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 V49: New Frontiers in Magic Angle Graphene and Moire MaterialsInvited Live
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Sponsoring Units: DCMP Chair: Ashvin Vishwanath, Harvard University |
Thursday, March 18, 2021 3:00PM - 3:36PM Live |
V49.00001: Correlated insulators and skyrmion superconductivity in magic angle twisted bilayer graphene Invited Speaker: Eslam Khalaf When two sheets of graphene are twisted relative to each other by an angle around 1 degree, a fascinating array of interaction-driven phases, including correlated insulators and superconductors, is observed. I will discuss the nature of these correlated insulators and how they give rise to superconductivity upon doping. Starting by projecting the Coulomb interaction onto the flat bands, I will show that the flatband-projected interacting Hamiltonian is characterized by a hidden approximate U(4)xU(4) symmetry which allows us to map the problem to that of a pair of tunnel-coupled multilayer Chern insulators with opposite Chern numbers. This mapping enables us to obtain the ground state at some integer fillings exactly. I will then discuss how superconductivity emerges upon doping such insulators via a topological mechanism based on pairing skyrmion textures. Remarkably, this new mechanism of superconductivity, which is distinct from weak coupling phonon-mediated pairing and unconventional pairing mechanisms in cuprates, arises solely from repulsive interactions. I will discuss how these insights not only clarify why certain correlated moire materials do not become superconducting, but they also point to promising new platforms where robust superconductivity is anticipated. |
Thursday, March 18, 2021 3:36PM - 4:12PM Live |
V49.00002: Moiré Magic 3.0 Invited Speaker: Pablo Jarillo-Herrero Moiré superlattices have recently emerged as a novel platform where correlated physics and superconductivity can be studied with unprecedented tunability. Although correlated effects have been observed in several other moiré systems, magic-angle twisted bilayer graphene (MATBG) remains the only one where robust superconductivity has been reproducibly measured. In this talk I will present a new moiré superconductor, mirror symmetric magic-angle twisted trilayer graphene (MATTG) with dramatically richer tunability in electronic structure and superconducting properties. Hall effect and quantum oscillations measurements as a function of density and electric field allow us to determine the system's tunable phase boundaries in the normal state. Zero magnetic field resistivity measurements then reveal that the existence of superconductivity is intimately connected to the broken symmetry phase emerging at two carriers per moiré unit cell. Strikingly, we find that the superconducting phase gets suppressed and bounded at the van Hove singularities (vHs) partially surrounding the broken-symmetry phase, which is difficult to reconcile with weak-coupling BCS theory. Moreover, the extensive in situ tunability of our system allows us to achieve the ultra-strong coupling regime, characterized by a Ginzburg-Landau coherence length reaching the average inter-particle distance and very large T_BKT/T_F ratios in excess of 0.1. These observations suggest that MATTG can be electrically tuned close to the two-dimensional BCS-BEC crossover. Our results establish a new generation of tunable moiré superconductors with the potential to revolutionize our fundamental understanding and the applications of strong coupling superconductivity. |
Thursday, March 18, 2021 4:12PM - 4:48PM Live |
V49.00003: Flat Bands and Correlated Electronic States in Two Dimensional Atomic Crystals Invited Speaker: Eva Andrei Stacking two-dimensional atomic crystals to form a moiré superstructure or applying an external periodic potential, can radically change the electronic properties. In particular, it is possible to engineer conditions leading to the creation of essentially flat energy bands with non-trivial topology, where the quenched kinetic energy facilitates the emergence of correlated electronic states, including superconductivity, Mott insulators or ferromagnetism. This talk will highlight two examples where the electronic ground state and Fermi surface topology depend sensitively on the filling of the flat bands: twisted graphene bilayers that develop a flat band at a magic twist-angle [1,2,3], and buckled graphene layers in which a strain-induced periodically modulated pseudo-magnetic field creates a post-graphene material with flat electronic bands [4]. |
Thursday, March 18, 2021 4:48PM - 5:24PM Live |
V49.00004: Moiré Magic Invited Speaker: Allan MacDonald Moiré materials are formed when two-dimensional crystals are overlaid with a small difference in lattice constant or orientation. When the two-dimensional crystals are semimetals or semiconductors, the low energy states of moiré materials are described by periodic continuum models and have the electronic properties of artificial crystals with lattice constants on the tens of nanometer scale, allowing the number of electrons per atom to be varied widely. My talk will focus on the particular case of graphene bilayers, which exhibit a rich set of strongly correlated electron states, including superconductors and orbital magnets, when twisted close to a magic relative orientation angle at which the electron velocity at the Fermi level vanishes. Electronic correlations in Magic Angle Twisted Bilayer Graphene (MAtBG) are strong because the low-energy moiré superlattice bands are very narrow, and unusual becasue of non-trivial band topology inherited from the isolated layer Dirac cones. I will discuss efforts in my group at the University of Texas to settle on answers to some of the following questions. Does the flat-band dispersion that remains near the magic twist angle play a key role in controlling the phase diagram? Why are insulating states at odd integer filling factors not always Chern insulators? Is superconductivity in MAtBG mediated by electron-phonon interactions, by quantum fluctuations of spin/valley/orbital degrees of freedom, or by some other mechanism? What is the pair wavefunction for Cooper pairs in MAtBG? Are there important similarities between the doped Mott insulator states of cuprates and MAtBG states close to integer moiré band filling? |
Thursday, March 18, 2021 5:24PM - 6:00PM Live |
V49.00005: Correlated insulating states in semiconductor moiré superlattices Invited Speaker: Jie Shan When two van der Waals materials of slightly different orientations or lattice constants are overlaid, a moiré pattern is formed. The moiré pattern introduces a new length scale, many times the lattice constant of the original materials, for Bragg scattering of Bloch electrons in each layer. This gives rise to moiré minibands and rich emergent phenomena. In this talk I will discuss recent experiments on transition metal dichalcogenide moiré systems, which exhibit a series of correlated insulating states from strong electronic correlation. We study the quantum phase diagram of the system by probing both the charge and magnetic response while continuously tuning the filling factor and other system parameters. |
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