Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session R49: Manipulation of Strongly Correlated Electronic Ground States by the Competition between the Contact and Long-Range Parts of the Coulomb InteractionInvited Live
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Sponsoring Units: DCMP Chair: Leonid Levitov, Massachusetts Institute of Technology MIT |
Thursday, March 18, 2021 8:00AM - 8:36AM Live |
R49.00001: Visualizing the Quantum Phases of Magic Angle Graphene Invited Speaker: Shahal Ilani Twisted bilayer graphene near the magic angle exhibits a remarkable array of quantum phases, whose nature and origin are still poorly understood. In this talk I will describe our nanotube-based scanning single-electron-transistor (SET) experiments that explore and visualize some of these phases. Nanotube SET is an extremely sensitive electrometer, capable of probing a variety of thermodynamic properties of electrons on the nanoscale. By measuring the electronic compressibility1, we reveal a cascade of spin/valley symmetry-breaking phase transitions. This cascade appears at temperatures well above the onset of the superconducting and correlated insulating phases, demonstrating that it forms the parent state out of which these phases emerge. Measurements of the electronic entropy2 reveal that magic angle graphene exhibits a curious analog of the Pomeranchuk effect in 3He. In 3He, counterintuitively, the liquid can solidify upon heating, owing to a large spin entropy of the solid. Here we measure a similar giant magnetic entropy (~1kB per moiré site) near a filling of one electron per moiré site. This entropy drives a Pomeranchuk-like transition from a rather convention metal to a correlated state with nearly-free magnetic moments. However, while in 3He it is easy to understand why the spins of localized atoms in the solid are practically free, it is very surprising to observe nearly-free moments in a metallic, compressible state, making the nature of this newly observed correlated state highly puzzling. |
Thursday, March 18, 2021 8:36AM - 9:12AM Live |
R49.00002: The role of electron-electron interactions in two-dimensional Dirac fermions: from realistic to toy models. Invited Speaker: Fakher Assaad In this talk I will review a body of quantum Monte Carlo results aimed at exploring the effect of correlations in Dirac fermions. Emphasis will be placed on the role of the long-ranged Coulomb repulsion at weak coupling and close to the Gross-Neveu transition [1]. I will then move on to more exotic interactions that allow for the dynamical generation of quantum spin Hall states. We will see that Skyrmion defects of the quantum spin Hall order parameter carry charge 2e and can condense to form an s-wave superconducting state [2,3] The relevance of this route to superconductivity for twisted bilayer graphene will be discussed. |
Thursday, March 18, 2021 9:12AM - 9:48AM Live |
R49.00003: Phase diagram of twisted graphene bilayers near a magic angle Invited Speaker: Francisco Guinea The way in which interactions in twisted bilayer graphene favor broken symmetry phases is analyzed. The long range electrostatic coupling between electrons gives rise to spin and/or valley polarized states for most fillings. Competition between phases with different polarizations give rise to discontinuous transitions and phase coexistence. Details of the phase diagram are determined by short range interactions, such as the intra-atomic Hubbard repulsion, and also by external effects, such as screening by the environment, strains, and the coupling to the substrate. The effect of acoustic phonons, which overlap with the continuum of electron-hole excitations, will also be discussed. |
Thursday, March 18, 2021 9:48AM - 10:24AM Live |
R49.00004: Critical role of device geometry for the phase diagram of twisted bilayer graphene Invited Speaker: Johannes Lischner In this talk, I will describe two approaches for understanding the electronic structure of twisted bilayer graphene. In the first approach, Wannier functions of flat band electrons are constructed and then matrix elements of the screened Coulomb interaction between those Wannier functions are calculated. It is demonstrated that screening by metallic gates results in significant qualitative changes of the effective Hamiltonian of the flat band electrons suggesting that the phase diagram of twisted bilayer graphene can be controlled through device engineering. |
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