from the breaking of time reversal symmetry in magnetic systems, offering a fertile ground for investigating the intricate relationship between magnetism and topological order. Endeavors have been undertaken to manipulate magnetism as a means to tune the topological electronic band structure. Notably, the well-established ferromagnetic Weyl semimetal, Co3Sn2S2, has garnered significant attention due to its intriguing magnetic anomalies persisting below the Curie temperature. Further investigations have revealed that the distribution of magnetic domains and domain walls plays a pivotal role in elucidating these anomalies. Herein, we report the observation of domain structures using a wide-field Kerr microscope and the manipulation of said structures employing a mid-infrared laser and magnetic field. This study not only sheds light on domain-related properties but also holds promise for uncovering exotic topological phenomena exhibited at domain boundaries.
", "doi": "10.7907/79g4-4392", "publication_date": "2024", "thesis_type": "phd", "thesis_year": "2024" }, { "id": "thesis:15081", "collection": "thesis", "collection_id": "15081", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:01052023-230400021", "type": "thesis", "title": "Electronic Correlations and Topology in Graphene Moir\u00e9 Multilayers and InAs/GaSb-Derivative Systems", "author": [ { "family_name": "Polski", "given_name": "Robert Michael", "orcid": "0000-0003-0887-8099", "clpid": "Polski-Robert-Michael" } ], "thesis_advisor": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Twisted bilayer graphene (TBG) near the magic angle exhibits a wide variety of correlated and topological phases such as superconductivity, correlated insulators, and orbital ferromagnetism. We show using electrical transport measurements that adding a layer of tungsten diselenide in proximity to twisted bilayer graphene stabilizes superconductivity to twist angles significantly below the magic angle despite the disappearance of correlated insulators and insulators at full moir\u00e9 filling. These findings--along with our report of a relationship between superconductivity and symmetry breaking Fermi surface reconstruction--suggest constraints on theories of the origin of superconductivity in TBG. In the context of this TBG-tungsten diselenide system, we study how the correlated phases evolve over a wide twist angle range and classify them into a hierarchy based on where they occur relative to the magic angle (or where bands have been maximally flattened). While effects such as orbital ferromagnetism near one electron per moir\u00e9 unit cell and gapped correlated insulators only exist in close proximity to the magic angle, superconductivity and high-temperature cascade transitions survive in a wider twist angle range.
\r\n\r\nWe also analyze the structures of twisted trilayer, quadrilayer, and pentalayer graphene (and all proximitized to tungsten diselenide) near their respective theoretical magic angles, revealing robust electron- and hole-side superconductivity in each heterostructure. We additionally find previously unreported insulating states in twisted trilayer and quadrilayer graphene along with an enlarged filling range of superconductivity in pentalayer. Our studies on twisted graphene multilayers beyond two layers allow us to generalize the correlated physics found in TBG and consider the role of the additional bands introduced.
\r\n\r\nIn the last part of this thesis, we measure the two-dimensional topological insulator candidate system InAs/GaSb with added stoichiometric impurities. Previous studies in pure InAs/GaSb structures have revealed low bulk resistivity and edge states that arise from trivial effects which can be easily mistaken for topological effects. Due, in part, to the strain effects of Indium impurities added to GaSb, our results show high bulk resistivity. We also, due to the wide gate-tunability in our devices, are able to measure the expected spin-orbit-split valence band structure. Our development of highly tunable InAs/GaSb-derivative structures paves the way for another look at two-dimensional topological insulator behavior in these systems and for their integration into superconducting devices.
", "doi": "10.7907/yhws-0f08", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15008", "collection": "thesis", "collection_id": "15008", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08252022-153300158", "primary_object_url": { "basename": "hirokawa_soichi_thesis.pdf", "content": "final", "filesize": 23936329, "license": "other", "mime_type": "application/pdf", "url": "/15008/4/hirokawa_soichi_thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Dynamics of Protein-Mediated Polymer Coupling and their Implications in Antibody Production and Emergent Patterning", "author": [ { "family_name": "Hirokawa", "given_name": "Soichi", "orcid": "0000-0001-5584-2676", "clpid": "Hirokawa-Soichi" } ], "thesis_advisor": [ { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" } ], "thesis_committee": [ { "family_name": "Schwab", "given_name": "Keith C.", "orcid": "0000-0001-8216-4815", "clpid": "Schwab-K-C" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" }, { "family_name": "Thomson", "given_name": "Matthew", "orcid": "0000-0003-1021-1234", "clpid": "Thomson-M-W" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "Proteins serve a wide range of functions in and out of the cell, from signaling and gene regulation to transport and structural reinforcement. These functions are usually carried out from interactions with other molecules in the surrounding medium such as other proteins, small molecules, or DNA. One such class of proteins are what I will call polymer-coupling proteins: these proteins intentionally link identical polymers or two regions of the same polymer together so that their coupled interactions critically affect the state of the biological system. A vast array of such proteins exist in nature with roles such as the looping of DNA to physically inhibit the expression of a gene or the formation of the cytoskeleton which provides a cell with its shape. In this thesis, I use in vitro experimental methods to explore two cases of coupling proteins and understand their roles not only in reorganizing their complementary polymers but influencing the final state of their respective systems.
\r\n\r\nIn Chapter 2, I examine the starting process for the assembly of an antibody-encoding gene in developing immune cells. Motivated by data suggesting that some antibodies are less likely to be made than others, I explore how the early steps of constructing an antibody-encoding gene affect this uneven frequency of assembly. To initiate recombination, the recombination-activating gene (RAG) protein complex simultaneously binds and cuts two well-recognized sequences neighboring two antibody-encoding gene segments in order to allow other proteins to combine these exposed segments together. The sequences to which the RAG protein performs its binding and cutting functions have certain identifiable sequence patterns but can still vary. Through a single-molecule experimental method known as tethered particle motion (TPM) I show how changes to the binding site sequence can enhance or diminish the propensity of the RAG protein to bind and cut the DNA and thus explore the consequences of these altered interactions in the unequal selection for certain antibody gene segments over others.
\r\n\r\nIn Chapter 3, I turn to questions of the emergence of order from self-organization in biological systems. From the molecular to the population scale, biology constantly demonstrates that with an injection of energy, systems can be driven out of equilibrium and allow for the organization of its constituents. A case of such organization in cells is the coupling of microtubules by motor proteins to create and maintain the mitotic spindle, a critical biological architecture for ensuring that each cell obtains a copy of the genome during division. In vitro experiments that exploit similar motor-microtubule interactions have become a convenient way to identify the effects of perturbing a key player such as motor properties or boundary conditions of the system on the spatiotemporal extent of organization. However, in many instances, the dynamics under which such cytoskeletal systems reduce their entropy over the course of creating order have not been carefully examined in experimental systems. Here, I use engineered light-dimerizable motors that can give rise to the formation of a highly connected network that compacts to form a dense, organized structure, and through the use of a noninvasive imaging technique observe how the polymers that make up the network continually reorganize in the bulk during a global contraction of the network.
", "doi": "10.7907/fpmm-a552", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15013", "collection": "thesis", "collection_id": "15013", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08292022-044824279", "primary_object_url": { "basename": "Honglie_Ning_2023_Thesis.pdf", "content": "final", "filesize": 27843215, "license": "other", "mime_type": "application/pdf", "url": "/15013/1/Honglie_Ning_2023_Thesis.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Ultrafast Optical Control of Order Parameters in Quantum Materials", "author": [ { "family_name": "Ning", "given_name": "Honglie", "orcid": "0000-0003-4867-0751", "clpid": "Ning-Honglie" } ], "thesis_advisor": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "thesis_committee": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" }, { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-Scott-K" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Developing protocols to realize quantum phases that are not accessible thermally and to manipulate material properties on demand is one of the central problems of modern condensed matter physics. Impulsive electromagnetic stimulus provides an extensive playground not only to exert desired control over the material macroscopic properties but also to optically detect the underlying microscopic mechanisms. Two indispensable components form the cornerstone to realize these goals: a meticulous comprehension of light-induced phenomena and a suitable and versatile platform.
\r\n\r\nAbundant photoinduced phenomena emerge upon light irradiation. A collective oscillation of order parameter can be launched and probed in the weak perturbation regime; further increasing light intensity can transiently modulate the free-energy landscape, inducing a suppression, enhancement, reversal, and switch of order parameters; in the strong non-perturbative excitation regime, the system can be driven nonlinearly with microscopic coupling parameters modified. Understanding these light driven emergent phenomena lays the foundation of optical control and novel functionalities.
\r\n\r\nQuantum materials, embodying a large portfolio of topological and strongly correlated compounds, afford an exceptional venue to realize optical control. Owing to the complex interplay between the charge, spin, orbital, and lattice degrees of freedom, a rich phase diagram can be generated with various phases that are selectively and independently accessible via optical perturbations. They hence offer a wealth of opportunities to not only improve our comprehension of the underlying physics but also develop the next generation of ultrafast technologies.
\r\n\r\nIn Chapter I of this thesis, I will first cover a multitude of light-induced emergent phenomena in quantum materials under the framework of time-dependent Landau theory, Keldysh theory, and Floquet theory, and then introduce several canonical microscopic models to quantitatively rationalize the intra- and interactions between different degrees of freedom in quantum materials. As the necessary theoretical background is established, three main experimental techniques that have been extensively utilized in my research: time-resolved reflectivity and Kerr effect, time-resolved second harmonic generation rotational anisotropy, and coherent phonon spectroscopy will be introduced in Chapter II. In Chapter III, I will demonstrate that a light-induced topological phase transition can be engendered concomitant with an inverse-Peierls structural phase transition in elemental Te. In Chapter IV, I will describe signatures of ultrafast reversal of excitonic order in excitonic insulator candidate Ta2NiSe5 and substantiate a manipulation of the reversal as well as the Higgs mode with tailored light pulses. In Chapter V, a light-induced switch of spin-orbit-coupled quadrupolar order in multiband Mott insulator Ca2RuO4 will be introduced. In Chapter VI, a Keldysh tuning of nonlinear carrier excitation and Floquet bandwidth renormalization in strongly driven Ca2RuO4 will be covered.
", "doi": "10.7907/yxa0-6884", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15127", "collection": "thesis", "collection_id": "15127", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04032023-062047194", "primary_object_url": { "basename": "SouvikBiswas_Thesis_Final.pdf", "content": "final", "filesize": 15418859, "license": "other", "mime_type": "application/pdf", "url": "/15127/1/SouvikBiswas_Thesis_Final.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Electro-Optic Excitations in van der Waals Materials for Active Nanophotonics", "author": [ { "family_name": "Biswas", "given_name": "Souvik", "orcid": "0000-0002-8021-7271", "clpid": "Biswas-Souvik" } ], "thesis_advisor": [ { "family_name": "Atwater", "given_name": "Harry Albert", "orcid": "0000-0001-9435-0201", "clpid": "Atwater-H-A" } ], "thesis_committee": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Faraon", "given_name": "Andrei", "orcid": "0000-0002-8141-391X", "clpid": "Faraon-A" }, { "family_name": "da Jornada", "given_name": "Felipe H.", "orcid": "0000-0001-6712-7151", "clpid": "da-Jornada-FElipe-H" }, { "family_name": "Atwater", "given_name": "Harry Albert", "orcid": "0000-0001-9435-0201", "clpid": "Atwater-H-A" } ], "local_group": [ { "literal": "Kavli Nanoscience Institute" }, { "literal": "div_eng" } ], "abstract": "van der Waals materials are emerging due to their unique properties such as atomic thickness, diverse quasiparticle optical resonances, and no requirement for lattice matching. While there is a vast variety of materials, semiconductors hold a special place for opto-electronic and linear/non-linear optical studies. Black phosphorus (BP), a 2D quantum-well with direct bandgap and puckered crystal structure, is a compelling platform for this research direction. In this thesis, we investigate fundamental optical excitations in novel low-dimensional quantum materials to achieve strong light-matter interaction and integrate with nanophotonic motifs for low-footprint, reconfigurable optical technology, focusing primarily on black phosphorus and transition metal dichalcogenides.
\r\n\r\nThe thesis begins with the 'thin film limit' of van der Waals materials, between 5 and 20 nm thickness range. Chapters 2 and 3 explore how few-layer black phosphorus hosts interband and intraband optical excitations that can be strongly modified with gate-controlled doping and electric field, displaying epsilon near zero and hyperbolic behavior in the mid and far-infrared. In atomic thickness, strongly bound excitonic quasiparticles dominate the optical response. In Chapter 4, we investigate electrically tunable excitons in tri-layer black phosphorus, demonstrating a reconfigurable birefringent material that, when coupled with a Fabry-Perot cavity, enables the realization of a versatile and broadband polarization modulator. In Chapter 5, we examine the ultimate limit of a monolayer, studying MoTe2 via photoluminescence measurements and first-principles GW+BSE calculations, highlighting the Rydberg series associated with the exciton and its gate-tunability to understand strong electron-exciton interactions. In Chapter 6, we show how such excitons in monolayer black phosphorus can be strongly quantum confined at natural edges of exfoliated flakes, leading to highly temporally coherent emission. This emission is gate-tunable and understood via transmission electron microscopy and first-principles GW+BSE calculations of phosphorene nanoribbons to be originating from atomic reconstructions of the edge coupled with strain and screening effects.
