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Ultrafast Optical Studies of Pressure-Tuned Spin-Orbit Materials


Li, Chen (2024) Ultrafast Optical Studies of Pressure-Tuned Spin-Orbit Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/79g4-4392.


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.

In 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.

The 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.

Sr2IrO4 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.

In 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.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Condensed matter experiment, correlated system, topological semimetal, nonlinear optical experiment, second harmonic generation, high pressure, diamond anvil cell
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Hsieh, David
Thesis Committee:
  • Alicea, Jason F. (chair)
  • Rosenbaum, Thomas F.
  • Falson, Joseph
  • Hsieh, David
Defense Date:19 June 2023
Non-Caltech Author Email:chenlicli2 (AT)
Funding AgencyGrant Number
Department of Energy (DOE)DE SC0010533
Brown Science Foundation. Brown Investigator AwardUNSPECIFIED
Institute of Quantum Information and Matter, an NSF Physics Frontiers Center, with support from the Moore FoundationPHY-1733907
Record Number:CaltechTHESIS:06012024-155841744
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for ch.3
Li, Chen0000-0001-6750-5925
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16460
Deposited By: Chen Li
Deposited On:04 Jun 2024 20:41
Last Modified:12 Jun 2024 23:06

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