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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2022)

My PhD experience has been focused on identifying and pursuing gaps in the understanding and implementation of spin-based phenomena. I primarily focus on Bi2Se3, due to its pervasiveness in the literature, safe and low-cost constituent elements, as well as the fact that its remarkably simple topological band structure functions as an ideal testbed for spin-momentumlocked physics.

The ideal spintronic material should have spin-orbit coupling that can be easily and reversibly tuned on ultrafast timescales. From the viewpoint of an ARPES experimentalist, ultrafast infrared pulses are a logical choice for implementing picosecond modifications. Furthermore, the effect of light on Rashba and topological systems has not fully transitioned from material characterization into the realm of engineered systems. For the first portion of my studies, I engineered Rashba-split quantum well states on the surface of a topological insulator and demonstrated the precise control of the spin and energy degrees of freedom as well as the density of states at the Fermi level. Chapters 2 through 5 represent the bulk of my PhD controlling surface band bending to generate ultrafast surface fields in topological insulators. These chapters follow a single coherent and chronological journey from my personal recognition of the effect (Ch. 2) to the exploration of the surface dipole effect on electron trajectory (Ch. 3) to the engineering of the SPV to drive ultrafast electric-field induced changes in quantum well states (Ch. 4 and Ch. 5). The unprecedented control of nearly all aspects of the band structure using photons presents a foundation for coupling spintronic and photonic systems, essential for integrating fiber optic communications with future spin-based devices.

Additionally, I pursue the issue no one wants to talk about for quantum material applications: scaling growth methods while preserving essential properties. This has been one of the main sticking points in not only 2D material fabrication (think mass graphene production or twistronic applications) but also for 3D bulk systems that depend heavily on single crystal quality. In my pursuit of understanding spin-orbit coupling in amorphous topological insulators, my collaborators and I discovered spin-momentum locking in amorphous Bi2Se3 grown through scalable room-temperature thermal evaporation. Chapter 6 details my work investigating strongly dispersive band structures in amorphous materials and the implications to amorphous topology and fermiology. These efforts demonstrate that spin-momentum locking can exist in the amorphous state, and provide an avenue for extracting the same momentum-dependent physics in crystals but for the much larger (and production-feasible)material category of non-crystalline matter.

Finally, Chapter 7 continues in my motivation to expand real-world applications for spin-polarized materials. Solid state spin-filters can be applied to free electrons in vacuum, an intriguing prospect that has implications for spin-resolved microscopy/spectroscopy techniques and has been pursued for decades. I present my work leading a team to design a prototype transmission spin-filter for free electrons. I go over the methods we have developed and optimized to measure electron transmission through membranes and our successful attempts at growing ultrathin free-standing magnetic films, resulting in a groundbreaking spin-filter prototype.