\r\n\r\nOverall, our work highlights the potential of van der Waals materials for various electro-optical excitations and their applications in active nanophotonics.\r\n
", "doi": "10.7907/tz4z-ed06", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15273", "collection": "thesis", "collection_id": "15273", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022023-010804337", "type": "thesis", "title": "Fractonic Orders from Lattice Models and Field Theories", "author": [ { "family_name": "Ma", "given_name": "Xiuqi", "orcid": "000-0001-8294-2277", "clpid": "Ma-Xiuqi" } ], "thesis_advisor": [ { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Ni", "given_name": "Yi", "orcid": "0000-0002-5287-4258", "clpid": "Ni-Yi" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Fracton models are characterized by exotic features such as point-like excitations with restricted mobility, sub-extensive ground state degeneracy and UV/IR mixing. They have been studied previously using exactly solvable lattice models, higher rank gauge theories, etc. In an effort to classify fracton models into phases (i.e., fractonic orders), the so-called foliation structure has been introduced and shown to exist in many previously known models. A natural question then arises concerning the feasibility of the foliation paradigm in general. In this thesis, I study fracton models beyond the foliation paradigm and give simple diagnostics for the absence of a foliation structure. New notions of fractonic orders therefore need to be conceived, and I present such a conception which is a generalization of the foliation RG.
\r\n\r\nIn Chapters 2 - 4, I introduce new fracton models obtained from infinite-component Chern-Simons (CS\u221e) theories. By calculating observables such as ground state degeneracy and planon braiding statistics, I prove that most CS\u221e theories are not foliated. A CS\u221e theory can also be gapless with certain choices of parameters, and I show that such a theory is a stable gapless fracton model. Furthermore, I discuss topological features of a large subclass of gapless CS\u221e theories and present fully continuous effective field theories for this subclass.
\r\n\r\nIn Chapters 5 - 6, I discuss a new notion of fractonic orders by studying the example of the Ising cage-net model. I begin by calculating the ground state degeneracy of the model, which shows that the model is not foliated. The calculation uses an operator algebra approach which relies only on intrinsic physical properties of the model rather than microscopic details, and I establish the framework of this approach conceptually and via examples. I then argue why this intrinsic approach, despite being a tool for calculation initially, may be a useful characterization of a fractonic order. Finally, I present a generalized foliation RG scheme, apply it to the Ising cage-net model, and discuss its limitations.
", "doi": "10.7907/g80m-dy31", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:15198", "collection": "thesis", "collection_id": "15198", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05202023-131846404", "type": "thesis", "title": "Ultrafast Dynamics of Photo-Doped Mott Antiferromagnets", "author": [ { "family_name": "Mehio", "given_name": "Omar", "orcid": "0000-0001-7923-2178", "clpid": "Mehio-Omar" } ], "thesis_advisor": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "thesis_committee": [ { "family_name": "Endres", "given_name": "Manuel A.", "orcid": "0000-0002-4461-224X", "clpid": "Endres-M" }, { "family_name": "Cushing", "given_name": "Scott K.", "orcid": "0000-0003-3538-2259", "clpid": "Cushing-Scott-K" }, { "family_name": "Lee", "given_name": "Patrick A.", "orcid": "0000-0001-7809-8157", "clpid": "Lee-Patrick-A" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Strong coupling between spin and charge degrees of freedom in two-dimensional spin-1/2 Mott antiferromagnets (AFMs) creates a rich platform to study quantum many-body physics. For decades, the consequences of these interactions have been intensely studied in thermal equilibrium, where the introduction of charge carriers through chemical doping has been shown to generate a vibrant phase diagram rich with unconventional types of charge, spin, and orbital ordering. In recent years, however, attention has grown to include the study of these materials as they are driven far from equilibrium using intense pulses of light produced by femtosecond laser sources. In addition to fundamental interest in the resultant dynamics, recent experimental and theoretical studies have suggested that driven Mott insulators can host states of matter that cannot be accessed in thermal equilibrium.
\r\n\r\nWhile many driving protocols have been developed---spanning from the selective excitation of bosonic modes to photon-dressing via coherent time-periodic driving---the simplest conceptual approach to engineering Mott insulators with light is known as photo-doping. In this procedure, the material is impulsively driven resonantly with a transition from a filled band to an empty band, transiently producing charge carriers. Given the impact of chemical doping in thermal equilibrium, photo-doping has garnered interest as an important tool in the study of driven Mott insulators. Early successes in the study of photo-doped Mott AFMs include the observation of ultrafast demagnetization and the prediction of non-thermal magnetic states, charge density waves, and superconductivity. Photo-doping thus holds promise to generate an out-of-equilibrium phase diagram that is equally rich to that found in equilibrium.
\r\n\r\nYet, many open questions about the basic properties of photo-doped Mott insulators remain unresolved. Whether charge instabilities exist as a result of interactions between the photo-dopants has yet to be examined. Moreover, while theoretical studies have suggested that antiferromagnetic correlations can enhance attractive interactions between photo-dopants, evidence of the resultant bound states remain elusive. Even the light-matter interactions that generate the photo-dopants are in need of investigation, as the fate of a Mott insulator driven by strong electric fields remains a fundamental open theoretical and experimental problem.
\r\n\r\nIn this thesis, I present a series of experiments designed to answer each of these questions. After describing the properties of Mott insulators in Chapter 1, I present the experimental details of the tools that enable these studies in Chapter 2. Taking a multi-messenger approach to ultrafast spectroscopy, a suite of ultrafast probes simultaneously track the spin and charge degrees of freedom to paint a holistic picture of the out-of-equilibrium state. In Chapter 3, I use ultrafast THz conductivity to establish the existence of an insulating photo-excited fluid of Hubbard excitons (HEs), which are bound states that are thought to form as a result of attractive spin-mediated interactions. This magnetic binding mechanism is studied in more detail in Chapter 4 by examining the properties of these HEs in the magnetic critical region of several materials that lie in different magnetic universality classes. In Chapter 5, I study the effects of HE formation on the ultrafast demagnetization that is known to occur following photo-doping. Finally, I turn my attention towards the photo-dopant generation mechanism in Chapter 6, exploring the effects of strong electric field driving in Mott insulators. I find signatures of the so-called Keldysh crossover from a multiphoton-absorption- to a quantum-tunneling-dominated pair production regime. Altogether, this work establishes photo-doped Mott insulators as a rich playground to engineer non-equilibrium phases of matter and study quantum many-body dynamics.
", "doi": "10.7907/fsbz-pd46", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" }, { "id": "thesis:14645", "collection": "thesis", "collection_id": "14645", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05272022-220331463", "primary_object_url": { "basename": "Wilbur_Shirley_Thesis.pdf", "content": "final", "filesize": 26187118, "license": "other", "mime_type": "application/pdf", "url": "/14645/1/Wilbur_Shirley_Thesis.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Structure of Entanglement in Fracton Phases of Matter", "author": [ { "family_name": "Shirley", "given_name": "Wilbur Eric", "orcid": "0000-0002-4970-1460", "clpid": "Shirley-Wilbur-Eric" } ], "thesis_advisor": [ { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" } ], "thesis_committee": [ { "family_name": "Preskill", "given_name": "John P.", "orcid": "0000-0002-2421-4762", "clpid": "Preskill-J" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "This thesis discusses recent contributions to the theory of gapped fracton phases of matter, utilizing exactly solvable Hamiltonian models as the primary tool of study. A large component of the work revolves around the notion of a foliation structure, which is a defining feature of the long-range entanglement in certain gapped fracton states. We introduce this concept, identify its presence in a handful of prominent fracton models, and explore its consequences in terms of entanglement entropy and fractional excitations. A second major theme of the thesis is the characterization of gapped fracton states via emergent gauge theories based on discrete subsystem symmetries. We introduce a variety of novel fractonic gauge theories including twisted and fermionic variants, identify their emergence in a bevy of well-known models, and classify them with the use of novel topological invariants. We also establish a link between subsystem symmetry and entanglement renormalization group flow in fractal spin liquids.
", "doi": "10.7907/7b2c-2e06", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14543", "collection": "thesis", "collection_id": "14543", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:04082022-171550192", "type": "thesis", "title": "Two-Dimensional Transition Metal Dichalcogenides for Ultrathin Solar Cells", "author": [ { "family_name": "Went", "given_name": "Cora Margaret", "orcid": "0000-0001-7737-3348", "clpid": "Went-Cora-Margaret" } ], "thesis_advisor": [ { "family_name": "Atwater", "given_name": "Harry Albert", "orcid": "0000-0001-9435-0201", "clpid": "Atwater-H-A" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" }, { "family_name": "Refael", "given_name": "Gil", "clpid": "Refael-G" }, { "family_name": "Atwater", "given_name": "Harry Albert", "orcid": "0000-0001-9435-0201", "clpid": "Atwater-H-A" } ], "local_group": [ { "literal": "Resnick Sustainability Institute" }, { "literal": "div_pma" } ], "abstract": "Ultrathin solar cells, with absorber layers less than one micron thick, have the potential to use orders of magnitude less high-quality semiconducting material than current silicon solar cells. This could be advantageous in applications that require high power output per unit weight, such as vehicle-integrated photovoltaics, or where reducing the capital cost of solar cell manufacturing is important. Transition metal dichalcogenides are a promising candidate for the semiconducting absorber layer of ultrathin solar cells due to their intrinsically passivated surfaces and their high absorption per unit thickness.
\r\n\r\nThis thesis explores two-dimensional transition metal dichalcogenides for ultrathin photovoltaics. We start with the simplest type of solar cell, which collects carriers via a Schottky junction formed by sandwiching the absorber layer between two metal contacts with different work functions. To enable this geometry and avoid Fermi-level pinning, we develop a new process for gently transferring van der Waals metal contacts onto transition metal dichalcogenides. We measure an open-circuit voltage of 250 mV and a power conversion efficiency of 0.5% in Schottky-junction solar cells. To improve upon this efficiency, we next make carrier-selective contact solar cells, which employ wide bandgap semiconductors to selectively collect electrons on one side and holes on the other side of the absorber layer. We measure an open-circuit voltage of 520 mV and a power conversion efficiency greater than 2% in devices based on perovskite solar cell geometries, with PTAA and C60 as selective contact layers. We demonstrate that short carrier lifetimes limit the voltage in these solar cells to 750 mV, well below the detailed balance voltage limit. This motivates a more thorough understanding of the carrier dynamics at play, and we use a new pump-probe optical microscopy technique, stroboSCAT, to spatiotemporally track heat and carrier evolution in transition metal dichalcogenides. When paired with a kinetic model, we show that this technique can be used to measure lifetimes and other important material parameters even in materials with low radiative efficiencies.
\r\n\r\nWe conclude by outlining future research directions towards achieving power conversion efficiencies greater than 10% in transition metal dichalcogenide solar cells.
", "doi": "10.7907/xrxk-3q08", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14372", "collection": "thesis", "collection_id": "14372", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09242021-222116257", "primary_object_url": { "basename": "Youngjoon_Choi_PhD_Thesis_Final.pdf", "content": "final", "filesize": 49417580, "license": "other", "mime_type": "application/pdf", "url": "/14372/1/Youngjoon_Choi_PhD_Thesis_Final.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "A Spectroscopic Study of Electronic Correlations in Twisted Bilayer Graphene by Scanning Tunneling Microscopy", "author": [ { "family_name": "Choi", "given_name": "Youngjoon", "orcid": "0000-0001-9783-5992", "clpid": "Choi-Youngjoon" } ], "thesis_advisor": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "thesis_committee": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Lee", "given_name": "Patrick A.", "orcid": "0000-0001-7809-8157", "clpid": "Lee-Patrick-A" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Twisted bilayer graphene around the magic angle has shown variety of correlated phases such as superconductivity, correlated insulators, and magnetism due to its flat band structure. The unconventional nature of the superconductivity and its pos- sible relation to high temperature superconductors have sparked a lot of theoretical and experimental efforts to understand the properties of the magic angle twisted bilayer graphene. While electrical transport measurements revealed the interesting phases, spectroscopic understanding is strongly needed to connect the phases with theoretical calculations. We present the spectroscopic studies of gate-tunable magic angle twisted bilayer graphene using scanning tunneling microscopy. We report that the band structure is significantly modified even at charge neutrality due to exchange interaction. We apply a perpendicular magnetic field and develop a novel method that enables scanning tunneling microscopy to reveal Landau fan diagrams. We discover topologically non-trivial states appearing at finite magnetic field, and from spectroscopy we are able to identify the mechanism. Finally, we verify inter- action driven band flattening experimentally in twisted bilayer graphene, which is responsible for creating strong correlations.
", "doi": "10.7907/ajgk-7246", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14576", "collection": "thesis", "collection_id": "14576", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05112022-213102696", "primary_object_url": { "basename": "Junyi_Shan_thesis_2022_full.pdf", "content": "final", "filesize": 5095753, "license": "other", "mime_type": "application/pdf", "url": "/14576/8/Junyi_Shan_thesis_2022_full.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Non-Thermal Optical Engineering of Strongly-Correlated Quantum Materials", "author": [ { "family_name": "Shan", "given_name": "Junyi", "orcid": "0000-0001-7665-2169", "clpid": "Shan-Junyi" } ], "thesis_advisor": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "thesis_committee": [ { "family_name": "Rosenbaum", "given_name": "Thomas F.", "orcid": "0009-0008-6152-666X", "clpid": "Rosenbaum-T-F" }, { "family_name": "Lee", "given_name": "Patrick A.", "orcid": "0000-0001-7809-8157", "clpid": "Lee-Patrick-A" }, { "family_name": "Falson", "given_name": "Joseph", "orcid": "0000-0003-3183-9864", "clpid": "Falson-Joseph" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "This thesis develops multiple optical engineering mechanisms to modulate the electronic, magnetic, and optical properties of strongly-correlated quantum materials, including polar metals, transition metal trichalcogenides, and copper oxides. We established the mechanisms of Floquet engineering and magnon bath engineering, and used optical probes, especially optical nonlinearity, to study the dynamics of these quantum systems.
