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 Z37: Topological phases through twist angle engineeringInvited
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Sponsoring Units: DCMP Chair: Pavel Volkov, Harvard University; University of Connecticut Room: Room 233 |
Friday, March 10, 2023 11:30AM - 12:06PM |
Z37.00001: TBD Invited Speaker: Dmitri K Efetov
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Friday, March 10, 2023 12:06PM - 12:42PM |
Z37.00002: Kondo Resonance, Pomeranchuk Effect, and Heavy Fermi Liquid in Twisted Bilayer Graphene - A Numerical Renormalization Group Study Invited Speaker: Zhida Song Low energy electron Hamiltonian in the magic-angle twisted bilayer graphene can be equivalently reformulated as a topological heavy fermion model [Phys. Rev. Lett. 129, 047601 (2022)]. It consists of effective localized f-electrons at AA-stacking regions and itinerant Dirac c-electrons. In this work, we applied systematic analytical and numerical renormalization group analyses to a single-impurity version of this model. We obtained a phase diagram consisting of a Fermi liquid phase in the Kondo regime, a Fermi liquid phase in the frozen impurity regime, and various local moment phases with different spin momenta. Remarkably, this single impurity phase diagram explains a series of experimental discoveries reported recently: (i) the zero-energy peak at fillings 1 ? |ν| |
Friday, March 10, 2023 12:42PM - 1:18PM |
Z37.00003: Charge excitations in graphene-based Moire materials Invited Speaker: Eslam Khalaf Graphene-based moire materials usually realize correlated insulators in the form of generalized spin, valley, or sublattice ferromagnets at different integer fillings. A key question is, what is the nature of charge carriers associated with doping these generalized Chern ferromagnets? In addition to single electron excitations, more exotic possibilities such as skyrmions are possible due to band topology where bands residing in opposite valleys or sublattices carry opposite Chern numbers. However, the description of skyrmions as real space topological textures is ill-suited to address momentum space features such as dispersion and the distribution of the Berry curvature which are crucial to understand their energy competition with single electrons and their relevance to low-energy physics. Here, I discuss a momentum-space formulation of the problem of charge excitations in a Chern ferromagnets by studying the bound states formed between single-electrons and spin flip excitations. Using this formulation, I will show what parameter regimes in different graphene-based Moire superlattice are most promising to hunt for non-trivial skyrmion-like charge excitations. |
Friday, March 10, 2023 1:18PM - 1:54PM |
Z37.00004: Non-Abelian topological superconductivity in maximally twisted double-layer spin-triplet valley-singlet superconductors Invited Speaker: Benjamin T Zhou Recent theoretical and experimental studies point to a novel spin-triplet valley-singlet (STVS) superconducting phase in certain two-valley electron liquids, including rhombohedral trilayer graphene, Bernal bilayer graphene and ZrNCl. This fully gapped phase is exotic in that it combines into Cooper pairs same-spin electrons from valleys centered around the opposing corners of a hexagonal Brillouin zone, but is, nevertheless, topologically trivial. Here, we predict that upon stacking two layers of an STVS material with an angular twist, a novel chiral topological phase -- an f±if'-wave superconductor -- emerges in the vicinity of the `maximal' twist angle of 30° where the system becomes an extrinsic quasi-crystal with 12-fold tiling. The resulting composite is a non-Abelian topological superconductor (TSC) with an odd number of chiral Majorana modes at its edges and a single Majorana zero mode (MZM) localized in the vortex core. Through symmetry analysis and detailed microscopic modelling based on a novel quasi-crystal band structure technique, we demonstrate that the non-Abelian TSC forms when the isolated Fermi pockets coalesce into a single connected Fermi surface around the center of the moiré Brillouin zone and is stable over a wide range of electron density. We further discuss how the energetics leading to the f±if'-wave phase results in anomalous π-periodic inter-layer Josephson effect, which can serve as a distinctive signature of the chiral phase. Distinct from the valley-preserving moiré physics in small-angle twisted graphene, our results establish the large-angle moiré physics arising near maximal twist as a new avenue toward intrinsic TSC with non-Abelian excitations. |
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