Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T55: Flat Bands in Two Dimensional SystemsRecordings Available
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Sponsoring Units: DCMP Chair: Jed Pixley, Rutgers University Room: Hyatt Regency Hotel -Adler |
Thursday, March 17, 2022 11:30AM - 11:42AM |
T55.00001: Theory of engineering flat bands in graphene using doubly-periodic electrostatic gating Nicholas M Hougland, David Pekker, Jeremy Levy, Ranjani Ramachandran We explore the use of applied electrical potentials to induce band flattening in graphene for bands near zero energy. We consider various families of doubly periodic potentials and simulate their effect on the electronic band structure using a tight-binding and a continuum approach. From these families, we find that an applied potential with symmetries of wallpaper group 17, in particular a Kagome potential, works best for inducing a high degree of band flattening for a range of realistic potential amplitudes and periods. Our work indicates that it should be possible to engineer the band structure of graphene using electrostatic gating, thus enabling a new approach to the development of graphene-based metamaterials. |
Thursday, March 17, 2022 11:42AM - 11:54AM |
T55.00002: Electrical transport in rhombohedral tri-layer graphene Xueshi Gao, Shi Che, Haidong Tian, Emilio A Codecido, Kenji Watanabe, Takashi Taniguchi, Chun Ning Lau Rhombohedral tri-layer graphene is a promising platform for investigating electron interaction. Collective phenomena such as superconductivity, magnetism and Mott-insulating states have been reported. Here we further study electronic properties in suspended and hBN-encapsulated rhombohedral tri-layer graphene experimentally. In this talk, I will present our latest transport studies as a function of magnetic field, charge density, displacement field, and temperature. |
Thursday, March 17, 2022 11:54AM - 12:06PM |
T55.00003: Kagome Flat-Band Acoustic Metamaterials Jiatong Yang, Radu Andrei, Syeda Minhal F Gardezi, Benjamin H November, Harris S Pirie, Jennifer E Hoffman The search for electronic flat bands in atomic lattices is difficult and time-consuming. Acoustic metamaterials, on the other hand, can be easily simulated, cheaply fabricated, and quickly measured, allowing us to search the phase space more rapidly and thoroughly for lattices that host acoustic flat bands. Here, we investigate an acoustic analogue to the kagome lattice made of air cavities in solid materials. We present both COMSOL Multiphysics simulations and experimental data imaging the dispersive bands of the kagome metamaterial in momentum space and the localized modes of the kagome flat band in real space. Taking advantage of the flexibility of metamaterials, we also designed a new set of lattice structures, the “n-extended kagome lattices,” by adding n additional cavities between kagome supercells. These modified kagome lattices host a singly degenerate, frequency isolated flat band with a significantly higher Q-factor than that of the kagome lattice. Such artificial lattices may be interesting candidates for developing strongly-correlated electron systems. |
Thursday, March 17, 2022 12:06PM - 12:18PM |
T55.00004: Flat bands and broken symmetry phases in 2D graphene triangulene crystals Goncalo Catarina, Ricardo Ortiz Cano, Joaquin Fernandez-Rossier Equilateral triangle-shaped graphene nanoislands with lateral dimension of n benzene rings are known as [n]-triangulenes. Individual [n]-triangulenes are open-shell molecules, with single-particle spectra that feature n-1 zero modes at half-filling, and a many-body ground state with total spin S=(n-1)/2. Their on-surface synthesis has been demonstrated for n=2,3,4,5,7. The Haldane phase for 1D chains of spin-1 [3]-triangulenes has been recently observed, showing the potential of triangulenes to create nontrivial quantum states. Here we present mean-field Hubbard calculations of several honeycomb triangulene 2D crystals. In the noninteracting limit, all these crystals feature narrow bands at the Fermi level. We study the emergence of broken symmetry phases as interactions are ramped up. |
Thursday, March 17, 2022 12:18PM - 12:30PM |
T55.00005: Vacancy-Engineered Flat-Band Superconductivity in Holey Graphene Matheus S Martins de Sousa, Wei Chen, Fanyao Qu, Fujun Liu A bipartite lattice with chiral symmetry is known to host zero energy flat bands if the numbers of the two sublattices are different. We demonstrate that this mechanism of producing flat bands can be realized on graphene by introducing periodic vacancies. Using first-principle calculations, we elaborate that even though the pristine graphene does not exactly preserve chiral symmetry, this mechanism applied to holey graphene still produces single or multiple bands as narrow as $\sim 0.5$eV near the Fermi surface throughout the entire Brillouin zone. Moreover, this mechanism can combine with vacancy-engineered nonsymmorphic symmetry to produce band structures with coexisting flat bands and nodal lines. A weak coupling mean-field treatment suggests the stabilization of superconductivity by these vacancy-engineered narrow bands. In addition, superconductivity occurs predominantly on the majority sublattices, with an amplitude that increases with the number of narrow bands. |