\r\n \r\nStrongly-correlated quantum materials host complex interactions between different degrees of freedom, offering a rich phase diagram to explore both in and out of equilibrium. While static tuning methods of the phases have witnessed great success, the emerging optical engineering methods have provided a more versatile platform. For optical engineering, the key to success lies in achieving the desired tuning while suppressing other unwanted effects, such as laser heating.
\r\n \r\nWe used sub-gap optical driving in order to avoid electronic excitation. Therefore, we managed to directly couple to low-energy excitation, or to induce coherent light-matter interactions. In order to elucidate the exact microscopic mechanisms of the optical engineering effects, we performed photon energy-dependent measurements and thorough theoretical analysis. To experimentally access the engineered quantum states, we leveraged various probe techniques, including the symmetry-sensitive optical second harmonic generation (SHG), and performed pump-probe type experiments to study the dynamics of quantum materials.
\r\n \r\nI will first introduce the background and the motivation of this thesis, with an emphasis on the principles of optical engineering within the big picture of achieving quantum material properties on demand (Chapter I). I will then continue to introduce the main probe technique used in this thesis: SHG. I will also introduce the experimental setups which we developed and where we conducted the works contained in this thesis (Chapter II). In Chapter III, I will introduce an often overlooked aspect of SHG studies -- using SHG to study short-range structural correlations. Chapter IV will contain the theoretical analysis and experimental realizations of using sub-gap and resonant optical driving to tune electronic and optical properties of MnPS\u2083. The main tuning mechanism used in this chapter is Floquet engineering, where light modulates material properties without being absorbed. In Chapter V, I will turn to another useful material property: magnetism. First I will describe the extension of the Floquet mechanism to the renormalization of spin exchange interaction. Then I will switch gears and describe the demagnetization in Sr\u2082Cu\u2083O\u2084Cl\u2082 by resonant coupling between photons and magnons. I will end the thesis with a brief closing remark (Chapter VI).
", "doi": "10.7907/0shs-fa90", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:14933", "collection": "thesis", "collection_id": "14933", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022022-002941282", "type": "thesis", "title": "Interface Optimization for Improved Photovoltaic Devices", "author": [ { "family_name": "Glaudell", "given_name": "Rebecca Denise", "orcid": "0000-0001-5701-2932", "clpid": "Glaudell-Rebecca-Denise" } ], "thesis_advisor": [ { "family_name": "Atwater", "given_name": "Harry Albert", "orcid": "0000-0001-9435-0201", "clpid": "Atwater-H-A" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Refael", "given_name": "Gil", "clpid": "Refael-G" }, { "family_name": "Scherer", "given_name": "Axel", "clpid": "Scherer-A" }, { "family_name": "Atwater", "given_name": "Harry Albert", "orcid": "0000-0001-9435-0201", "clpid": "Atwater-H-A" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The wide band gaps and superior conductivity of ZnS\u2093Se\u2081\u208b\u2093 semiconductors to amorphous Si suggest an alternative carrier-selective contact in silicon heterojunction solar cells. Electron-selective ZnS\u2093Se\u2081\u208b\u2093 front contacts on p-type c-Si solar cells are explored by simulating in Sentaurus TCAD a large design parameter space informed by experimentally determined optoelectronic properties. Comparable performance to experimental and simulated p-SHJ reference devices is shown, with a champion simulated device efficiency of 20.8%. X-ray photoelectron spectroscopy is used to measure band offsets at interfaces for the aforementioned ZnS\u2093Se\u2081\u208b\u2093-c-Si photovoltaic devices as well as various carrier-selective contacts and passivation layers for GaAs photovoltaic devices.
", "doi": "10.7907/kxdy-h496", "publication_date": "2022", "thesis_type": "phd", "thesis_year": "2022" }, { "id": "thesis:13839", "collection": "thesis", "collection_id": "13839", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07092020-003626793", "type": "thesis", "title": "Investigation of the Physical Properties of Dirac Materials", "author": [ { "family_name": "Chen", "given_name": "Chien-Chang", "orcid": "0000-0003-0959-5584", "clpid": "Chen-Chien-Chang" } ], "thesis_advisor": [ { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" } ], "thesis_committee": [ { "family_name": "Eisenstein", "given_name": "James P.", "clpid": "Eisenstein-J-P" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "This thesis focuses on the investigation of two types of Dirac materials: topological insulators (TI) and graphene. Both materials have received much attention and stimulated intense research activities over the last decade. Although massless Dirac electron are wonderful, there will be more industrial applications if we can open the gap and make Dirac electrons massive. For topological insulators, we focus on studies of the TI/Magnetic TI (MTI) bilayer structures to induce a gap on the surface state. For graphene, the author focuses on the Moir\u00e9 pattern and interlayer interaction.
\r\n\r\nFor bilayer TI/MTI samples, they were investigated with scanning tunneling microscopy and spectroscopy (STM/STS), and with electrical transport measurements by means of a Physical Property Measurement System (PPMS). Details of the experimental setups for this research and their upgrades were described. For the current STM system, both the tube scanner and sample stage in the STM head had been redesigned and rebuilt, which led to better XYZ fine approach control, improved wire protection, and enhanced noise shielding. A new back gate capability was added to the sample stage. A customized commercial STM system has been commissioned, which is expected to provide a better sample holder with improved vacuum seals and easier temperature control, as well as more convenient approaches to loading samples and switching STM or AFM (atomic force microscope) tips. For PPMS, an optical probe had been designed and constructed, which enabled light-induced effects on the electrical transport properties of TIs. A new custom-made glove box has been installed, which provides a computer-controlled and self-circling gas environment to minimize the concentration of air while reduces the waste of argon. The glove box is also easy to use. This upgrade helps expand our abilities to conduct research more efficiently.
\r\n\r\nSTM/STS studies of both the binary and ternary types of magnetic topological insulators (MTIs) are presented. For both binary and ternary bilayer TI/MTI systems, the majority of the density of states (DOS) spectra evolved with the temperature. At room temperature, all samples showed massless Dirac spectra. However, for temperatures below 200 K, all bilayer samples with the top pure TI layer thinner than 5QL revealed opening of a surface gap. Generally, binary TI/MTI samples exhibited smaller gapped domains, which was consistent with the finding of nearly negligible hysteretic behavior for Hall resistance vs, magnetic field sweeps at low temperatures. In contrast, ternary TI/MTI samples exhibited larger gapped domains, which implied longer range ferromagnetic order and was indeed corroborated by the apparent hysteretic behavior in the electrical transport measurements at low temperatures. Additionally, the application of c-axis magnetic fields led to slighter larger surface gaps and more uniform gap distributions, which further confirmed the physical origin of the surface gap as magnetic in nature. Besides the U or V-shaped DOS spectra, double-peak or single peak impurity resonances were also observed. These spatially localized minority spectra were found to mostly appear along the boundaries of gapped and gapless domains. Moreover, the number of impurities was founded to reach a maximum around 240 K, which corresponded to the onset temperature of localized surface gaps.
\r\n\r\nDetailed studies of the electrical transport properties of both the binary and ternary MTIs by the PPMS provided a comparison between the macroscopic information thus obtained with the microscopic information derived from STS studies. Binary TI/MTI showed an anonymous Hall effect (AHE) at 25 K while ternary TI/MTI showed AHE around 20 K. Binary TI/MTI systems exhibited weak localization (WL) behavior in the longitudinal resistance vs. magnetic field data at 2 K. The binary TI/MTI samples with a thinner top pure TI layer revealed sharper and stronger WL behavior. In contrast, for the 3QL-TI/6QL-MTI ternary sample, weak antilocalization (WAL) behavior was present for all temperatures, while WL also showed up below 13 K. The Hall resistance vs. magnetic field data for all samples of ternary TI/MTI bilayers and ternary MTI monolayer samples revealed strong hysteresis at low temperatures, in contrast to the negligible hysteretic behavior in all binary TI/MTI samples. Finally, circularly polarized light was found to enhance the AHE of the bilayer ternary TI/MTI sample while weakening that of the monolayer ternary MTI. These experimental phenomena may be mostly attributed to the different band structures and Fermi levels among the binary and ternary TI/MTI samples. In particular, we note that the observation of quantum anomalous Hall effect (QAHE) only in ternary MTI monolayers at extremely low temperatures (at T \u2264 30 mK < < Tcbulk ~ 30 K) may be attributed to the finite contributions of bulk carriers to excess conduction unless T \u2192 0.
\r\n\r\nSimulations have been carried out to account for the Moir\u00e9 patterns of graphene on Cu (111), graphene on Cu (100), twisted bilayer graphene, and Cr-doped topological insulators. The physical origin for empirically observed structural superlubricity between graphene layers has also been modeled by simulations based on the density functional theory (DFT).
\r\n\r\nFinally, the key findings of this thesis work and the suggested future research directions are summarized.
\r\n", "doi": "10.7907/bgw9-d234", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14210", "collection": "thesis", "collection_id": "14210", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05312021-191637587", "primary_object_url": { "basename": "Thesis.pdf", "content": "final", "filesize": 5790355, "license": "other", "mime_type": "application/pdf", "url": "/14210/2/Thesis.pdf", "version": "v7.0.0" }, "type": "thesis", "title": "Exciton Dynamics Studies from First-Principles Calculations: Radiative Recombination, Exciton-Phonon Interactions, and Ultrafast Exciton Relaxation", "author": [ { "family_name": "Chen", "given_name": "Hsiao-Yi", "orcid": "000-0003-1962-5767", "clpid": "Chen-Hsiao-Yi" } ], "thesis_advisor": [ { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" }, { "family_name": "Bernardi", "given_name": "Marco", "orcid": "0000-0001-7289-9666", "clpid": "Bernardi-Marco" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Excitons are bound electron-hole pairs that dominate the optical response of semi-conductors and insulators, especially in nanoscale and wide bandgap materials where the Coulomb interaction is weakly screened. Excitons can enhance light-matter coupling at certain wavelengths, thus making their host materials candidates for optoelectronic, photovoltaic, and quantum technology devices. For instance, two-dimensional transition metal dichalcogenides have a large and tunable optical response and hold promise for next-generation ultrathin light-emitting diodes. It is remarkable that exciton properties such as the binding energy and radiative lifetime can vary by orders of magnitude in different materials and can be further tuned by material properties like defects and lattice vibrations. Therefore, quantitative studies of exciton interactions and dynamics can advance understanding of the optical response of complex materials and play a role in the design of future devices. Among theoretical studies, numerical approaches based on density functional theory (DFT) can quantitatively address the electronic structure in real materials and their response to external perturbations, enabling accurate calculations of the conductivity and dielectric properties. These first-principle methods, which employ numerical quantum mechanics and use only the atomic structure of the material as input (making no use of empirical parameters) have revolutionized studies of materials and condensed matter physics. Over the last few years, first-principles methods for studies of excitons have focused on the GW-Bathe-Salpeter equation (GW-BSE) method to compute exciton energies and optical absorption spectra. However, going beyond calculations of exciton energetics to address the exciton dynamical processes remains challenging and is an exciting new frontier of first-principles studies.
\r\n\r\nThis thesis develops theory and novel numerical approaches to study exciton radiative and nonradiative interactions from first-principles. For the radiative processes, we demonstrate a systematic derivation of exciton radiative lifetimes in materials ranging from bulk to nanostructures and molecules. The results correctly reproduce the observed power-law temperature dependence of the radiative lifetimes. To benchmark our calculations, we study exciton radiative lifetimes in gas-phase molecules, obtaining excellent agreement between theory and experiment. Our framework is then applied in three different studies. First, we extend the radiative lifetime formula to account for the dependence on light polarization and valley occupation and investigate exciton recombination in two-dimensional transition metal dichalcogenides (2D-TMDs). We show that excitons emit light anisotropically upon recombination when they are in any quantum superposition state of the K and K' inequivalent valleys. When averaged over the emission angle and exciton momentum, our new treatment recovers the temperature-dependent radiative lifetimes derived in early literature. Second, we use the exciton energy and radiative lifetimes to identify the atomic structure of the defects in monolayer hexagonal boron nitride (h-BN). In the study, we narrow down the potential structures to nine candidates and identify the highest-likelihood structure as the VNNB defect, consisting of a nitrogen vacancy plus a carbon replacing boron in h-BN. Finally, we generalize the discussion of isotropic bulk system to accurately compute the exciton radiative lifetimes in bulk uniaxial crystals, focusing on wurtzite GaN. Our computed radiative lifetimes are in very good agreement with experiments at low temperature. We show that taking into account excitonic effect and spin-orbit coupling (to include the exciton fine structure) is essential for computing accurate radiative lifetimes. A model for exciton dissociation into free carriers allows us to compute the radiative lifetimes up to room temperature.
