Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session F67: Broken Symmetries and Electron Pairing in Magic-Angle Flat BandsInvited
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Sponsoring Units: DCMP Chair: Pablo Jarillo-Herrero, Massachusetts Institute of Technology MIT Room: Four Seasons 2-3 |
Tuesday, March 3, 2020 8:00AM - 8:36AM |
F67.00001: Superconductors, Orbital Magnets, and Correlated States in Magic Angle Bilayer Graphene Invited Speaker: Dmitri Efetov Superconductivity often occurs close to broken-symmetry parent states and is especially common in doped magnetic insulators. When twisted close to a magic relative orientation angle near 1 degree, bilayer graphene has flat moire superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics, notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moire band filling factors nu = 0, +(-) 1, +(-) 2, +(-) 3, and superconductivity below critical temperatures as high as 3 K close to - 2 filling. We also observe three new superconducting domes at much lower temperatures close to the nu = 0 and nu = +(-) 1 insulating states. Interestingly, at nu = +(-) 1 we find states with non-zero Chern numbers. For nu = - 1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field above 3.6 tesla is applied, consistent with a field driven phase transition. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moire flat bands, including near charge neutrality. |
Tuesday, March 3, 2020 8:36AM - 9:12AM |
F67.00002: Competing Orders, Nematicity and Novel Josephson Effects in Magic-Angle Graphene Superlattices Invited Speaker: Yuan Cao The emergence of two-dimensional materials have provided physicists with unprecedented way of studying the motion of electrons in a superconductor. Although superconductivity itself has been studied for more than a century, the recent advances of “twistronics” research in graphene superlattices brings fundamentally new physics into the picture [1,2]. In this talk I will present that demonstrate some peculiar aspects of magic-angle twisted bilayer graphene (MATBG) which has recently been discovered to exhibit correlated insulating phase and unconventional superconductivity. In an in-plane magnetic field, MATBG exhibits a highly anisotropic critical magnetic field within the sample plane that strongly suggests nematicity in the order parameter of the superconductivity. Furthermore, the phase diagrams of some MATBG samples show a distinctive kink in Tc in the ‘underdoped’ region which is reminiscent of quantum critical point behavior in some cuprates such as YBa2Cu3O6+δ[3]. These observations signify the similarity between MATBG and established strongly-correlated systems, and might provide key insight into the underlying mechanism that is responsible for the emergence of 2D unconventional superconductivity. In a second experiment, we performed transport experiments on versatile gate-defined planar junction devices made from MATBG. Our results show that using this device geometry, all-graphene Josephson junctions can be realized by electrostatic gating with configurable tunneling barrier type and strength. Furthermore, we find peculiar Fraunhofer pattern in a magnetic field that exemplifies the intrinsic low dimensionality of the superconducting phase. These results pioneer in novel superconducting devices based on twisted bilayer graphene and might be utilized in future magnetic field sensing applications. |
Tuesday, March 3, 2020 9:12AM - 9:48AM |
F67.00003: Engineering Correlation and Topology in Two-Dimensional Moire Superlattices Invited Speaker: Feng Wang Van der Waals heterostructures of atomically thin crystals offer an exciting new platform to design novel electronic and optical properties. In this talk, I will describe how to engineer correlated and topological physics using moire superlattice in two dimensional heterostructures. I will show that we can realize and control extremely rich condensed matter physics, ranging from correlated Mott insulator and superconductivity to ferromagnetism and topological Chern insulator, in a single device featuring the ABC trilayer graphene and boron nitride moire superlattices. |
Tuesday, March 3, 2020 9:48AM - 10:24AM |
F67.00004: Strong coupling phases of partially filled twisted bilayer graphene narrow bands. Invited Speaker: Oskar Vafek States favored by Coulomb interactions projected onto the basis of the four narrow bands of the “magic angle” twisted bilayer graphene will be presented. Due to the unusual shape and symmetry of the Wannier orbitals [1], the resulting strong coupling problem is qualitatively different from the much studied (topologically) trivial narrow band i.e. a solid in an atomic limit. Instead, a dramatically new form of the interaction Hamiltonian is obtained [2]. It contains terms beyond the usual Hubbard term, leading to different strong coupling phases as in the atomic limit. Specifically, the usual anti-ferromagnetic super-exchange mechanism fails and turns ferromagnetic with an approximate spin-valley SU(4) symmetry. |
Tuesday, March 3, 2020 10:24AM - 11:00AM |
F67.00005: Correlated insulating states and broken symmetries in magic angle twisted bilayer graphene Invited Speaker: Ming Xie Magic angle twisted bilayer graphene exhibits insulating and superconducting behaviors when its low energy flat bands are partially filled[1]. Insulating states have been seen near all integer band filling factors, which strongly suggests that they are associated with states that break spin/valley flavor but not translational symmetries. I will first present our understanding of the nature of the insulating states[1] based partially on self-consistent Hartree-Fock (SCHF) calculations, which show that the insulating states can be understood qualitatively in terms of broken symmetries that gap the continuum model’s Dirac points as the twist angle is approached[2] from above and the Dirac velocities approach zero. Remote conduction and valence bands play an important quantitative role. Screening of Coulomb interactions due to the response of the metallic gates weakens Coulomb interactions, and can among other effects enlarge the stability region of superconducting states. Away from integer filling the bilayer system is metallic. I will argue on the basis of recent experiments that spin/valley flavor symmetries are often broken even in metallic and superconducting states and extend our SCHF calculations to metallic cases in an effort to shed light on electronic properties in the metallic regime. I will also discuss our recent effort using exact diagonalization method calculations based on a continuum-model defined low energy Hilbert space, which confirm some of the main results obtained using SCHF and point to some limitations. The exact diagonalization calculations suggest that Coulomb interaction alone are not enough to yield Cooper pairing, and that phonon induced effect attractive interactions have to be included in order to have a Cooper instability. |
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