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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2023)

In the last 5 years, the search to understand how correlated phases emerge in condensedmatter systems has shifted away from the cuprate high temperature superconductors and towards the realm of two dimensional materials, with twisted graphene being a particular point of focus. Due to the extremely high level of tunability of both interaction strength and electron kinetic energy in twisted graphene, these systems are an excellent playground to understand the relationship between correlations, electronic structure, symmetry breaking, and quantum phase transitions. This thesis is an examination of the electronic properties of graphene devices in monolayer and twisted geometries, studied through the lens of angle-resolved photoemission spectroscopy (ARPES).

This thesis is organized as follows. Chapter 1 provides a high-level introduction to correlationsand symmetry breaking, followed in Chapter 2 by an introduction to the electronic structure of graphene, twisted bilayer and trilayer graphene. Chapter 3 describes the experimental techniques: first the mechanics of ARPES used to measure electronic structure, and then the methods for fabricating twisted graphene devices with a particular focus on van der Waals heterostrucure samples for ARPES. Chapter 4 presents the ARPES results in monolayer graphene devices, showing how correlations and electron-electron interactions in particular drive electron-hole symmetry breaking. Chapter 5 investigates both the layer- and doping- tunability of electron-electron interactions in bilayer graphene devices at intermediate twist angles, and explores a regime where the signatures of symmetry breaking already present in the system can become further enhanced. Chapter 6 details Future directions for ARPES on twisted trilayer graphene, discussing how periodic strain produced by alignment with an hBN substrate can support certain correlated phases and suppress others.

    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2022)
    The formation of a charge density wave state is characterized by an order parameter. The way it is established provides unique information on both the role that correlation plays in driving the charge density wave formation and the mechanism behind its formation. Here we use time and angle resolved photoelectron spectroscopy to optically perturb the charge-density phase in 1T-TiSe[Formula: see text] and follow the recovery of its order parameter as a function of energy, momentum and excitation density. Our results reveal that two distinct orders contribute to the gap formation, a CDW order and pseudogap-like order, manifested by an overall robustness to optical excitation. A detailed analysis of the magnitude of the the gap as a function of excitation density and delay time reveals the excitonic long-range nature of the CDW gap and the short-range Jahn-Teller character of the pseudogap order. In contrast to the gap, the intensity of the folded Se[Formula: see text]* band can only give access to the excitonic order. These results provide new information into the the long standing debate on the origin of the gap in TiSe[Formula: see text] and place it in the same context of other quantum materials where a pseudogap phase appears to be a precursor of long-range order.
      Cover page: Revealing the order parameter dynamics of 1T-TiSe2 following optical excitation
      • 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
        LBL Publications (2023)
        Near the magic angle, strong correlations drive many intriguing phases in twisted bilayer graphene (tBG) including unconventional superconductivity and chern insulation. Whether correlations can tune symmetry breaking phases in tBG at intermediate (≳ 2°) twist angles remains an open fundamental question. Here, using ARPES, we study the effects of many-body interactions and displacement field on the band structure of tBG devices at an intermediate (3°) twist angle. We observe a layer- and doping-dependent renormalization of bands at the K points that is qualitatively consistent with moiré models of the Hartree-Fock interaction. We provide evidence of correlation-enhanced inversion symmetry-breaking, manifested by gaps at the Dirac points that are tunable with doping. These results suggest that electronic interactions play a significant role in the physics of tBG even at intermediate twist angles and present a new pathway toward engineering band structure and symmetry-breaking phases in moiré heterostructures.
          Cover page: Layer-Dependent Interaction Effects in the Electronic Structure of Twisted Bilayer Graphene Devices.
          • 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
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