\r\n\r\nIn the study of exciton non-radiative process, we focus on the exciton-phonon (ex-ph)interaction, which plays an important role to understand the dynamics of excitons in materials. We establish and implement a first-principle formalism to compute the ex-ph coupling constants by combining the electron-phonon couplings and the exciton wavefunctions from the GW-BSE approach. Using the computed ex-ph coupling matrix elements, we calculate the ex-ph relaxation times as a function of exciton energy, momentum, temperature, and phonon mode in bulk h-BN. Our calculations reveal the dominant ex-ph coupling with the longitudinal optical (LO) mode and identify the threshold for LO phonon emission with an associated \u223c15 fs LO emission characteristic time. In addition, we derive the phonon-assisted photoluminescence(PL) from the ex-ph interaction and correctly reproduce the PL spectrum observed in h-BN at both 8 K and 100 K. Based on our successful study of ex-ph interactions in bulk h-BN, we extend the discussion to materials with strong spin-orbit coupling. We investigate the bright exciton linewidth broadening and PL in monolayer WSe2. The numerical results show an increase of linewidth by 20 meV from 0 K to 250 K as observed in early experiments and identify the main PL peak as a consequence of LA phonon emission while the side band is due to optical phonons. Lastly, we present results from a joint theory-experiment study of the ultrafast exciton dynamics in WSe2. We develop a Boltzmann equation for excitons and employ it to model ultrafast exciton relaxation due to ex-ph processes. The simulation and experiment both show a ~70 fs time delay for the electron intervalley scattering from the K- to the Q-valley due to exciton dynamical effects. We also develop accurate simulations of time-domain angle-resolved photoemission (ARPES) experiments, which are becoming a powerful experimental probe of exciton dynamics in condensed matter. In summary, this thesis work paves the way to quantitative studies of exciton radiative and non-radiative processes, as well as exciton ultrafast dynamics, and quantitative modeling of pump-probe experiments in materials with strongly bound excitons.
", "doi": "10.7907/4edg-jw48", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:14250", "collection": "thesis", "collection_id": "14250", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072021-061251562", "primary_object_url": { "basename": "Swati_ThesisJune8.pdf", "content": "final", "filesize": 85448266, "license": "other", "mime_type": "application/pdf", "url": "/14250/1/Swati_ThesisJune8.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Light Induced Dynamics in Quantum Matter", "author": [ { "family_name": "Chaudhary", "given_name": "Swati", "clpid": "Swati Chaudhary" } ], "thesis_advisor": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Endres", "given_name": "Manuel A.", "orcid": "0000-0002-4461-224X", "clpid": "Endres-M" }, { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "This thesis presents studies of different schemes to probe and manipulate quantum matter using light with an aim to discover novel routes to efficiently control the properties of quantum materials. A special focus is placed on developing new schemes utilizing light-matter interactions (1) to modify exchange interactions in magnetic insulators, and (2) to probe and modify band topology in quantum matter.
\r\n\r\nIn part II, new schemes are presented to probe local band topology of Bloch bands. First, we study the effects of time-dependent band topology on adiabatic evolution of a Bloch wavepacket. We find that it results in an electric-field analog in semi-classical equation of motion, and can be demonstrated in a honeycomb lattice by varying the sublattice offset energy. We then extend these methods to include non-adiabatic processes, and found interesting connections between the anomalous drift during band excitation and a quantum geometric quantity known as shift-vector. We generalize the concept of shift-vector to include different kinds of band transition protocols beyond light-induced dipole transitions. The idea of electric-field analog and the shift-vector are then combined to develop a novel charge pumping scheme. Motivated by these interesting consequences of band topology in non-adiabatic processes, we study shift-current response in moir\u00e9 materials, and find that the highly topological nature of flat bands along with their very large unit cells significantly enhances these shift-vector related effects. This response also displays a strong dependence on interaction-induced changes in the band structure and quantum geometric quantities. These results suggest that shift-current response can possibly serve as a very reliable probe for interactions in twisted bilayer graphene. In addition to studying consequences of band topology on single-particle transport, we also consider Berry curvature effects on exciton transport. We find that the non-trivial band topology of underlying electron and hole bands allows us to manipulate excitons with a uniform electric field. We examine the conditions necessary to observe such transport and propose that transition metal dichalcogenide heterobilayers with moir\u00e9 structure can prove an ideal platform for these effects.
\r\n\r\nIn part III, we propose novel drive protocols based on manipulating orbital and lattice degrees of freedom in quantum materials with light. We found that light induced changes in orbital hybridization and their electronic energies results in a significant change in exchange interactions in quantum magnets. We also accounted for the role of ligands in periodically driven quantum magnets, and found that the predictions made by the minimal model based on direct-hopping can be wrong in certain regimes of drive parameters. This understanding of light induced modifications in ligand-mediated exchange interactions was used to explain the phase shift observed in coherent phonon oscillations of CrSiTe\u2083 upon the onset of short-range spin correlations. We also demonstrate that light induced coherent lattice vibrations can provide a new route to realize space-time symmetry protected topological phases. Our results suggest that manipulating additional degrees of freedom (not included in commonly employed minimal models of periodically driven systems) with light can provide novel routes for ultrafast control of quantum materials.
", "doi": "10.7907/a8z1-5c40", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:13839", "collection": "thesis", "collection_id": "13839", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:07092020-003626793", "type": "thesis", "title": "Investigation of the Physical Properties of Dirac Materials", "author": [ { "family_name": "Chen", "given_name": "Chien-Chang", "orcid": "0000-0003-0959-5584", "clpid": "Chen-Chien-Chang" } ], "thesis_advisor": [ { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" } ], "thesis_committee": [ { "family_name": "Eisenstein", "given_name": "James P.", "clpid": "Eisenstein-J-P" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "This thesis focuses on the investigation of two types of Dirac materials: topological insulators (TI) and graphene. Both materials have received much attention and stimulated intense research activities over the last decade. Although massless Dirac electron are wonderful, there will be more industrial applications if we can open the gap and make Dirac electrons massive. For topological insulators, we focus on studies of the TI/Magnetic TI (MTI) bilayer structures to induce a gap on the surface state. For graphene, the author focuses on the Moir\u00e9 pattern and interlayer interaction.
\r\n\r\nFor bilayer TI/MTI samples, they were investigated with scanning tunneling microscopy and spectroscopy (STM/STS), and with electrical transport measurements by means of a Physical Property Measurement System (PPMS). Details of the experimental setups for this research and their upgrades were described. For the current STM system, both the tube scanner and sample stage in the STM head had been redesigned and rebuilt, which led to better XYZ fine approach control, improved wire protection, and enhanced noise shielding. A new back gate capability was added to the sample stage. A customized commercial STM system has been commissioned, which is expected to provide a better sample holder with improved vacuum seals and easier temperature control, as well as more convenient approaches to loading samples and switching STM or AFM (atomic force microscope) tips. For PPMS, an optical probe had been designed and constructed, which enabled light-induced effects on the electrical transport properties of TIs. A new custom-made glove box has been installed, which provides a computer-controlled and self-circling gas environment to minimize the concentration of air while reduces the waste of argon. The glove box is also easy to use. This upgrade helps expand our abilities to conduct research more efficiently.
\r\n\r\nSTM/STS studies of both the binary and ternary types of magnetic topological insulators (MTIs) are presented. For both binary and ternary bilayer TI/MTI systems, the majority of the density of states (DOS) spectra evolved with the temperature. At room temperature, all samples showed massless Dirac spectra. However, for temperatures below 200 K, all bilayer samples with the top pure TI layer thinner than 5QL revealed opening of a surface gap. Generally, binary TI/MTI samples exhibited smaller gapped domains, which was consistent with the finding of nearly negligible hysteretic behavior for Hall resistance vs, magnetic field sweeps at low temperatures. In contrast, ternary TI/MTI samples exhibited larger gapped domains, which implied longer range ferromagnetic order and was indeed corroborated by the apparent hysteretic behavior in the electrical transport measurements at low temperatures. Additionally, the application of c-axis magnetic fields led to slighter larger surface gaps and more uniform gap distributions, which further confirmed the physical origin of the surface gap as magnetic in nature. Besides the U or V-shaped DOS spectra, double-peak or single peak impurity resonances were also observed. These spatially localized minority spectra were found to mostly appear along the boundaries of gapped and gapless domains. Moreover, the number of impurities was founded to reach a maximum around 240 K, which corresponded to the onset temperature of localized surface gaps.
\r\n\r\nDetailed studies of the electrical transport properties of both the binary and ternary MTIs by the PPMS provided a comparison between the macroscopic information thus obtained with the microscopic information derived from STS studies. Binary TI/MTI showed an anonymous Hall effect (AHE) at 25 K while ternary TI/MTI showed AHE around 20 K. Binary TI/MTI systems exhibited weak localization (WL) behavior in the longitudinal resistance vs. magnetic field data at 2 K. The binary TI/MTI samples with a thinner top pure TI layer revealed sharper and stronger WL behavior. In contrast, for the 3QL-TI/6QL-MTI ternary sample, weak antilocalization (WAL) behavior was present for all temperatures, while WL also showed up below 13 K. The Hall resistance vs. magnetic field data for all samples of ternary TI/MTI bilayers and ternary MTI monolayer samples revealed strong hysteresis at low temperatures, in contrast to the negligible hysteretic behavior in all binary TI/MTI samples. Finally, circularly polarized light was found to enhance the AHE of the bilayer ternary TI/MTI sample while weakening that of the monolayer ternary MTI. These experimental phenomena may be mostly attributed to the different band structures and Fermi levels among the binary and ternary TI/MTI samples. In particular, we note that the observation of quantum anomalous Hall effect (QAHE) only in ternary MTI monolayers at extremely low temperatures (at T \u2264 30 mK < < Tcbulk ~ 30 K) may be attributed to the finite contributions of bulk carriers to excess conduction unless T \u2192 0.
\r\n\r\nSimulations have been carried out to account for the Moir\u00e9 patterns of graphene on Cu (111), graphene on Cu (100), twisted bilayer graphene, and Cr-doped topological insulators. The physical origin for empirically observed structural superlubricity between graphene layers has also been modeled by simulations based on the density functional theory (DFT).
\r\n\r\nFinally, the key findings of this thesis work and the suggested future research directions are summarized.
\r\n", "doi": "10.7907/bgw9-d234", "publication_date": "2021", "thesis_type": "phd", "thesis_year": "2021" }, { "id": "thesis:13781", "collection": "thesis", "collection_id": "13781", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06052020-175219708", "primary_object_url": { "basename": "PhD_Thesis_Harpreet_Arora_Final.pdf", "content": "final", "filesize": 43517279, "license": "other", "mime_type": "application/pdf", "url": "/13781/14/PhD_Thesis_Harpreet_Arora_Final.pdf", "version": "v9.0.0" }, "type": "thesis", "title": "Superconductivity in Graphene Hetero-Structures: From Fundamental Physics to Functional Devices", "author": [ { "family_name": "Arora", "given_name": "Harpreet Singh", "orcid": "0000-0002-7674-735X", "clpid": "Arora-Harpreet-Singh" } ], "thesis_advisor": [ { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "thesis_committee": [ { "family_name": "Roukes", "given_name": "Michael Lee", "orcid": "0000-0002-2916-6026", "clpid": "Roukes-M-L" }, { "family_name": "Rosenbaum", "given_name": "Thomas F.", "orcid": "0009-0008-6152-666X", "clpid": "Rosenbaum-T-F" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "local_group": [ { "literal": "div_eng" } ], "abstract": "While graphene has been dubbed as a \"wonder material\" because of its amazing characteristics, such as the ability to conduct electricity better than copper and being two hundred times stronger than steel, until recently, the key quantum phenomenon of superconductivity was missing from the list of properties exhibited by graphene. In 2018, an astonishing discovery showed that by placing two sheets of graphene on top of each other in a structure known as Twisted Bilayer Graphene, it is possible to realize superconductivity when the rotation angle between the sheets is close to the \"Magic Angle\" value of 1.1\u00b0. More surprisingly, superconductivity in the initial reports was observed in close proximity to insulating states - resembling the phase diagram of High Tc superconductors. This sparked a fierce debate about its origin and its possible relation to High Tc superconductors. In this thesis, we show that by carefully engineering the dielectric environment of TBG, it is possible to stabilize superconductivity in non-magic angle TBG devices without the presence of any insulating states. This discovery imposes severe constraints on the origin of superconductivity in TBG. We also report, for the first time, the successful induction of spin-orbit coupling in TBG and discuss its implications.
\r\n\r\nSuperconductivity can also be induced into graphene via coupling to conventional superconductors, and the strength of the induced supercurrent depends strongly on temperature. We employ this thermal dependence by integrating graphene into superconducting circuits that serves two purposes a) to investigate graphene's thermal behavior at milliKelvin temperatures and b) to utilize its extremely low heat capacity in making functional devices that have the potential to achieve ultra-high thermal sensitivity.
