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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 (2021)

Novel electronic phases induced by unusual electronic structures have attracted central attention in condensed matter physics communities. One of such exotic electronic structures is a dispersionless flat band. The fundamental physics and possibility of the induced phases, including high-temperature superconductivity, magnetism, and fractional quantum Hall effects, have been discussed for past decades, and such a flat band has been considered as the driver of new exotic phenomena. Hence, the system with a flat band could be fertile to many novel states of the matter. However, due to the limited number of the systems hosting the flat bands, the detail mechanisms of the flat band-induced unconventional states remain still elusive. In this dissertation, we provide the new platforms to explore what phases can be driven in the proximity of a flat band when it is coupled to a strong correlation or topological physics. We show the experimental electronic structures on two different systems, measured using angle-resolved photoemission spectroscopy (ARPES).

Chapter 1 briefly introduces the basic concept of flat band systems with examples of the unconventional states with flat bands, establishing the flat band as the tool to achieve various quantum states of matter. We also provide the motivation of each study in this dissertation. Chapter 2 introduces an overview of the primary experimental technique used in this dissertation, ARPES, a direct probe of the electronic structure. Additionally, we briefly introduce the principle of density functional theory calculation.Chapters 3, 4, and 5 describe three different studies. In Chapter 3, we build the correlation by coupling different oxides and creating the states at the interface. Unlike conventional semiconductors, oxide interface systems host strong correlations. If some unconventional phases are driven by the combination of flat band and correlation effects, this system can be the analog of cuprate high-$T_\text{c}$ superconductors. Chapters 4 and 5 are the case of topological materials. We establish a new playground where the topological physics can couple to the flat band. We investigate the bulk electronic properties of a new topological semimetal in Chapter 4. On the other hand, we focus on the surface localized states of the same material in Chapter 5. Finally, in Chapter 6, we briefly summarize the main findings and offer some concluding remarks.

    • 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 Semimetal
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2019)
      The emergence of saddle-point Van Hove singularities (VHSs) in the density of states, accompanied by a change in Fermi surface topology, Lifshitz transition, constitutes an ideal ground for the emergence of different electronic phenomena, such as superconductivity, pseudo-gap, magnetism, and density waves. However, in most materials the Fermi level, [Formula: see text], is too far from the VHS where the change of electronic topology takes place, making it difficult to reach with standard chemical doping or gating techniques. Here, we demonstrate that this scenario can be realized at the interface between a Mott insulator and a band insulator as a result of quantum confinement and correlation enhancement, and easily tuned by fine control of layer thickness and orbital occupancy. These results provide a tunable pathway for Fermi surface topology and VHS engineering of electronic phases.
        Cover page: Controlling a Van Hove singularity and Fermi surface topology at a complex oxide heterostructure interface
        • 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 (2021)
          The BaAl4 prototype crystal structure is the most populous of all structure types, and is the building block for a diverse set of sub-structures including the famous ThCr2Si2 family that hosts high-temperature superconductivity and numerous magnetic and strongly correlated electron systems. The MA4 family of materials (M = Sr, Ba, Eu; A = Al, Ga, In) themselves present an intriguing set of ground states including charge and spin orders, but have largely been considered as uninteresting metals. We predict the exemplary compound BaAl4 to harbor a three-dimensional Dirac spectrum with non-trivial topology and possible nodal lines crossing the Brillouin zone, wherein one pair of semi-Dirac points with linear dispersion along the kz direction and quadratic dispersion along the kx/ky direction resides on the rotational axis with C4v point group symmetry. An extremely large, unsaturating positive magnetoresistance in BaAl4 despite an uncompensated band structure is revealed, and quantum oscillations and angle-resolved photoemission spectroscopy measurements confirm the predicted multiband semimetal structure with pockets of Dirac holes and a Van Hove singularity (VHS) remarkably consistent with the theoretical prediction. We thus present BaAl4 as a topological semimetal, casting its prototype status into a role as a building block for a vast array of topological materials.
            Cover page: Crystalline symmetry-protected non-trivial topology in prototype compound BaAl4
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2022)
            Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in twisted graphene and moiré heterostructures has spurred interest into whether it stems from single-particle effects or from correlations, which are core to the emergence of intriguing phases in moiré systems. Here, we report an effective way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field-effect devices. As the chemical potential moves from the hole to the electron-doped side, we see an increased strength of electronic correlations manifested by an increase in the band velocity and inverse quasiparticle lifetime. These results suggest that electronic correlations intrinsically drive the electron-hole asymmetry in graphene and by leveraging this asymmetry can provide alternative avenues to generate exotic phases in twisted moiré heterostructures.
              Cover page: Correlation-driven electron-hole asymmetry in graphene field effect devices
              • Article
              • Peer Reviewed
              UC Berkeley Previously Published Works (2016)
              Recent progress in the field of topological states of matter has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs; refs ,,,), followed by closely related ternary compounds and predictions of several weak TIs (refs ,,). However, both the conceptual richness of Z2 classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z2 topological insulator is theoretically predicted and experimentally confirmed in the β-phase of quasi-one-dimensional bismuth iodide Bi4I4. The electronic structure of β-Bi4I4, characterized by Z2 invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements performed on the (001) surface reveal a highly anisotropic band-crossing feature located at the  point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.
                Cover page: A novel quasi-one-dimensional topological insulator in bismuth iodide β-Bi4I4
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