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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Article
  • Peer Reviewed
Proceedings of the Annual Meeting of the Berkeley Linguistics Society, Volume 38 (2014)
    Cover page: Multiple Interpretations and Constraints of Causative Serial Verb Constructions in Korean
    • Thesis
    • Peer Reviewed
    UC Berkeley Electronic Theses and Dissertations (2017)

    Quantum confinement of Dirac fermions is an important frontier in graphene research. In this dissertation, we report on our use of a scanning tunneling microscope (STM) to investigate electrostatically confined Dirac fermions in graphene quantum dots. We first describe a technique for patterning embedded gates in backgated graphene/hexagonal boron nitride (hBN) heterostructures by STM manipulation of defect charges within the hBN substrate. In conjunction with a tunable backgate, this allows us to engineer p-n junctions in monolayer and bilayer graphene whose geometries can be flexibly designed with nanoscale precision. Using scanning tunneling spectroscopy (STS), we image and spatially characterize the behavior of Dirac fermions in the vicinity of p-n junctions and show that circular p-n junctions in monolayer and bilayer graphene act as gate-tunable quantum dots with unique energy spectra. For monolayer graphene quantum dots, comparison with theoretical simulations of the massless Dirac equation enables us to identify each experimentally observed spectroscopic peak as a quantum dot eigenstate with a unique set of quantum numbers. In bilayer graphene, we demonstrate a gate-tunable evolution of locally gated graphene from classical dots to quantum dots and achieve control over the number of massive Dirac fermions contained in a quantum dot by using the STM tip as a top gate. Furthermore, we explore the electronic properties of quantum double dots and non-circular monolayer graphene p-n junctions using spatially resolved STS. Our work yields insight into the spatial behavior of Dirac fermions under the influence of local electrostatic potentials and provides a platform for further experimental investigation of physics related to p-n junctions in graphene.

      Cover page: Scanning Tunneling Microscopy of Graphene Quantum Dots
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2016)
      Electrostatic confinement of charge carriers in graphene is governed by Klein tunnelling, a relativistic quantum process in which particle-hole transmutation leads to unusual anisotropic transmission at p-n junction boundaries. Reflection and transmission at these boundaries affect the quantum interference of electronic waves, enabling the formation of novel quasi-bound states. Here we report the use of scanning tunnelling microscopy to map the electronic structure of Dirac fermions confined in quantum dots defined by circular graphene p-n junctions. The quantum dots were fabricated using a technique involving local manipulation of defect charge within the insulating substrate beneath a graphene monolayer. Inside such graphene quantum dots we observe resonances due to quasi-bound states and directly visualize the quantum interference patterns arising from these states. Outside the quantum dots Dirac fermions exhibit Friedel oscillation-like behaviour. Bolstered by a theoretical model describing relativistic particles in a harmonic oscillator potential, our findings yield insights into the spatial behaviour of electrostatically confined Dirac fermions.
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2021)
        Experimental realizations of graphene-based stadium-shaped quantum dots (QDs) have been few and have been incompatible with scanned probe microscopy. Yet, the direct visualization of electronic states within these QDs is crucial for determining the existence of quantum chaos in these systems. We report the fabrication and characterization of electrostatically defined stadium-shaped QDs in heterostructure devices composed of monolayer graphene (MLG) and bilayer graphene (BLG). To realize a stadium-shaped QD, we utilized the tip of a scanning tunneling microscope to charge defects in a supporting hexagonal boron nitride flake. The stadium states visualized are consistent with tight-binding-based simulations but lack clear quantum chaos signatures. The absence of quantum chaos features in MLG-based stadium QDs is attributed to the leaky nature of the confinement potential due to Klein tunneling. In contrast, for BLG-based stadium QDs (which have stronger confinement) quantum chaos is precluded by the smooth confinement potential which reduces interference and mixing between states.
          Cover page: Imaging Quantum Interference in Stadium-Shaped Monolayer and Bilayer Graphene Quantum DotsCreative Commons 'BY-NC' version 4.0 license
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