", "doi": "10.7907/nc05-gr15", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13768", "collection": "thesis", "collection_id": "13768", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022020-110730808", "type": "thesis", "title": "Development of Tools for Probing Order in Single Crystals Using Electron and Photon Spectroscopy", "author": [ { "family_name": "Deshpande", "given_name": "Tejas Makarand", "orcid": "0000-0003-0326-1372", "clpid": "Deshpande-Tejas-Makarand" } ], "thesis_advisor": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "thesis_committee": [ { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Eisenstein", "given_name": "James P.", "orcid": "0000-0001-5460-0464", "clpid": "Eisenstein-J-P" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Discovering novel quantum phases of matter\u2013from emergent behavior of strongly-correlated electrons in solid-state systems to superfluidity in quantum degenerate liquids\u2013has been a cornerstone of condensed matter physics for many decades. In the most recent decades, however, the discovery of topological phases has emphasized the importance of symmetry, in addition to the conventional paradigm of symmetry breaking, in the definition of the order parameter, \u03a8, and hence the quantum phase it represents. Naturally, novel experimental tools, capable of coupling to said order parameter, directly or indirectly, are required to discover conventionally elusive quantum phases. In this thesis, I will discuss experimental techniques, using both photon and electron spectroscopy, to study exotic electronic phases in single crystals. The thesis will be divided into two unequal parts: (a) the development of a high-energy-resolution sub-Kelvin angle-resolved photoemission spectroscopy apparatus to study 3D time-reversal invariant topological superconductors, and (b) the experiments exploiting the non-linear and time-resolved aspects of femtosecond lasers to study a broad class of many-body systems.
", "doi": "10.7907/tt7b-fm83", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13660", "collection": "thesis", "collection_id": "13660", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:03172020-153749505", "primary_object_url": { "basename": "Thesis_Final_Chen-Chih Hsu.pdf", "content": "final", "filesize": 6521176, "license": "other", "mime_type": "application/pdf", "url": "/13660/13/Thesis_Final_Chen-Chih Hsu.pdf", "version": "v13.0.0" }, "type": "thesis", "title": "Physics and Applications of Graphene-Based Nanostructures and Nano-Meta Materials", "author": [ { "family_name": "Hsu", "given_name": "Chen-Chih", "orcid": "0000-0003-1130-5240", "clpid": "Hsu-Chen-Chih" } ], "thesis_advisor": [ { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" } ], "thesis_committee": [ { "family_name": "Roukes", "given_name": "Michael Lee", "orcid": "0000-0002-2916-6026", "clpid": "Roukes-M-L" }, { "family_name": "Yeh", "given_name": "Nai-Chang", "orcid": "0000-0002-1826-419X", "clpid": "Yeh-Nai-Chang" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Graphene, a single layer of carbon atoms forming a honeycomb lattice structure, has been considered a wonder material for both scientific research and technological applications. Structural distortions in nano-materials can induce dramatic changes in their electronic properties. In particular, strained graphene can result in both charging effects and pseudo-magnetic fields, so that controlled strain on a perfect graphene lattice can be tailored to yield desirable electronic properties.
\r\n\r\nIn the first part of this thesis (Chapter 2 to 5), we explore a new approach to manipulating the topological states in monolayer graphene via nanoscale strain engineering. By placing strain-free monolayer graphene on architected nanostructures to induce global inversion symmetry breaking, we demonstrate the development of giant pseudo-magnetic fields, global valley polarization, and periodic one-dimensional topological channels for protected propagation of chiral modes in strained graphene. We have also observed pseudo-magnetic field-induced quantum oscillations and valley Hall signals, including quantum valley Hall effect, by transport measurements at 1.8K.
\r\n\r\nThe second part of this thesis focuses on the development and applications of other graphene-based nanostructures. We report PECVD techniques for the synthesis of various graphene and graphene-based nanostructures, including horizontal growth of graphene sheets, vertical growth of graphene nanostructures such as graphene nanostripes with large aspect ratios, and direct and selective deposition of multi-layer graphene on nanostructured substrates. By properly controlling the gas environment of the plasma, it is found that no active heating is necessary for the PECVD growth processes and that high-yield growth can take place in a single step on a variety of surfaces, including metallic, semiconducting, and insulating materials.
", "doi": "10.7907/6T02-4X35", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:13734", "collection": "thesis", "collection_id": "13734", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05292020-005036817", "primary_object_url": { "basename": "Singh_Ashmeet_2020.pdf", "content": "final", "filesize": 3661638, "license": "other", "mime_type": "application/pdf", "url": "/13734/1/Singh_Ashmeet_2020.pdf", "version": "v3.0.0" }, "type": "thesis", "title": "Quantum Mechanical Vistas on the Road to Quantum Gravity", "author": [ { "family_name": "Singh", "given_name": "Ashmeet", "orcid": "0000-0002-4404-1416", "clpid": "Singh-Ashmeet" } ], "thesis_advisor": [ { "family_name": "Carroll", "given_name": "Sean M.", "orcid": "0000-0002-4226-5758", "clpid": "Carroll-S-M" } ], "thesis_committee": [ { "family_name": "Preskill", "given_name": "John P.", "orcid": "0000-0002-2421-4762", "clpid": "Preskill-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Brandao", "given_name": "Fernando", "orcid": "0000-0003-3866-9378", "clpid": "Brand\u00e3o-F-G-S-L" }, { "family_name": "Carroll", "given_name": "Sean M.", "orcid": "0000-0002-4226-5758", "clpid": "Carroll-S-M" } ], "local_group": [ { "literal": "Walter Burke Institute for Theoretical Physics" }, { "literal": "div_pma" } ], "abstract": "In this thesis, we lay out the goal, and a broad outline, for a program that takes quantum mechanics in its minimal form to be the fundamental ontology of the universe. Everything else, including features like space-time, matter and gravity associated with classical reality, are emergent from these minimal quantum elements. We argue that the Hilbert space of quantum gravity is locally finite-dimensional, in sharp contrast to that of conventional field theory, which could have observable consequences for gravity. We also treat time and space on an equal footing in Hilbert space in a reparametrization invariant setting and show how symmetry transformations, both global and local, can be treated as unitary basis changes.
\r\n\r\nMotivated by the finite-dimensional context, we use Generalized Pauli Operators as finite-dimensional conjugate variables and define a purely Hilbert space notion of locality based on the spread induced by conjugate operators which we call \"Operator Collimation.\" We study deviations in the spectrum of physical theories, particularly the quantum harmonic oscillator, induced by finite-dimensional effects, and show that by including a black hole-based bound in a lattice field theory, the quantum contribution to the vacuum energy can be suppressed by multiple orders of magnitude.
\r\n\r\nWe then show how one can recover subsystem structure in Hilbert space which exhibits emergent quasi-classical dynamics. We explicitly connect classical features (such as pointer states of the system being relatively robust to entanglement production under environmental monitoring and the existence of approximately classical trajectories) with features of the Hamiltonian. We develop an in-principle algorithm based on extremization of an entropic quantity that can sift through different factorizations of Hilbert space to pick out the one with manifest classical dynamics. This discussion is then extended to include direct sum decompositions and their compatibility with Hamiltonian evolution.
\r\n\r\nFollowing this, we study quantum coarse-graining and state-reduction maps in a broad context. In addition to developing a first-principle quantum coarse-graining algorithm based on principle component analysis, we construct more general state-reduction maps specified by a restricted set of observables which do not span the full algebra (as could be the case of limited access in a laboratory or in various situations in quantum gravity). We also present a general, not inherently numeric, algorithm for finding irreducible representations of matrix algebras.
\r\n\r\nThroughout the thesis, we discuss implications of our work in the broader goal of understanding quantum gravity from minimal elements in quantum mechanics.
", "doi": "10.7907/m1vx-d174", "publication_date": "2020", "thesis_type": "phd", "thesis_year": "2020" }, { "id": "thesis:11538", "collection": "thesis", "collection_id": "11538", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05232019-192004916", "primary_object_url": { "basename": "Lin__Cheng_Ju__2019.pdf", "content": "final", "filesize": 18638108, "license": "other", "mime_type": "application/pdf", "url": "/11538/1/Lin__Cheng_Ju__2019.pdf", "version": "v4.0.0" }, "type": "thesis", "title": "Surviving Quantum Chaos: Weak Thermalization, Prethermalization and Quantum Many-Body Scar States", "author": [ { "family_name": "Lin", "given_name": "Cheng-Ju", "orcid": "0000-0001-7898-0211", "clpid": "Lin-Cheng-Ju" } ], "thesis_advisor": [ { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Quantum chaos and the eigenstate thermalization hypothesis are based on the assumption of the validity of random matrix theory description on the spectrum and eigenstates. They provide the foundation and descriptions for the typical dynamics and thermalization in generic closed quantum systems. In this thesis, we investigate situations where the systems show atypical dynamics or anomalous thermalization, conflicting with the usual expectations from quantum chaos and eigenstate thermalization hypothesis.\r\nWe first examine weak thermalization in a nonintegrable spin chain. The system shows long-lived strong oscillations and relaxes to the thermal equilibrium weakly. We identify the dynamics describable by quasiparticles and recognize the oscillation frequency to be the quasiparticle mass gap. We also estimate the damping time for the oscillations.\r\nNext, we study prethermalization, a phenomenon where a system relaxes to an intermediate almost-equilibrium stage before reaching the true thermal equilibrium. We study a nonintegrable spin chain in the strong coupling limit, where an almost-conserved quantity emerges and gives rise to the prethermalization.\r\nWe also study a newly proposed diagnostic for quantum chaos: out-of-time-ordered correlators. Contrasting to the chaotic systems, we inspect their behaviors in various noninteracting integrable models.\r\nFinally, we dig into the quantum many-body scar states in the PXP model which describes a Rydberg atom chain. These special states do not satisfy the random matrix theory description nor the eigenstate thermalization hypothesis, therefore defying quantum chaos.", "doi": "10.7907/DKYP-PH92", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11546", "collection": "thesis", "collection_id": "11546", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05252019-154108242", "type": "thesis", "title": "Electrically-Tunable Light-Matter Interactions in Quantum Materials", "author": [ { "family_name": "Whitney", "given_name": "William Schuyler", "orcid": "0000-0001-5269-2967", "clpid": "Whitney-William-Schuyler" } ], "thesis_advisor": [ { "family_name": "Atwater", "given_name": "Harry Albert", "clpid": "Atwater-H-A" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "clpid": "Hsieh-David" }, { "family_name": "Atwater", "given_name": "Harry Albert", "clpid": "Atwater-H-A" }, { "family_name": "Schwab", "given_name": "Keith C.", "clpid": "Schwab-K-C" }, { "family_name": "Rossman", "given_name": "George Robert", "clpid": "Rossman-G-R" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Dynamic control of the flow of light at the nanoscale is critical for next-generation optoelectronic devices that will enable the technologies of the future. Ultra-thin, layered materials are promising building blocks for this functionality, as they are easily fabricated into atom-scale structures, and their optical properties change dramatically under applied electric fields. Many of these material systems, like topological insulators \u2013 a subset of layered materials that host spin-polarized surface states, promise more exotic functionality as well. The emerging field of nanophotonics in quantum materials is a route not only to an improved material platform for optoelectronics, but also to new physics, and the potential new device paradigms that follow.
\r\n\r\nIn this work we describe investigations of electrically-tunable light-matter interactions in two different layered materials: few-layer black phosphorus, and bismuth antimony telluride. In few-layer black phosphorus, we demonstrate several in-plane anisotropic optoelectronic phenomena, including Pauli-blocking of intersubband optical transitions under carrier injection, a quantum-confined Stark effect, and a change of quantum well selection rules under applied electric field. We further describe how these optoelectronic phenomena drive anisotropic birefringence and dichroism in few-layer black phosphorus. Lastly, we present theory describing amplitude, phase and polarization control in a black phosphorus integrated microcavity device, with applications that include metasurface beam-steering and more.
\r\n\r\nWe next present experiments demonstrating field-effect control of optical transitions in bismuth antimony telluride. These measurements evidence the merits of topological insulators as optoelectronic materials, and highlight a pathway towards future exploration of spin-plasmon excitations in bismuth antimony telluride.
\r\n\r\nLastly, we present a summary of pending work, including initial results of an ongoing study of plasmon excitations in few-layer black phosphorus, and a perspective on next steps for both these projects and nanophotonics in quantum materials at large.
", "doi": "10.7907/KKB7-KN62", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11147", "collection": "thesis", "collection_id": "11147", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:08162018-100820958", "type": "thesis", "title": "Transport Signatures of Spin-Orbit Coupling in Graphene-Based Materials", "author": [ { "family_name": "Tu", "given_name": "Min-Feng", "orcid": "0000-0001-6292-627X", "clpid": "Tu-Min-Feng" } ], "thesis_advisor": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" }, { "family_name": "Mong", "given_name": "Roger S.", "orcid": "0009-0000-7182-5681", "clpid": "Mong-Roger-S" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "Topological materials have been a fastest growing research topic in the recent decade. Out of the numerous new phases proposed and/or discovered, \"topological insulators\" (TIs) are one of the most promising materials that could lead to further advances in high-performance electronics and to applications in quantum computing. Similar to the ordinary semiconductors, TIs have a bulk gap; yet they host robust edge/surface states which are protected from non-magnetic disorder and interactions while the gap remains open. This feature is a manifestation of the non-trivial topology of TIs, the crucial feature that distinguishes them from ordinary semiconductors. Although the search for more topological materials continues, discovered TI currently are limited by practical difficulties that prevent industrialization.