    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2022)
    A flat band structure in momentum space is considered key for the realization of novel phenomena. A topological flat band, also known as a drumhead state, is an ideal platform to drive new exotic topological quantum phases. Using angle-resolved photoemission spectroscopy experiments, we reveal the emergence of a highly localized surface state in a topological semimetal BaAl4 and provide its full energy and momentum space topology. We find that the observed surface state is localized in momentum, inside a square-shaped bulk Dirac nodal loop, and in energy, leading to a flat band and a peak in the density of state. These results imply this class of materials as an experimental realization of drumhead surface states and provide an important reference for future studies of the fundamental physics of correlated quantum effects in topological materials.
      Cover page: Observation of a Flat and Extended Surface State in a Topological SemimetalCreative Commons 'BY' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      The appearance of topologically protected spin-momentum locked surface states in topological insulators gives rise to robust room temperature spin currents making them ideal candidates for the realization of spintronic devices. New methods are needed to access and manipulate such currents with timescales that are compatible with modern electronics. Here we reveal that an optically induced long-lived (~10 ns), spin-polarized surface state excitation in topological insulators can be easily tuned in both magnitude and duration. Time-resolved angle-resolved photoemission spectroscopy, together with a quantitative model, reveals the ideal conditions for a surface photovoltage in two different topological insulators. Our model predicts that the reported effects are an intrinsic property of topological insulators, as long as the chemical potential falls within the band gap. This work demonstrates that persistent excited topological surface states are photon-accessible and easily tuned in both magnitude and duration, merging photonics- and spintronics-based devices in the same material.
        Cover page: Manipulating long-lived topological surface photovoltage in bulk-insulating topological insulators Bi2Se3 and Bi2Te3
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2024)
        Rich momentum-dependent electronic structure naturally arises in solids with long-range crystalline symmetry. Reliable and scalable quantum technologies rely on materials that are either not perfect crystals or non-crystalline, breaking translational symmetry. This poses the fundamental questions of whether coherent momentum-dependent electronic states can arise without long-range order, and how they can be characterized. Here we investigate Bi2Se3, which exists in crystalline, nanocrystalline, and amorphous forms, allowing direct comparisons between varying degrees of spatial ordering. Through angle-resolved photoemission spectroscopy, we show for the first time momentum-dependent band structure with Fermi surface repetitions in an amorphous solid. The experimental data is complemented by a model that accurately reproduces the vertical, dispersive features as well as the replication at higher momenta in the amorphous form. These results reveal that well-defined real-space length scales are sufficient to produce dispersive band structures, and that photoemission can expose the imprint of these length scales on the electronic structure.
          Cover page: Establishing coherent momentum-space electronic states in locally ordered materials
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2022)
          Van der Waals ferromagnets are thrilling materials from both a fundamental and technological point of view. VI3 is an interesting example, with a complex magnetism that differentiates it from the first reported Cr based layered ferromagnets. Here, we show in an indirect way through angle resolved photoemission spectroscopy experiments, the importance of spin-orbit coupling setting the electronic properties of this material. Our light polarized photoemission measurements point to a ground state with a half-filled e ± ′ doublet, where a gap opening is triggered by spin-orbit coupling enhanced by electronic correlations.
            Cover page: Polarization dependent photoemission as a probe of the magnetic ground state in the van der Waals ferromagnet VI3Creative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Santa Barbara Previously Published Works (2023)
            Crystalline symmetries have played a central role in the identification and understanding of quantum materials. Here we investigate whether an amorphous analogue of a well known three-dimensional strong topological insulator has topological properties in the solid state. We show that amorphous Bi2Se3 thin films host a number of two-dimensional surface conduction channels. Our angle-resolved photoemission spectroscopy data are consistent with a dispersive two-dimensional surface state that crosses the bulk gap. Spin-resolved photoemission spectroscopy shows this state has an anti-symmetric spin texture, confirming the existence of spin-momentum locked surface states. We discuss these experimental results in light of theoretical photoemission spectra obtained with an amorphous topological insulator tight-binding model, contrasting it with alternative explanations. The discovery of spin-momentum locked surface states in amorphous materials opens a new avenue to characterize amorphous matter, and triggers the search for an overlooked subset of quantum materials outside of current classification schemes.
              Cover page: Observation of spin-momentum locked surface states in amorphous Bi2Se3Creative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2022)
              The study of quantum phase transitions that are not clearly associated with broken symmetry is a major effort in condensed matter physics, particularly in regard to the problem of high-temperature superconductivity, for which such transitions are thought to underlie the mechanism of superconductivity itself. Here we argue that the putative quantum critical point in the prototypical unconventional superconductor CeCoIn5 is characterized by the delocalization of electrons in a transition that connects two Fermi surfaces of different volumes, with no apparent broken symmetry. Drawing on established theory of f-electron metals, we discuss an interpretation for such a transition that involves the fractionalization of spin and charge, a model that effectively describes the anomalous transport behavior we measured for the Hall effect.
                Cover page: Evidence for a delocalization quantum phase transition without symmetry breaking in CeCoIn5
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