Thursday, March 17, 2022 12:30PM - 12:42PM |
T55.00006: High order Van Hove singularities and flat bands Robert S Markiewicz, Baadur Singh, Christopher Lane, Arun Bansil Beyond the two-dimensional (2D) saddle-point Van Hove singularities (VHSs) with logarithmic divergences in the density of states (DOS), recent studies have identified high-order VHSs with faster-than-logarithmic divergences that can amplify electron correlation effects [1-3]. Here we demonstrate a close relationship between the high-order VHSs and flat bands and show that this provides an estimate of the electronic dimensionality of the flat bands. We find that 3D materials can host high-order VHSs with similar power-law divergences to those near the cuprates [3]. |
Thursday, March 17, 2022 12:42PM - 12:54PM |
T55.00007: Topological Flat Bands in Spin-orbit Assisted Orbital Frustrated Systems Nandini Trivedi, Zachariah M Addison We expand the concept of frustration in Mott insulators and quantum spin liquids to semimetals with flat bands. In this approach topological flat bands are engineered through models with large onsite potentials, V, that split the spin and orbital degrees of freedom on the lattice into multi-degenerate band multiplets. Kinetic processes, t, that forbid intra-multiplet hopping, but allow inter-multiplet hopping frustrate the degrees of freedom on the lattice giving rise to flat bands with small bandwidths of order t2/V compared to general models for which bandwidths are of order t. We demonstrate this spin-orbit assisted orbital frustration on lattices in one and two dimensions with a variety of onsite potential spin-orbit structure. Spin-orbit assisted orbital frustration provides a new route that goes beyond the flat bands of quantum Hall effects and those generated from moire potentials as in twisted bilayer graphene. We provide an alternative mechanism that can guide the search for a richer materials' platform, including cold atoms systems, that can achieve topological flat bands poised at the brink of instabilities toward novel correlated and fractionalized metallic phases. |
Thursday, March 17, 2022 12:54PM - 1:06PM |
T55.00008: Electronic Structure of Multilayer Rhombohedral Graphite Studied Using STM Erin Grimes, Edward Seifert, Benjamin M Hunt, Randall M Feenstra Multilayer rhombohedral graphite is predicted to possess an electronic structure that contains low-energy, dispersionless flat surface bands and a gapped bulk. Here we present a study of the surface states of rhombohedral graphite of various thicknesses using Scanning Tunneling Microscopy/Spectroscopy (STM/STS). In the density of states of all samples we observe a van Hove singularity (vHS) that we attribute to the top surface flat band. This vHS increases in height and decreases in width as sample thickness increases. Peaks that correspond to higher energy band edges are also observed.In our thickest 14-layer device, we study a natural boundary between hexagonal and rhombohedral graphite. These individual graphite stacking orders are believed to be topologically trivial and non-trivial, respectively. Our experimental results reveal a splitting of the van Hove singularity in the local density of states over a distance of 15 nm as the boundary transitions from rhombohedral to hexagonal stacking. Tight-binding calculations reveal that this splitting may be explained through the introduction of gradual stacking faults throughout the boundary. |
Thursday, March 17, 2022 1:06PM - 1:18PM |
T55.00009: Landau Level Quantization in Acoustic Rippled Graphene Yiting Huang, Benjamin H November, Jenny Hoffman, Walker Gillett, Harry Pirie Atomic lattices hosting electronic flat bands give rise to strong electron correlations and novel emergent phases. However, the discovery of flat-band quantum materials is costly and slow, while the use of acoustic metamaterials to prototype flat-band lattices is an efficient alternative approach. While the flat bands generated by twisted bilayer graphene have already been reproduced in acoustic metamaterials [1], the flat bands emerging from strain-induced Landau levels in buckled graphene [2] have not been explored in a classical system. Acoustic metamaterial graphene under a triaxial strain gauge produces a uniform pseudo-magnetic field and corresponding Landau levels. Here, we study a graphene lattice of air cavities in 3D-printed plastic under alternating pseudo-magnetic field. Specifically, we present COMSOL simulations and experimental images of the Landau levels and the quantum-Hall-like edge states. We reproduce the expected switching of sub-lattice polarization at zeroth Landau level as well as the linear dependence of the Landau level spacing that has been reported in quantum graphene. |
Thursday, March 17, 2022 1:18PM - 1:30PM |