\r\n\r\nIn this thesis, we study graphene, which is the first proposed TI candidate in the history, and its derivatives. With the intrinsic spin-orbital coupling (SOC) on graphene, one can open a topologically nontrivial band gap at the Dirac cones, although the SOC of the carbon atoms is exceedingly small for topological insulation to be observed in experiments. Many proposals exist to enhance the SOC on graphene by doping with adatoms, changing the functionality of the surface, placing graphene on top of other strong SOC materials, etc. However, few proposed TI signatures have been found experimentally. Furthermore, measuring these intrinsic SOCs through magnetoconductance is challenging due to their relatively weak signatures in transport. This work addresses the challenges in transport measurements from both analytical and numerical approaches on various graphene-based materials. Graphene\u2019s Dirac band structure and open geometry underlie its exciting prospects for engineering new physics via impurity-induced spin-orbit coupling. As a tantalizing example, previous theory works predicted a robust quantum-spin-Hall phase in graphene covered with dilute heavy adatoms such as In, Tl, and Os, although experiments to date have not detected the required enhancement of spin-orbit coupling. Motivated by these experiments, we explore the consequences of adatom-generated spin-orbit couplings on magneto-transport in graphene. We attack the problem using diagrammatic techniques and the Landauer-Buttiker transport simulation informed by microscopics, and study various coverages, chemical potentials, and disorder types. We find that the induced spin-orbit couplings can contribute to magneto-conductance differently from conventional intrinsic and Rasbha spin-orbit couplings. Our results provide a possible rationale for the absence of spin-orbit signatures in recent experiments, and also highlight a roadmap for their discovery\r\nin future work.
\r\n\r\nIn addition to the adatom-dedoped graphene, we also study graphene placing on top of strong SOC substrate, WS2, by jointing theory, numerics, and experiment. We demonstrate, in experiment, a clear weak anti-localization (WAL) effect arising from induced Rashba spin\u2013orbit coupling (SOC) in WS2-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single layer graphene flake, WAL in WS2-covered graphene occurs over a wide range of carrier densities on both the electron and hole sides.
", "doi": "10.7907/KCPE-1F36", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11726", "collection": "thesis", "collection_id": "11726", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06082019-034706928", "primary_object_url": { "basename": "einav_tal_2019_thesis.pdf", "content": "final", "filesize": 40337719, "license": "other", "mime_type": "application/pdf", "url": "/11726/1/einav_tal_2019_thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Taming the Molecular Dance: Harnessing Statistical Mechanics to Quantitatively Characterize Allosteric Systems", "author": [ { "family_name": "Einav", "given_name": "Tal", "orcid": "0000-0003-0777-1193", "clpid": "Einav-Tal" } ], "thesis_advisor": [ { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Phillips", "given_name": "Robert B.", "orcid": "0000-0003-3082-2809", "clpid": "Phillips-R" }, { "family_name": "Bois", "given_name": "Justin S.", "orcid": "0000-0001-7137-8746", "clpid": "Bois-J-S" }, { "family_name": "Eisenstein", "given_name": "James P.", "orcid": "0000-0001-5460-0464", "clpid": "Eisenstein-J-P" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "The pace of biological research continues to grow at a staggering pace as high-throughput experimental techniques rapidly increase our ability to sequence DNA, quantify cell behavior, and image molecules of all types within the cellular milieu. Given this surge in experimental prowess, the time is ripe to examine how well our conceptual cartoons of biological phenomena can not only recapitulate the data but also successfully predict the outcomes of future experiments.
\r\n\r\nOne of the fundamental challenges in biology is that the space of possible molecules is overwhelmingly large. The number of variants of a moderately-sized protein (20^300) is larger than the number of atoms in the universe, as is the space of possible bacterial genomes, protein interaction networks, and effector functions; progress in any of these fronts requires a theory-experiment dialogue that can extrapolate our small drop of data to explain large swaths of parameter space.
\r\n\r\nMy thesis strives towards this goal by analyzing a number of central molecular players in biology including enzymes (biological catalysts that accelerate chemical reactions), transcription factors (proteins that bind to DNA and regulate its expression), and ion channels (signaling proteins that regulate ion transport). I develop a quantitative description in each context by harnessing the statistical mechanical Monod-Wyman-Changeux model of allostery which coarse-grains the behavior of a multi-state system into two effective states, demonstrating that these seemingly diverse molecules are all governed by the same fundamental equation.
\r\n\r\nWrit large, there are two overarching goals encompassed by these projects. The first is to translate our biological knowledge into concrete physical models, enabling us to quantitatively describe how the key molecular components in each system interact to carry out their function. The second goal is to analyze how mutations can be mapped into the fundamental biophysical parameters governing each system. In my opinion, predicting the effects of mutations remains one of the great unsolved problems in biology, and it has been incredibly exciting to make progress on this front.
\r\n\r\nLooking back at my amazing graduate school experience, one of the most surprising aspects of my PhD was how closely each of my projects revolved around experiments. I entered graduate school as a theoretical physicist expecting to work on esoteric mathematical models, yet the direct connection with data provided a window into the exhilarating world of biology. While I have never physically manipulated these biological systems in the lab, my models allow me to push and prod and examine their behavior from the most mundane to the utterly extreme limits. Through modeling, I test our assumptions of how these systems work and tease out insights into their underlying biophysical mechanism. Most importantly, these models enable me to harness the incredible wealth of hard-won data to weave a few more threads of understanding into our tapestry of how these incredible living systems operate.
", "doi": "10.7907/S4CV-T162", "publication_date": "2019", "thesis_type": "phd", "thesis_year": "2019" }, { "id": "thesis:11032", "collection": "thesis", "collection_id": "11032", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06062018-171256773", "type": "thesis", "title": "Thermalization in Periodically-Driven Interacting Quantum Systems", "author": [ { "family_name": "Seetharam", "given_name": "Karthik Iyengar", "orcid": "0000-0003-1928-8019", "clpid": "Seetharam-Karthik-Iyengar" } ], "thesis_advisor": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "thesis_committee": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_eng" } ], "abstract": "Periodically-driven (Floquet) quantum systems are ubiquitous in science and technology. For example, when a laser illuminates a material or an AC voltage is applied to a device, the system is well-described by a time-periodic Hamiltonian. In recent years, periodic driving has been proposed, not just as a tool to excite and probe devices, but actually as a mechanism of engineering new phases of matter, some of which have no equilibrium analog. However, with this promise comes a serious problem. Intuitively, if energy is injected into and distributed throughout a system, it is no surprise that it tends to heat up indefinitely to infinite temperature.
\r\n\r\nIn this thesis, we study the mechanisms of heating, i.e. the process of thermalization, in Floquet systems and propose methods to control them. Specifically, for non-interacting Floquet systems that are coupled to external bosonic and fermionic baths (e.g. laser-driven electrons in a semiconductor that interact with phonons and an external lead), we classify the relevant scattering processes that contribute to cooling/heating in the Floquet bands and suggest methods to suppress heating via bandwidth-restrictions on the baths. We find that is possible, with appropriate dissipative engineering, to stabilize a controlled incompressible nonequilibrium steady-state resembling a ground state - a state we term the \"Floquet insulator.\" We extend this analysis to include short-range interactions that contribute additional heating processes and show, under the same framework, that heating can be controlled with dissipation. In the process, we develop a simple effective model for the Floquet band densities that captures the essence of all the Floquet scattering processes and that is useful for ballparking experimentally-relevant estimates of heating. Next, we turn our attention to strongly-interacting closed Floquet systems and study how heating emerges through a proliferation of resonances. We find a novel integrable point governing the strong-interaction limit of the Floquet system and examine the breakdown of integrability via the proliferation of resonances. We observe two distinct scaling regimes, attributed to non-thermal and thermal behavior, and discover a power-law scaling of the crossover between them as a function of system size. The lingering ergodicity-breaking effects of the conserved quantities in the vicinity (in parameter space) of the integrable point at finite size is a phenomena we term \"near-integrability.\" These results suggest that small quantum systems, which are accessible currently in many platforms (e.g. trapped ions, cold atoms, superconducting devices), intrinsically host non-thermal states that one may be able to utilize to avoid heating. Furthermore, our results suggest a \"dual\" interpretation, in the thermodynamic limit, that a periodically-driven system exhibits prethermalization as a power-law in interaction strength.
", "doi": "10.7907/3G0V-TW52", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10860", "collection": "thesis", "collection_id": "10860", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05082018-133854488", "type": "thesis", "title": "Towards a Theory of Quantum Gravity Through Geometrization of Quantum Mechanics", "author": [ { "family_name": "Cao", "given_name": "ChunJun", "orcid": "0000-0002-5761-5474", "clpid": "Cao-ChunJun" } ], "thesis_advisor": [ { "family_name": "Carroll", "given_name": "Sean M.", "orcid": "0000-0002-4226-5758", "clpid": "Carroll-S-M" } ], "thesis_committee": [ { "family_name": "Preskill", "given_name": "John P.", "orcid": "0000-0002-2421-4762", "clpid": "Preskill-J" }, { "family_name": "Cheung", "given_name": "Clifford W.", "orcid": "0000-0002-9983-9425", "clpid": "Cheung-Clifford" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Carroll", "given_name": "Sean M.", "orcid": "0000-0002-4226-5758", "clpid": "Carroll-S-M" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "In this thesis, we adapt an approach by assuming quantum mechanics as a fundamental theory of nature and attempt to recover familiar concepts such as space-time geometry and gravity from quantum wavefunctions and their unitary evolutions. More specifically, we explore a number of approaches in \"geometrizing\" quantum systems using techniques such as tensor networks and manifold learning. We find that consistency conditions in quantum gravity can be used to put constraints on tensor network models that approximate the anti-de Sitter/Conformal Field Theory correspondence. Furthermore, quantum circuits and tensor networks can also be used to describe cosmological models and reproduce important features of space-time configurations such as de Sitter space. We find that a generic framework using quantum circuit to describe cosmology puts an upper bound on the number of e-folds during the inflationary phase of the Universe's expansion. In addition to tensor network models, we also propose a Bulk Entanglement Gravity framework that analyzes the entanglement data of a quantum state in a Hilbert space without any a priori assumptions on geometry, such as the likes of a boundary conformal field theory. We find that from an amorphous configuration, one can directly recover geometry of bulk space-time from a generic class of wavefunctions that is fully characterized in this thesis via quantum entropy cone techniques. We find that under a number of assumptions, it is possible to derive linearized Einstein's equation from a version of Jacobson's entanglement equilibrium conditions for an emergent spacetime geometry in the weak field limit near Minkowski space. We show that non-local entanglement perturbations display features of wormhole-like configurations. We also clarify connections between Bulk Entanglement Gravity and highly generic features in quantum error correction codes that can be used to derive gravity.
", "doi": "10.7907/SCD0-VB49", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10872", "collection": "thesis", "collection_id": "10872", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05102018-115838454", "primary_object_url": { "basename": "Wang-2018-Thesis.pdf", "content": "final", "filesize": 11832256, "license": "other", "mime_type": "application/pdf", "url": "/10872/15/Wang-2018-Thesis.pdf", "version": "v6.0.0" }, "type": "thesis", "title": "Antiferromagnetic Quantum Phase Transitions: Continuous Tuning and Direct Probes of Competing States", "author": [ { "family_name": "Wang", "given_name": "Yishu", "orcid": "0000-0003-1259-8073", "clpid": "Wang-Yishu" } ], "thesis_advisor": [ { "family_name": "Rosenbaum", "given_name": "Thomas F.", "orcid": "0009-0008-6152-666X", "clpid": "Rosenbaum-T-F" } ], "thesis_committee": [ { "family_name": "Rosenbaum", "given_name": "Thomas F.", "orcid": "0009-0008-6152-666X", "clpid": "Rosenbaum-T-F" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Nadj-Perge", "given_name": "Stevan", "orcid": "0000-0002-2394-9070", "clpid": "Nadj-Perge-S" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_pma" } ], "abstract": "Antiferromagnets are choice systems to study quantum critical behavior. Unlike ferromagnets, they can experience continuous quantum phase transitions when tuned by pressure. However, the lack of a net magnetization renders experimental approaches difficult and often indirect. Here I demonstrate that both non-resonant and resonant x-ray magnetic diffraction under pressure provide the highly-desired direct probe for microscopic insights into the disappearance of the magnetic order, as well as the evolution of the charge and structural degrees of freedom. In Mo3Sb7, where spins are itinerant with small magnetic moments, we have discovered the doubling of the superconducting transition temperature under pressure and relate it to a lattice change from tetragonal to cubic structure. In MnP, a spiral magnetic order with tightened pitch was revealed in the high-pressure phase near a superconducting state at \u223c7 GPa. As the spiral pitch changes, fluctuations move from antiferromagnetic to ferromagnetic at long and short wavelengths, respectively, thereby potentially pro- moting spin-fluctuation-mediated superconductivity of different symmetries. In the all-in-all-out (AIAO) pyrochlore antiferromagnet Cd2Os2O7, we discovered an anti- ferromagnetic quantum critical point at 35.8 GPa using new techniques for resonant x-ray magnetic diffraction under pressure. The continuous suppression of AIAO antiferromagnetic order to zero temperature is accompanied by inversion symmetry breaking of the lattice, dividing the P \u2212 T phase space into three regions of different time reversal and spatial inversion symmetries. While phase lines of opposite curvature indicate a striking departure from a mean-field form at high pressure, the intertwined spin, charge, and phonon fluctuation modes point to a strong-coupled scenario of quantum criticality.
