Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session K37: The interplay between correlation and topology in TBG moire systemsInvited
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Sponsoring Units: DCMP Chair: Eun-Ah Kim, Cornell University Room: Room 233 |
Tuesday, March 7, 2023 3:00PM - 3:36PM |
K37.00001: Fractional Chern insulators in magic-angle twisted bilayer graphene Invited Speaker: Amir Yacoby Fractional Chern insulators (FCIs) are lattice analogues of fractional quantum Hall states that may provide a new avenue toward fractionalized and non-abelian excitations even at zero magnetic field. Early theoretical studies have predicted their existence in systems with energetically flat Chern bands and highlighted the critical role of a particular quantum band geometry. Magic angle twisted bilayer graphene (MATBG) supports flat Chern bands at zero magnetic field, and therefore offers a promising route toward stabilizing zero-field FCIs. Here we describe our recent observation of FCI states at low magnetic fields in MATBG enabled by high-resolution local compressibility measurements. Our findings highlight the interplay between symmetry, topology and interactions that leads to a competition between ordered electronic solid phases and fractionalized electronic liquid phases. Our findings strongly suggest that FCIs may be realized at zero magnetic field and pave the way for the exploration and manipulation of anyonic excitations in moiré systems. |
Tuesday, March 7, 2023 3:36PM - 4:12PM |
K37.00002: Unconventional Superconductivity in Hofstadter Electronic Bands Invited Speaker: Luiz H Santos The interplay between correlations and topology in fractal Hofstadter electronic bands has been traditionally associated with the quantum Hall effect. Two-dimensional heterostructures with large unit cells, such as moiré quantum materials, have recently emerged as platforms realizing the Hofstadter regime when the magnetic flux per unit cell is comparable to the flux quantum. Motivated by such developments, I will present recent theoretical progress in understanding a new class of quantum states that result when Hofstadter electrons form Cooper pairs. A general classification of Hofstadter superconductors based on the representations of the magnetic translation symmetries will be presented, uncovering a rich phase diagram of symmetry-broken phases characterized by multi-component order parameters. Further, I will present a weak-coupling renormalization group analysis revealing how repulsive electronic interactions projected on Van Hove singularities can promote unconventional superconductivity, bringing to light microscopic mechanisms for chiral topological superconductors and pair-density wave states. This approach suggests a new venue for realizing long-sought unconventional symmetry breaking and topological orders using quantum Hofstadter materials. |
Tuesday, March 7, 2023 4:12PM - 4:48PM |
K37.00003: Visualizing Many Body Quantum States in Magical Flat Bands Invited Speaker: Ali Yazdani The emergence of strongly correlated insulating, superconducting, and topological electronic phases at partial filling of the flat bands of magic angle bilayer graphene (MATBG) has been the subject of intense reserach in condensed matter physics. There are many theorethical proposals for the nature of correlated insulating phases, such as various broken symmetry phases involving spin or valley degree of freedom. The relation between these proposed phases and the emergence of superconductivity in this system remains also to be understood.Recently, we have demonstrated the power spectroscopic mapping with the STM in detecting broken symmetry phases and their topological excitations in Landau levels of monolayer graphene using the scanning tunneling microscope (STM).[1] The zeroth Landau level of graphene supports a variety of novel valley and spin polarized/coherent phases that can be directly visualized with the STM. Application of such visualization techniques to MATBG provides a powerful new tool to experimentally detect broken symmetry phases and to enable identification of the ground states in this system as function of filling. [2] I will present high-resolution low temperture STM spectrocoic maps of MATBG as function of partial filiing of its flat band and analyize them in the context of various proposed ground states for this system. |
Tuesday, March 7, 2023 4:48PM - 5:24PM |
K37.00004: Enter New Abstract Title HereInteractions in Flat Bands with and without Touching points: Topological Heavy Fermions Beyond Twisted Bilayer Graphene Invited Speaker: Andrei B Bernevig The interacting and noninteracting physics of the topologically nontrivial flat bands in twisted bilayer graphene can be mapped into that of a heavy localized fermion carrying the interaction and a topological conduction semi metal carrying the topology. We show new progress in this mapping, with new application to twisted trilayers, magnetic field, and an (unlikely) scenario of electron phonon - induced superconductivity, all of which can be analytically tracked in this formalism. We then move to show that, far from being confined to the TBG, the mapping of topological flat bands to heavy fermions and topological semi metals is a generic principle in interacting flat band electrons. We provide these maps for the Lieb lattice as well as for other twisted and untwisted lattices. |
Tuesday, March 7, 2023 5:24PM - 6:00PM |
K37.00005: Fractionalization in Fractional Correlated Insulating States at $npm 1/3$ filled moir'e graphene systems Invited Speaker: Dan Mao Fractionalization without time-reversal symmetry breaking is a long sought-after goal that manifests non-trivial correlation effects. While exactly solvable models offered many new theoretical insights, the physical realization of time-reversal symmetric fractionalization remained out of reach. The earlier proposal of correlated insulating states at $pm 1/3$ filling in twisted bilayer graphene and recent experimental observations of insulating states at those fillings provide a new platform to realize time-reversal symmetric fractionalized states. However, the nature of fractional excitations and the effect of quantum fluctuation on the fractional correlated insulating states are unknown. We show that excitations of the broken-symmetry fractional correlated insulator phases in the strong coupling limit carry fractional charges and exhibit fracton-like behavior with restricted mobility. Upon introduction of quantum fluctuations, the resonance of ``lemniscate" structured operators quantum melts the broken-symmetry states into a new quantum fluid state of ``quantum lemniscate liquid (QLL)" or a novel charge ordering state ``quantum lemniscate solid (QLS)". We propose experimental strategies to observe these fractionalized excitations and discuss the theoretical implications of the QLL/QLS phase. |
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