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 B49: Chern Insulators in Topological Flat bands: Twisted Bilayer Graphene and BeyondInvited Live
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Sponsoring Units: DCMP Chair: Eva Andrei, Rutgers University, New Brunswick |
Monday, March 15, 2021 11:30AM - 12:06PM Live |
B49.00001: Magic Angle Bilayer Graphene - Superconductors, Orbital Magnets, Correlated States and beyond Invited Speaker: Dmitri K. Efetov
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Monday, March 15, 2021 12:06PM - 12:42PM Live |
B49.00002: Correlations and topology in the magic angle twisted bilayer graphene Invited Speaker: Oskar Vafek When the twist angle of a bilayer graphene is near the ``magic'' value, there are four narrow bands near the neutrality point, each two-fold spin degenerate. These bands are separated from the rest of the bands by energy gaps. In the first part of the talk, the topology of the narrow bands will be discussed, as well as the associated obstructions --or lack there of -- to construction of a complete localized basis [1,3]. |
Monday, March 15, 2021 12:42PM - 1:18PM Live |
B49.00003: Correlation and Topology in Magic Angle Twisted Bilayer Graphene Invited Speaker: Ali Yazdani Interactions among electrons and the topology of their energy bands can create novel quantum phases of matter. The discovery of electronic bands with flat energy dispersion in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for new correlated and topological electronic phases. We have developed new scanning tunneling microscopy (STM) and spectroscopy (STS) techniques to probe the nature of electronic correlations and to detect novel topological phases in two-dimensional systems, such as MATBG. Density-tuned STS studies have enabled us to study the properties of MATBG as function of carrier concentration revealing key and new properties of this novel material. These measurements establish that MATBG is a strong correlated system at all partial filling of its flat bands. [1] The strength of the interactions, which can be measured in our experiments, is found to be larger than the flat bandwidth in the non-interacting limit. We demonstrate that these interactions drive a cascade of transitions at each integer filling of these bands, creating likely the insulating states at low temperatures that are spin or valley polarized.[2] Most recently, we developed a new STS technique to detect topological phases and their associated Chern numbers and used it to show that strong interactions drive the formation of unexpected topological insulating phases in MATBG [3]. These phases, which are stabilized by a weak magnetic field, are rare examples of when topology emerges from interaction between electrons. I will describe these experiments, and other ongoing efforts, that illustrate the power of atomic scale experiments in revealing novel physics of electrons in moiré superlattices. |
Monday, March 15, 2021 1:18PM - 1:54PM Live |
B49.00004: Isospin analog of the Pomeranchuk effect in twisted bilayer graphene Invited Speaker: Yu Saito In bilayer graphene rotationally faulted to 1.1 degrees, interlayer tunneling and rotational misalignment conspire to create a pair of low energy flat bands causing strongly correlated phenomena. An emerging question in twisted bilayer graphene is the role of isospin ferromagnetism. In this talk, I will describe experiments probing the finite-temperature phase diagram of isospin symmetry breaking in high quality twisted bilayer graphene using transport and thermodynamic measurements. We find that low-temperature transport at superlattice filling factor \nu=-1 shows no sign of a commensurate correlated phase, but a resistivity peak appears at a high temperature that resembles behavior observed near commensurate \nu where the low-temperature phase is a correlated insulator. Tilted field magnetotransport and direct measurements of the in-plane magnetic moment show that the resistivity peak near nu = -1 is adiabatically connected to a metamagnetic phase transition at which the system develops finite isospin polarization. These data are suggestive of a Pomeranchuk-type mechanism, in which the entropy of disordered isospin moments in the ferromagnetic phase stabilizes it relative to the unpolarized Fermi liquid phase at elevated temperatures. Direct thermodynamic measurements of the entropy, S indeed find it to be large, S ~ 1k_B per moiré unit cell, for nu ~ ± 1, and we find that a fraction of S is suppressed by an in-plane magnetic field consistent with an isospin contribution. In contrast to Helium-3, no discontinuities are observed in the thermodynamic quantities across this transition, implying that the magnetic transitions are continuous in nature. Our findings imply a small isospin stiffness, with implications for the nature of finite temperature transport as well as the mechanisms underlying isospin ordering and superconductivity twisted bilayer graphene and related systems. |
Monday, March 15, 2021 1:54PM - 2:30PM Live |
B49.00005: Chern Insulators and Topological Flat-bands in Magic-angle Twisted Bilayer Graphene Invited Speaker: Shuang Wu Magic-angle twisted bilayer graphene (MATBG) hosts low energy flat bands whose quenched kinetic energy facilitates interaction-induced instabilities when the Fermi level is swept through. Beyond many-body interactions, the existence of non-trivial band-topology in MATBG is expected to play a crucial role in the emergence of correlated phases. However, questions concerning the nature of these phases and their connection to the putative non-trivial band-topology are largely unanswered. Here we report on magneto-transport and Hall density measurements that reveal a succession of doping-induced Lifshitz transitions where discontinuous changes in the Fermi surface topology are accompanied by van Hove singularities (VHS) that facilitate the emergence of correlation-induced gaps and topologically non-trivial sub-bands. In the presence of a magnetic field the topology of the sub-bands at filling of 1, 2, 3 carriers per moiré cell, is revealed through well quantized Hall plateaus signaling the appearance of Chern insulators with Chern numbers, C=3, 2, 1, respectively. Surprisingly, for magnetic fields exceeding 5T we observe a new VHS at a filling of 3.5 electrons per moiré cell, suggesting the possibility of a topological sub-band at a fractional moiré filling. |
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