", "doi": "10.7907/VTHP-7645", "publication_date": "2018", "thesis_type": "phd", "thesis_year": "2018" }, { "id": "thesis:10334", "collection": "thesis", "collection_id": "10334", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06092017-141136995", "type": "thesis", "title": "Nonlinear and Ultrafast Optical Investigations of Correlated Materials", "author": [ { "family_name": "Chu", "given_name": "Hao", "clpid": "Chu-Hao" } ], "thesis_advisor": [ { "family_name": "Hsieh", "given_name": "David", "clpid": "Hsieh-David" } ], "thesis_committee": [ { "family_name": "Hsieh", "given_name": "David", "clpid": "Hsieh-David" }, { "family_name": "Eisenstein", "given_name": "James P.", "clpid": "Eisenstein-J-P" }, { "family_name": "Yeh", "given_name": "Nai-Chang", "clpid": "Yeh-Nai-Chang" }, { "family_name": "Refael", "given_name": "Gil", "clpid": "Refael-G" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_eng" } ], "abstract": "This thesis comprises studies of 3d-5d transition metal oxides with various degrees of electronic correlation using nonlinear harmonic generation rotational anisotropy as well as time-resolved optical reflectivity methods. Specifically, we explored photo-induced phase transition in Ca2RuO4 and Sr2IrO4, discovered novel electronic phases in doped Sr2IrO4 and Sr3Ir2O7, and investigated different types of antiferromagnetic orders in transition metal trichalcogenides MPX3.
\r\n", "doi": "10.7907/Z9VD6WHV", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10174", "collection": "thesis", "collection_id": "10174", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05152017-144541997", "primary_object_url": { "basename": "nanoscopic-atomic-lattices_jmuniz_final_revised.pdf", "content": "final", "filesize": 13416649, "license": "other", "mime_type": "application/pdf", "url": "/10174/1/nanoscopic-atomic-lattices_jmuniz_final_revised.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Nanoscopic Atomic Lattices with Light-Mediated Interactions", "author": [ { "family_name": "Muniz Silva", "given_name": "Juan Andres", "orcid": "0000-0001-6756-6218", "clpid": "Muniz-Silva-Juan-Andres" } ], "thesis_advisor": [ { "family_name": "Kimble", "given_name": "H. Jeff", "clpid": "Kimble-H-J" } ], "thesis_committee": [ { "family_name": "Kimble", "given_name": "H. Jeff", "clpid": "Kimble-H-J" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Painter", "given_name": "Oskar J.", "orcid": "0000-0002-1581-9209", "clpid": "Painter-O" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_pma" } ], "abstract": "Integrating ultracold atoms with nanophotonics enables the exploration of new paradigms in quantum optics and many body physics. Advanced fabrication capabilities for low-loss dielectric materials provide powerful tools to engineer light-matter coupling of photons and atoms. For example, dispersion-engineered photonic crystal waveguides (PCWs) permit not only stable trapping and probing of atoms via interactions with guided mode (GM) light, but also the possibility to study the physics of strong photon-mediated interactions between atoms. This thesis describes the design of a quasi-one-dimensional structure, the alligator photonic crystal waveguide (APCW), which has already allowed for the observation of some of those features.
\r\n\r\nFurthermore, external illumination schemes allow for the trapping and transport of atoms near the dielectric device. Here, atoms loaded into a one-dimensional optical lattice are transported through the APCW. As the atoms trapped in the lattice approach the APCW, the combination of lattice and GM potential can smoothly guide atoms into the gap between the two dielectric nanobeams. Therefore, the transmission of a weak guided mode probe is modulated at the rate determined by the lattice moving through the APCW. In the near future, single atoms can then be transferred from the moving lattice into optical traps formed in each unit cell by GMs of the APCW. Moreover, a characterization of a simple 2D photonic crystal slabs design is presented.
", "doi": "10.7907/Z9W66HTK", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:10174", "collection": "thesis", "collection_id": "10174", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05152017-144541997", "primary_object_url": { "basename": "nanoscopic-atomic-lattices_jmuniz_final_revised.pdf", "content": "final", "filesize": 13416649, "license": "other", "mime_type": "application/pdf", "url": "/10174/1/nanoscopic-atomic-lattices_jmuniz_final_revised.pdf", "version": "v5.0.0" }, "type": "thesis", "title": "Nanoscopic Atomic Lattices with Light-Mediated Interactions", "author": [ { "family_name": "Muniz Silva", "given_name": "Juan Andres", "orcid": "0000-0001-6756-6218", "clpid": "Muniz-Silva-Juan-Andres" } ], "thesis_advisor": [ { "family_name": "Kimble", "given_name": "H. Jeff", "clpid": "Kimble-H-J" } ], "thesis_committee": [ { "family_name": "Kimble", "given_name": "H. Jeff", "clpid": "Kimble-H-J" }, { "family_name": "Chen", "given_name": "Xie", "orcid": "0000-0003-2215-2497", "clpid": "Chen-Xie" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Painter", "given_name": "Oskar J.", "orcid": "0000-0002-1581-9209", "clpid": "Painter-O" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_pma" } ], "abstract": "Integrating ultracold atoms with nanophotonics enables the exploration of new paradigms in quantum optics and many body physics. Advanced fabrication capabilities for low-loss dielectric materials provide powerful tools to engineer light-matter coupling of photons and atoms. For example, dispersion-engineered photonic crystal waveguides (PCWs) permit not only stable trapping and probing of atoms via interactions with guided mode (GM) light, but also the possibility to study the physics of strong photon-mediated interactions between atoms. This thesis describes the design of a quasi-one-dimensional structure, the alligator photonic crystal waveguide (APCW), which has already allowed for the observation of some of those features.
\r\n\r\nFurthermore, external illumination schemes allow for the trapping and transport of atoms near the dielectric device. Here, atoms loaded into a one-dimensional optical lattice are transported through the APCW. As the atoms trapped in the lattice approach the APCW, the combination of lattice and GM potential can smoothly guide atoms into the gap between the two dielectric nanobeams. Therefore, the transmission of a weak guided mode probe is modulated at the rate determined by the lattice moving through the APCW. In the near future, single atoms can then be transferred from the moving lattice into optical traps formed in each unit cell by GMs of the APCW. Moreover, a characterization of a simple 2D photonic crystal slabs design is presented.
", "doi": "10.7907/Z9W66HTK", "publication_date": "2017", "thesis_type": "phd", "thesis_year": "2017" }, { "id": "thesis:9172", "collection": "thesis", "collection_id": "9172", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:09232015-145320310", "primary_object_url": { "basename": "thesis.pdf", "content": "final", "filesize": 13477843, "license": "other", "mime_type": "application/pdf", "url": "/9172/1/thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Coulomb Drag and Tunneling Studies in Quantum Hall Bilayers", "author": [ { "family_name": "Nandi", "given_name": "Debaleena", "clpid": "Nandi-Debaleena" } ], "thesis_advisor": [ { "family_name": "Eisenstein", "given_name": "James P.", "orcid": "0000-0001-5460-0464", "clpid": "Eisenstein-J-P" } ], "thesis_committee": [ { "family_name": "Eisenstein", "given_name": "James P.", "orcid": "0000-0001-5460-0464", "clpid": "Eisenstein-J-P" }, { "family_name": "Schwab", "given_name": "Keith C.", "orcid": "0000-0001-8216-4815", "clpid": "Schwab-K-C" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_pma" } ], "abstract": "The bilayer quantum Hall state at total filling factor \u03bdT=1, where the total electron density matches the degeneracy of the lowest Landau level, is a prominent example of Bose-Einstein condensation of excitons. A macroscopically ordered state is realized where an electron in one layer is tightly bound to a \"hole\" in the other layer. If exciton transport were the only bulk transportmechanism, a current driven in one layer would spontaneously generate a current of equal magnitude and opposite sign in the other layer. The Corbino Coulomb drag measurements presented in this thesis demonstrate precisely this phenomenon.
\r\n\r\nExcitonic superfluidity has been long sought in the \u03bdT=1 state. The tunneling between the two electron gas layers exihibit a dc Josephson-like effect. A simple model of an overdamped voltage biased Josephson junction is in reasonable agreement with the observed tunneling I-V. At small tunneling biases, it exhibits a tunneling \"supercurrent\". The dissipation is carefully studied in this tunneling \"supercurrent\" and found to remain small but finite.
", "doi": "10.7907/Z97H1GG0", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:9763", "collection": "thesis", "collection_id": "9763", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05262016-092645359", "primary_object_url": { "basename": "Titum_Paraj_2016_thesis.pdf", "content": "final", "filesize": 9149970, "license": "other", "mime_type": "application/pdf", "url": "/9763/1/Titum_Paraj_2016_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Disorder Driven Transitions in Non-Equilibrium Quantum Systems", "author": [ { "family_name": "Titum", "given_name": "Paraj", "orcid": "0000-0002-7792-1532", "clpid": "Titum-Paraj" } ], "thesis_advisor": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "thesis_committee": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Lindner", "given_name": "Netanel H.", "orcid": "0000-0003-1879-3902", "clpid": "Lindner-N-H" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_pma" } ], "abstract": "This thesis presents studies of the role of disorder in non-equilibrium quantum systems. The quantum states relevant to dynamics in these systems are very different from the ground state of the Hamiltonian. Two distinct systems are studied, (i) periodically driven Hamiltonians in two dimensions, and (ii) electrons in a one-dimensional lattice with power-law decaying hopping amplitudes. In the first system, the novel phases that are induced from the interplay of periodic driving, topology and disorder are studied. In the second system, the Anderson transition in all the eigenstates of the Hamiltonian are studied, as a function of the power-law exponent of the hopping amplitude.
\r\n\r\nIn periodically driven systems the study focuses on the effect of disorder in the nature of the topology of the steady states. First, we investigate the robustness to disorder of Floquet topological insulators (FTIs) occurring in semiconductor quantum wells. Such FTIs are generated by resonantly driving a transition between the valence and conduction band. We show that when disorder is added, the topological nature of such FTIs persists as long as there is a gap at the resonant quasienergy. For strong enough disorder, this gap closes and all the states become localized as the system undergoes a transition to a trivial insulator.
\r\n \r\nInterestingly, the effects of disorder are not necessarily adverse, disorder can also induce a transition from a trivial to a topological system, thereby establishing a Floquet Topological Anderson Insulator (FTAI). Such a state would be a dynamical realization of the topological Anderson insulator. We identify the conditions on the driving field necessary for observing such a transition. We realize such a disorder induced topological Floquet spectrum in the driven honeycomb lattice and quantum well models.
\r\n \r\nFinally, we show that two-dimensional periodically driven quantum systems with spatial disorder admit a unique topological phase, which we call the anomalous Floquet-Anderson insulator (AFAI). The AFAI is characterized by a quasienergy spectrum featuring chiral edge modes coexisting with a fully localized bulk. Such a spectrum is impossible for a time-independent, local Hamiltonian. These unique characteristics of the AFAI give rise to a new topologically protected nonequilibrium transport phenomenon: quantized, yet nonadiabatic, charge pumping. We identify the topological invariants that distinguish the AFAI from a trivial, fully localized phase, and show that the two phases are separated by a phase transition.
\r\n \r\nThe thesis also present the study of disordered systems using Wegner's Flow equations. The Flow Equation Method was proposed as a technique for studying excited states in an interacting system in one dimension. We apply this method to a one-dimensional tight binding problem with power-law decaying hoppings. This model presents a transition as a function of the exponent of the decay. It is shown that the the entire phase diagram, i.e. the delocalized, critical and localized phases in these systems can be studied using this technique. Based on this technique, we develop a strong-bond renormalization group that procedure where we solve the Flow Equations iteratively. This renormalization group approach\r\nprovides a new framework to study the transition in this system.
