Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session S44: Strongly Correlated & Topological Electronic States in Magic Angle Bilayer GrapheneInvited Live Streamed
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Sponsoring Units: DCMP DMP Chair: Ali Yazdani, Princeton University Room: McCormick Place W-375C |
Thursday, March 17, 2022 8:00AM - 8:36AM |
S44.00001: Local thermodynamic measurements of topological states in magic angle graphene Invited Speaker: Andrew T Pierce The discovery of robust superconductivity and correlated ground states in magic-angle twisted graphene devices has generated great interest due in part to the possibility of realizing topological phases and unconventional superconductors in a single material system. In this talk I will describe local compressibility measurements of topological states in magic-angle twisted bilayer graphene, including fractional Chern insulators, which arise due to the interplay of electron-electron interactions and band topology. I will also present recent results on magic-angle twisted trilayer graphene, in which a rich sequence of transitions near charge neutrality is observed, and discuss how these and other ongoing experiments shed new light on the electronic properties of magic-angle graphene systems. |
Thursday, March 17, 2022 8:36AM - 9:12AM |
S44.00002: Exact Diagonalization for Magic-Angle Twisted Bilayer Graphene Invited Speaker: Pawel Potasz We present new theoretical results about correlated states observed in magic angle twisted bilayer graphene obtained using finite-size exact-diagonalization calculations. Starting from the continuum model of flat moir´e bands, we investigate interaction effects in effective model with allowed fluctuations of occupation numbers within two flat bands of magic angle twisted bilayer graphene and self-energies of fully occupied frozen remote valence bands in graphene's negative-energy sea. We consider filling factors above |2|, which includes a filling factor range, where superconductivity is strongest. Spin and valley polarization for both, integer and non-integer filling factors, is determined. A comparison of exact-diagonalization calculations with Hartree-Fock method results allows us to determine the role of correlation effects. We analyze the response of the system to an external field that couples to sublattice polarization. An intepretation of the systems as generalized ferromagnet with filling factor dependent anisotropy energy related to spin, valley, and sublattice polarization is discussed. |
Thursday, March 17, 2022 9:12AM - 9:48AM |
S44.00003: Spectroscopic Evidence for Correlated Chern Insulators and Unconventional Superconductivity in Magic-Angle Graphene Invited Speaker: Kevin P Nuckolls The flat electronic bands of magic-angle twisted bilayer graphene (MATBG) are highly conducive to producing a wide variety of correlated ground states, including correlated insulating, superconducting, and magnetic phases [1]. In this talk, I will discuss a set of experiments that leverage the spectroscopic resolution of the scanning tunneling microscope (STM) to probe these correlated phases. First, I will introduce a local spectroscopic technique using the STM that we use to detect a sequence of topological insulators in MATBG that break time-reversal symmetry spontaneously [2]. Second, I will discuss a series of experiments that uncover key spectroscopic signatures of unconventional superconductivity in MATBG, which strongly parallel phenomena observed in many known correlated superconductors [3]. Although transport studies of superconductivity in MATBG have suggested an unconventional pairing mechanism, conclusive evidence for any mechanism beyond the Bardeen-Cooper-Schrieffer (BCS) paradigm requires spectroscopic resolution. Tunnelling spectroscopy shows that the spectra in MATBG below the critical transition temperature Tc are inconsistent with those of a conventional s-wave superconductor, but rather resemble those of a nodal superconductor with an anisotropic pairing mechanism. Combining these tunneling measurements with point-contact spectroscopy measurements not only allows us to identify a large discrepancy between the tunnelling gap and the gap extracted from Andreev reflection spectroscopy, but also allows us to disentangle superconductivity from a precursor pseudogap phase that persists well-above the critical temperature and magnetic field of MATBG. In total, these findings provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG. |
Thursday, March 17, 2022 9:48AM - 10:24AM |
S44.00004: Interaction-driven band flattening and correlated phases in magic-angle graphene superlattices Invited Speaker: Stevan Nadj-Perge Flat electronic bands, characteristic of 'magic-angle' twisted bilayer and trilayer graphene, host many correlated phenomena. Nevertheless, many properties of flat bands and emerging symmetry-broken phases in these systems are still poorly understood. In this talk, I will discuss scanning tunneling microscopy experiments that examine filling-dependent band deformations in these systems and emerging correlated phases. In bilayers, we observe flattening of the bands that is appreciable even when the angle is well above the magic angle value, and so the material is nominally in a weakly correlated regime. In trilayers, we additionally see significant shifting between flat and Dirac-like bands. Our observations are consistent with a model that suggests that a substantial enhancement of the density of states caused by the band flattening triggers a cascade of symmetry-breaking transitions. In the second part of the talk, I will discuss recent insights into correlated phases emerging from these band structure effects. |
Thursday, March 17, 2022 10:24AM - 11:00AM |
S44.00005: Orbital Chern insulators at integer and half-integer fillings of a moiré superlattice Invited Speaker: Hryhoriy Polshyn Realizing topological phases at zero magnetic field has been a longstanding goal since Haldane’s theoretical proposal of the quantum anomalous Hall (QAH) state. My talk will focus on QAH states that emerge in twisted bilayer and twisted monolayer-bilayer (tMBG) graphene systems. In contrast to magnetically doped topological insulators, the QAH states in these moiré systems are driven by intrinsic strong interactions, which polarize the electrons into a single moiré miniband with Chern number of C = 1 or 2. Remarkably, the magnetization of these “orbital Chern insulators” (OCI) arises predominantly from the orbital motion of the electrons rather than the electron spin. I will discuss a novel effect originating from the curious magnetic properties of OCIs that enables non-volatile electrical switching of the magnetic and topological orders [1]. Finally, I will also present recent studies of the OCIs that emerge at ν=3/2 and 7/2 filling of the moiré superlattice unit cell in tMBG [2]. At ν=7/2, we observe a strong anomalous Hall effect with a jump of △Rxy≈1.2h/e2 and a Streda formula behavior consistent with C=1, all of which is strong evidence for the underlying QAH state. Our observation of Chern insulators at half-integer superlattice fillings suggests a spontaneous doubling of the superlattice unit cell, in addition to spin- and valley-ferromagnetism. This is confirmed by Hartree-Fock calculations, which find a topological charge density wave ground state at half-filling of the underlying C=2 band. |
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