T55.00010: Dissipation Induced Flat Bands in Two Dimensions Spenser Talkington, Martin Claassen Flat bands are an ideal environment to realize strongly-correlated electron systems and to engineer unconventional phases, as their behavior is entirely governed by electronic interactions. Two-dimensional moiré lattices, such as magic-angle twisted bilayer graphene, have recently emerged as a promising route to realize flat bands, however moiré lattices are not the only platform with which to realize flat bands. Here we present a general scenario where flat bands emerge via the introduction of dissipation into systems that have dispersive bands in the zero-dissipation limit. To this end, we develop a Lindbladian approach to study dissipative electronic band structures, which arise from the coupling of a two-dimensional system to a substrate. We reveal a "flat-band condition" on the form of dissipation, under which flat bands with long lifetimes emerge from a broad class of electronic band structures. We analyze ramifications for a paradigmatic model of a Chern insulator, and finally discuss possible experimental signatures of these dissipation-induced flat bands. |
Thursday, March 17, 2022 1:30PM - 1:42PM |
T55.00011: Excitonic metal and non-Fermi liquid behaviour in twisted double bilayer graphene Unmesh Ghorai, Rajdeep Sensarma We show that twisted double bilayer graphene, whose band structure shows low energy electron and hole pockets, forms an excitonic metal near the charge neutrality point at very low temperatures ~ a few K. This leads to interesting implications for low-temperature transport in the system as a function of carrier density. Above the transition temperature, the density fluctuations in this critical metal lead to non-Fermi liquid features, which can also be seen in the temperature dependence of transport in the system. Similar features are also observed in recent experiments on these systems. |
Thursday, March 17, 2022 1:42PM - 1:54PM |
T55.00012: Topological flat bands in skyrmion-semiconductor heterostructures Nisarga Paul, Liang Fu, Yang Zhang Chiral magnets have received lots of attention in the past decade or so since the observation of magnetic skyrmions and the topological Hall effect, and work continues with the aim of skyrmion-based information devices. On the other hand, flat Chern bands in moire systems have garnered immense interest recently due to the possibility of realizing a host of strongly correlated phases. Here we explore the intersection and demonstrate the possibility of realizing flat Chern bands in chiral magnet-semiconductor heterostructures hosting skyrmions [1]. We find an almost perfectly flat band for generic skyrmion textures at a ``magic" value of out-of-plane magnetization m=0.2 for parabolic electrons and generally for Dirac electrons. We also point to MoS2/CrBr3 heterostructures as a material realization of such a flat Chern band due to a giant proximity exchange splitting of 14 meV we find from first-principles calculations. |
Thursday, March 17, 2022 1:54PM - 2:06PM |
T55.00013: Optical Spectral Weight and Tc Bounds for 2D Flat Band Superconductors Nishchhal Verma, Tamaghna Hazra, Mohit Randeria Motivated by Moiré materials, we investigate flat band superconductivity in 2D and obtain exact analytical results without making mean-field approximations. We review exact bounds for the 2D Tc in terms of the optical sum rule for multi-band superconductors [1] with arbitrary interactions, with the single assumption that the external vector potential couples to the kinetic energy. We then generalize these results to models with local attractive interactions where the low-energy physics is described by effective Hamiltonians without any kinetic energy. We first analyze trivial flat band systems for which we obtain upper and lower bounds [2] on the optical spectral weight that involve the "quantum geometry" of the flat band wavefunctions. We next extend our analysis to topological flat bands where one has an obstruction to constructing localized Wannier functions respecting all symmetries. Finally, we will discuss generalizations of our results to systems with strong-spin orbit coupling. |
Thursday, March 17, 2022 2:06PM - 2:18PM |
T55.00014: Bound on resistivity in flatband materials due to the quantum metric Johannes Mitscherling, Tobias Holder Recently, the quantum metric attracted a lot of attention by providing a lower bound to the superfluid stiffness of the superconducting state in flat-band systems. However until now, this central quantity of band theory could not be related to many response coefficients due to its non-classical origin. In this talk, I show that the linear electrical conductivity involves a new interband contribution based on the quantum metric. In systems with highly quenched bandwidth this quantum metric contribution can even dominate the longitudinal conductivity. Furthermore, for topological flat bands with nonzero Chern number the quantum metric contribution is bounded from below, i.e. I propose a new geometric bound on the longitudinal resistivity. These effects might be observable in highly tunable rhombohedral trilayer graphene flakes. |
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