Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F44: Symmetry Breaking and Fractional Chern Insulators in Moire Flat BandsInvited Live Streamed
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Sponsoring Units: DCMP Chair: Oskar Vafek, Florida State University Room: McCormick Place W-375C |
Tuesday, March 15, 2022 8:00AM - 8:36AM |
F44.00001: TBD Invited Speaker: Andrea Young
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Tuesday, March 15, 2022 8:36AM - 9:12AM |
F44.00002: Fractional Chern insulators in magic-angle twisted bilayer graphene Invited Speaker: Yonglong Xie Fractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue toward manipulating non-abelian excitations. Early theoretical studies have predicted their existence in systems with energetically flat Chern bands and highlighted the critical role of a particular quantum band geometry. Magic angle twisted bilayer graphene (MATBG) supports flat Chern bands at zero magnetic field, and therefore offers a promising route toward stabilizing zero-field FCIs. In this talk, I will describe our recent local compressibility measurements with a scanning single electron transistor that allow us to unambiguously identify the existence of FCIs in MATBG. I will also discuss the critical role of quantum band geometry in stabilizing FCIs. |
Tuesday, March 15, 2022 9:12AM - 9:48AM |
F44.00003: Kekulé spiral order at all nonzero integer fillings in twisted bilayer graphene Invited Speaker: Nick Bultinck Many features of Magic-Angle Twisted Bilayer Graphene (MATBG) which are observed in experiment can be explained theoretically using a strong-coupling picture. Many, but not all. We propose that adding physically realistic strains (such as those observed in scanning tunneling microscopy experiments) to the Bistritzer-MacDonald (BM) model can remedy some of the shortcomings of the strong-coupling theory. In particular, the interacting phase diagram of the strained BM model hosts a semi-metal at the charge neutrality point instead of a strong insulator - a property shared by most MATBG devices realized in the lab. At the other integer fillings away from the charge neutrality point we find that small strains stabilize a new type of symmetry-breaking order, which corresponds to a √3 x √3 or Kekulé distortion on the graphene scale which modulates on the moiré scale with a wavevector that is generally incommensurate with the superlattice period. This symmetry-breaking order preserves time-reversal symmetry and produces a charge gap at electron or hole fillings where insulating states are most commonly seen in experiment. |
Tuesday, March 15, 2022 9:48AM - 10:24AM |
F44.00004: Correlated topological phases in moire graphene systems Invited Speaker: Senthil Todadri The nearly flat bands of many moire graphene systems are topological. The resulting interplay of band topology and strong correlations make these systems suitable platforms for many interesting phenomena, After reviewing the physics leading to orbital ferromagnetism and an associated quantum anomalous Hall effect in some of these systems. I will discuss theoretically the possibile reaiization of other correlated topological phases. Examples include fractional Chern insulators (possibly at zero external magnetic field), and interaction-driven time-reversal symmetric topological insulators. Time permitting I will also discuss the theoretical challenges and existing progress in describing strongly correlated systems in a partially full topological band. |
Tuesday, March 15, 2022 10:24AM - 11:00AM |
F44.00005: Tuning electron correlation in magic-angle twisted bilayer graphene using Coulomb screening Invited Speaker: Jia Li To control the strength of interactions is essential for studying quantum phenomena emerging in systems of correlated fermions. We introduce a device geometry where magic-angle twisted bilayer graphene is placed in close proximity to a Bernal bilayer graphene, separated by a 3 nm thick barrier. Using charge screening from the Bernal bilayer, the strength of electron-electron Coulomb interaction within the twisted bilayer can be continuously tuned. Transport measurements show that tuning Coulomb screening has opposite effects on the insulating and superconducting states: as Coulomb interaction is weakened by screening, the insulating states become less robust, whereas the stability of superconductivity at the optimal doping is enhanced. The results provide important constraints for theoretical models to understand the mechanism for superconductivity in magic-angle twisted bilayer graphene. |
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