", "doi": "10.7907/Z9MK69VV", "publication_date": "2016", "thesis_type": "phd", "thesis_year": "2016" }, { "id": "thesis:8872", "collection": "thesis", "collection_id": "8872", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05202015-155217795", "primary_object_url": { "basename": "AGoban_thesis.pdf", "content": "final", "filesize": 65110077, "license": "other", "mime_type": "application/pdf", "url": "/8872/1/AGoban_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Strong Atom-Light Interactions Along Nanostructures: Transition from Free-space to Nanophotonic Interfaces", "author": [ { "family_name": "Goban", "given_name": "Akihisa", "orcid": "0000-0002-1766-6779", "clpid": "Goban-Akihisa" } ], "thesis_advisor": [ { "family_name": "Kimble", "given_name": "H. Jeff", "clpid": "Kimble-H-J" } ], "thesis_committee": [ { "family_name": "Kimble", "given_name": "H. Jeff", "clpid": "Kimble-H-J" }, { "family_name": "Painter", "given_name": "Oskar J.", "orcid": "0000-0002-1581-9209", "clpid": "Painter-O" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_pma" } ], "abstract": "An exciting frontier in quantum information science is the integration of otherwise \"simple\" quantum elements into complex quantum networks. The laboratory realization of even small quantum networks enables the exploration of physical systems that have not heretofore existed in the natural world. Within this context, there is active research to achieve nanoscale quantum optical circuits, for which atoms are trapped near nano-scopic dielectric structures and \"wired\" together by photons propagating through the circuit elements. Single atoms and atomic ensembles endow quantum functionality for otherwise linear optical circuits and thereby enable the capability of building quantum networks component by component. Toward these goals, we have experimentally investigated three different systems, from conventional to rather exotic systems : free-space atomic ensembles, optical nano fibers, and photonics crystal waveguides. First, we demonstrate measurement-induced quadripartite entanglement among four quantum memories. Next, following the landmark realization of a nanofiber trap, we demonstrate the implementation of a state-insensitive, compensated nanofiber trap. Finally, we reach more exotic systems based on photonics crystal devices. Beyond conventional topologies of resonators and waveguides, new opportunities emerge from the powerful capabilities of dispersion and modal engineering in photonic crystal waveguides. We have implemented an integrated optical circuit with a photonics crystal waveguide capable of both trapping and interfacing atoms with guided photons, and have observed the collective effect, superradiance, mediated by the guided photons. These advances provide an important capability for engineered light-matter interactions, enabling explorations of novel quantum transport and quantum many-body phenomena.", "doi": "10.7907/Z9T151KX", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:10337", "collection": "thesis", "collection_id": "10337", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06072015-223040119", "primary_object_url": { "basename": "ShuPingLeeThesisv2.pdf", "content": "final", "filesize": 4200459, "license": "other", "mime_type": "application/pdf", "url": "/10337/1/ShuPingLeeThesisv2.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Signatures of Topological Superconductors", "author": [ { "family_name": "Lee", "given_name": "Shu-Ping", "orcid": "0000-0002-6199-2408", "clpid": "Lee-Shu-Ping" } ], "thesis_advisor": [ { "family_name": "Alicea", "given_name": "Jason", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" } ], "thesis_committee": [ { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Eisenstein", "given_name": "James P.", "orcid": "0000-0001-5460-0464", "clpid": "Eisenstein-J-P" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_pma" } ], "abstract": "Topological superconductors are particularly interesting in light of the active ongoing experimental efforts for realizing exotic physics such as Majorana zero modes. These systems have excitations with non-Abelian exchange statistics, which provides a path towards topological quantum information processing. Intrinsic topological superconductors are quite rare in nature. However, one can engineer topological superconductivity by inducing effective p-wave pairing in materials which can be grown in the laboratory. One possibility is to induce the proximity effect in topological insulators; another is to use hybrid structures of superconductors and semiconductors.
\r\n\r\nThe proposal of interfacing s-wave superconductors with quantum spin Hall systems provides a promising route to engineered topological superconductivity. Given the exciting recent progress on the fabrication side, identifying experiments that definitively expose the topological superconducting phase (and clearly distinguish it from a trivial state) raises an increasingly important problem. With this goal in mind, we proposed a detection scheme to get an unambiguous signature of topological superconductivity, even in the presence of ordinarily detrimental effects such as thermal fluctuations and quasiparticle poisoning. We considered a Josephson junction built on top of a quantum spin Hall material. This system allows the proximity effect to turn edge states in effective topological superconductors. Such a setup is promising because experimentalists have demonstrated that supercurrents indeed flow through quantum spin Hall edges. To demonstrate the topological nature of the superconducting quantum spin Hall edges, theorists have proposed examining the periodicity of Josephson currents respect to the phase across a Josephson junction. The periodicity of tunneling currents of ground states in a topological superconductor Josephson junction is double that of a conventional Josephson junction. In practice, this modification of periodicity is extremely difficult to observe because noise sources, such as quasiparticle poisoning, wash out the signature of topological superconductors. For this reason, We propose a new, relatively simple DC measurement that can compellingly reveal topological superconductivity in such quantum spin Hall/superconductor heterostructures. More specifically, We develop a general framework for capturing the junction's current-voltage characteristics as a function of applied magnetic flux. Our analysis reveals sharp signatures of topological superconductivity in the field-dependent critical current. These signatures include the presence of multiple critical currents and a non-vanishing critical current for all magnetic field strengths as a reliable identification scheme for topological superconductivity.
\r\n\r\nThis system becomes more interesting as interactions between electrons are involved. By modeling edge states as a Luttinger liquid, we find conductance provides universal signatures to distinguish between normal and topological superconductors. More specifically, we use renormalization group methods to extract universal transport characteristics of superconductor/quantum spin Hall heterostructures where the native edge states serve as a lead. Interestingly, arbitrarily weak interactions induce qualitative changes in the behavior relative to the free-fermion limit, leading to a sharp dichotomy in conductance for the trivial (narrow superconductor) and topological (wide superconductor) cases. Furthermore, we find that strong interactions can in principle induce parafermion excitations at a superconductor/quantum spin Hall junction.
\r\n\r\nAs we identify the existence of topological superconductor, we can take a step further. One can use topological superconductor for realizing Majorana modes by breaking time reversal symmetry. An advantage of 2D topological insulator is that networks required for braiding Majoranas along the edge channels can be obtained by adjoining 2D topological insulator to form corner junctions. Physically cutting quantum wells for this purpose, however, presents technical challenges. For this reason, I propose a more accessible means of forming networks that rely on dynamically manipulating the location of edge states inside of a single 2D topological insulator sheet. In particular, I show that edge states can effectively be dragged into the system's interior by gating a region near the edge into a metallic regime and then removing the resulting gapless carriers via proximity-induced superconductivity. This method allows one to construct rather general quasi-1D networks along which Majorana modes can be exchanged by electrostatic means.
\r\n\r\nApart from 2D topological insulators, Majorana fermions can also be generated in other more accessible materials such as semiconductors. Following up on a suggestion by experimentalist Charlie Marcus, I proposed a novel geometry to create Majorana fermions by placing a 2D electron gas in proximity to an interdigitated superconductor-ferromagnet structure. This architecture evades several manufacturing challenges by allowing single-side fabrication and widening the class of 2D electron gas that may be used, such as the surface states of bulk semiconductors. Furthermore, it naturally allows one to trap and manipulate Majorana fermions through the application of currents. Thus, this structure may lead to the development of a circuit that enables fully electrical manipulation of topologically-protected quantum memory. To reveal these exotic Majorana zero modes, I also proposed an interference scheme to detect Majorana fermions that is broadly applicable to any 2D topological superconductor platform.
\r\n", "doi": "10.7907/Z90R9MB4", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8872", "collection": "thesis", "collection_id": "8872", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:05202015-155217795", "primary_object_url": { "basename": "AGoban_thesis.pdf", "content": "final", "filesize": 65110077, "license": "other", "mime_type": "application/pdf", "url": "/8872/1/AGoban_thesis.pdf", "version": "v2.0.0" }, "type": "thesis", "title": "Strong Atom-Light Interactions Along Nanostructures: Transition from Free-space to Nanophotonic Interfaces", "author": [ { "family_name": "Goban", "given_name": "Akihisa", "orcid": "0000-0002-1766-6779", "clpid": "Goban-Akihisa" } ], "thesis_advisor": [ { "family_name": "Kimble", "given_name": "H. Jeff", "clpid": "Kimble-H-J" } ], "thesis_committee": [ { "family_name": "Kimble", "given_name": "H. Jeff", "clpid": "Kimble-H-J" }, { "family_name": "Painter", "given_name": "Oskar J.", "orcid": "0000-0002-1581-9209", "clpid": "Painter-O" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Alicea", "given_name": "Jason F.", "orcid": "0000-0001-9979-3423", "clpid": "Alicea-J" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_pma" } ], "abstract": "An exciting frontier in quantum information science is the integration of otherwise \"simple\" quantum elements into complex quantum networks. The laboratory realization of even small quantum networks enables the exploration of physical systems that have not heretofore existed in the natural world. Within this context, there is active research to achieve nanoscale quantum optical circuits, for which atoms are trapped near nano-scopic dielectric structures and \"wired\" together by photons propagating through the circuit elements. Single atoms and atomic ensembles endow quantum functionality for otherwise linear optical circuits and thereby enable the capability of building quantum networks component by component. Toward these goals, we have experimentally investigated three different systems, from conventional to rather exotic systems : free-space atomic ensembles, optical nano fibers, and photonics crystal waveguides. First, we demonstrate measurement-induced quadripartite entanglement among four quantum memories. Next, following the landmark realization of a nanofiber trap, we demonstrate the implementation of a state-insensitive, compensated nanofiber trap. Finally, we reach more exotic systems based on photonics crystal devices. Beyond conventional topologies of resonators and waveguides, new opportunities emerge from the powerful capabilities of dispersion and modal engineering in photonic crystal waveguides. We have implemented an integrated optical circuit with a photonics crystal waveguide capable of both trapping and interfacing atoms with guided photons, and have observed the collective effect, superradiance, mediated by the guided photons. These advances provide an important capability for engineered light-matter interactions, enabling explorations of novel quantum transport and quantum many-body phenomena.", "doi": "10.7907/Z9T151KX", "publication_date": "2015", "thesis_type": "phd", "thesis_year": "2015" }, { "id": "thesis:8462", "collection": "thesis", "collection_id": "8462", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022014-093929164", "type": "thesis", "title": "Electronic States in Disordered Topological Insulators", "author": [ { "family_name": "Kim", "given_name": "Kun Woo", "clpid": "Kim-Kun-Woo" } ], "thesis_advisor": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "thesis_committee": [ { "family_name": "Cross", "given_name": "Michael Clifford", "clpid": "Cross-M-C" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_eng" } ], "abstract": "We present a theoretical study of electronic states in topological insulators with impurities. Chiral edge states in 2d topological insulators and helical surface states in 3d topological insulators show a robust transport against nonmagnetic impurities. Such a nontrivial character inspired physicists to come up with applications such as spintronic devices [1], thermoelectric materials [2], photovoltaics [3], and quantum computation [4]. Not only has it provided new opportunities from a practical point of view, but its theoretical study has deepened the understanding of the topological nature of condensed matter systems. However, experimental realizations of topological insulators have been challenging. For example, a 2d topological insulator fabricated in a HeTe quantum well structure by Konig et al. [5] shows a longitudinal conductance which is not well quantized and varies with temperature. 3d topological insulators such as Bi2Se3 and Bi2Te3 exhibit not only a signature of surface states, but they also show a bulk conduction [6]. The series of experiments motivated us to study the effects of impurities and coexisting bulk Fermi surface in topological insulators. We first address a single impurity problem in a topological insulator using a semiclassical approach. Then we study the conductance behavior of a disordered topological-metal strip where bulk modes are associated with the transport of edge modes via impurity scattering. We verify that the conduction through a chiral edge channel retains its topological signature, and we discovered that the transmission can be succinctly expressed in a closed form as a ratio of determinants of the bulk Green's function and impurity potentials. We further study the transport of 1d systems which can be decomposed in terms of chiral modes. Lastly, the surface impurity effect on the local density of surface states over layers into the bulk is studied between weak and strong disorder strength limits.", "doi": "10.7907/BSH1-AA62", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" }, { "id": "thesis:8462", "collection": "thesis", "collection_id": "8462", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06022014-093929164", "type": "thesis", "title": "Electronic States in Disordered Topological Insulators", "author": [ { "family_name": "Kim", "given_name": "Kun Woo", "clpid": "Kim-Kun-Woo" } ], "thesis_advisor": [ { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "thesis_committee": [ { "family_name": "Cross", "given_name": "Michael Clifford", "clpid": "Cross-M-C" }, { "family_name": "Motrunich", "given_name": "Olexei I.", "orcid": "0000-0001-8031-0022", "clpid": "Motrunich-Olexei" }, { "family_name": "Hsieh", "given_name": "David", "orcid": "0000-0002-0812-955X", "clpid": "Hsieh-David" }, { "family_name": "Refael", "given_name": "Gil", "orcid": "0009-0007-4566-8441", "clpid": "Refael-G" } ], "local_group": [ { "literal": "Institute for Quantum Information and Matter" }, { "literal": "div_eng" } ], "abstract": "We present a theoretical study of electronic states in topological insulators with impurities. Chiral edge states in 2d topological insulators and helical surface states in 3d topological insulators show a robust transport against nonmagnetic impurities. Such a nontrivial character inspired physicists to come up with applications such as spintronic devices [1], thermoelectric materials [2], photovoltaics [3], and quantum computation [4]. Not only has it provided new opportunities from a practical point of view, but its theoretical study has deepened the understanding of the topological nature of condensed matter systems. However, experimental realizations of topological insulators have been challenging. For example, a 2d topological insulator fabricated in a HeTe quantum well structure by Konig et al. [5] shows a longitudinal conductance which is not well quantized and varies with temperature. 3d topological insulators such as Bi2Se3 and Bi2Te3 exhibit not only a signature of surface states, but they also show a bulk conduction [6]. The series of experiments motivated us to study the effects of impurities and coexisting bulk Fermi surface in topological insulators. We first address a single impurity problem in a topological insulator using a semiclassical approach. Then we study the conductance behavior of a disordered topological-metal strip where bulk modes are associated with the transport of edge modes via impurity scattering. We verify that the conduction through a chiral edge channel retains its topological signature, and we discovered that the transmission can be succinctly expressed in a closed form as a ratio of determinants of the bulk Green's function and impurity potentials. We further study the transport of 1d systems which can be decomposed in terms of chiral modes. Lastly, the surface impurity effect on the local density of surface states over layers into the bulk is studied between weak and strong disorder strength limits.", "doi": "10.7907/BSH1-AA62", "publication_date": "2014", "thesis_type": "phd", "thesis_year": "2014" } ]