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: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: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: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: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: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" } ]