Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A01: Correlations and Topological StatesFocus

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Sponsoring Units: DCMP Chair: Efstratios Manousakis, Florida State Univ Room: BCEC 106 
Monday, March 4, 2019 8:00AM  8:12AM 
A01.00001: Study of the Dirac/Weyl candidates (Ce,Nd)Sb(Se,Te) KuanWen Chen, You Lai, Kaya Wei, Marein Rahn, YuChe Chiu, David E Graf, Marc Janoschek, Luis Balicas, Ryan Baumbach The nonsymmorphic compound CeSbTe [1] was recently shown to have coexisting complex magnetism and Weyl/Dirac states, where an applied magnetic field transforms the magnetic order and potentially provides a switch between different electronic states. Here we present results for other members of this family compounds, including CeSbSe [2], NdSbSe, and NdSbTe. CeSbSe exhibits complex magnetic ordering at T_{M} = 3 K and the application of a magnetic field results in a cascade of magnetically ordered states for H < 1.8 T which are characterized by fractional integer size steps: i.e., a possible devil’s staircase is observed. NdSbSe and NdSbTe show a low T buildup of magnetic entropy in the heat capacity, also suggesting complex magnetism. These compounds offer an opportunity to study the connection between the complex magnetic orders, Kondo lattices and topological properties. 
Monday, March 4, 2019 8:12AM  8:24AM 
A01.00002: Importance of electron correlations in understanding the photoelectron spectroscopy and the Weyl character of MoTe_{2} Efstratios Manousakis, Niraj Aryal We study the role of electron correlations in the type II Weyl semimetallic candidate γMoTe_{2} by using density functional theory (DFT) where the onsite Coulomb repulsion (Hubbard U) for the Mo 4d states is included within the DFT+U scheme. We find that inclusion of Hubbard U is important to describe both the lightpolarization dependence of the ARPES and the angular dependence of the Fermi surface as measured by quantum oscillation experiments. We also show that though the number and position of Weyl points change nonlinearly with U, the Weyl physics remains robust for a wide range of U values. Our calculations also indicate that $\gamma$MoTe$_2$ is in the vicinity of a correlationsinduced Lifshitz transition which can be probed experimentally by small amount of doping and its interplay with the Weyl physics might be intriguing. 
Monday, March 4, 2019 8:24AM  8:36AM 
A01.00003: Abelian Topological Phases with Background Electromagnetic Field on Lattice, and DeligneBeilinson Double Cohomology in Continuum JingYuan Chen Constructing exactly soluble lattice model is an important approach towards understanding topological phases of matter. The coupling of exactly soluble lattice topological models to continuous background gauge field is less wellstudied compared to discrete background gauge fields. The former is however responsible for important topological phenomena such as the Hall conductivity and the spincharge relation. In this talk, I introduce a systematic approach to this problem for abelian topological phases, which is to exactly retrieve the spacetime lattice model from the corresponding ChernSimons theory in the continuum, via the latter's formal structure known as DeligneBeilinson double cohomology. 
Monday, March 4, 2019 8:36AM  8:48AM 
A01.00004: Majorana zero mode on the edge of an interacting quantum wire subject to spinorbit interaction and transverse magnetic field Akira Furusaki, Oleg Starykh We revisit the problem of an interacting quantum wire subject to spinorbit interaction and transverse magnetic field. Previous studies showed that the spin sector of the model is equivalent to that of a spin chain with uniform DM interaction (D) in a transverse magnetic field (h), the ground state of which contains two ordered Ising phases and a critical TomonagaLuttinger liquid phase, depending on the ratio D/h. Importantly, the charge sector of the wire is a gapless TomonagaLuttinger liquid. We show that a quantum wire with an open boundary supports a zeroenergy bound state localized at the edge, provided that the spin sector of the problem is massive. We argue that as long as the charge sector is gapless, the wire is in a topological phase and that the bound state is a Majorana zero mode, and discuss physical implications of this finding. 
Monday, March 4, 2019 8:48AM  9:00AM 
A01.00005: Topological exciton insulator phase in twodimensional semiconductor systems WenKai Lou, Wen Yang, Kai Chang Exciton insulator was firstly proposed by Prof. Mott in 1961.This concept has been widely studied theoretically and confirmed experimentally in recent years. Here, We demonstrate theoretically the existence of topological exciton insulating phases in twodimensional (2D) semiconductor systems. We consider two kinds of systems: InAs/GaSb quantum wells [1] and 2D Van der Waals heterostructures[2]. In InAs/GaSb quantum wells, i.e., a 2D topological insulator, we demonstrate theoretically that the ground state of the system is a topological exciton insulator when the Coulomb interaction between electrons and holes is included. For a 2D VdH system, we find that a perpendicular electric field can decrease the bandgap, which even becomes smaller than the exciton binding energy, leading to the formation of exciton insulator phase. Due to large exciton binding energy, the exciton insulator phase in the 2D VdH system could be observed at room temperature. 
Monday, March 4, 2019 9:00AM  9:12AM 
A01.00006: Full Commuting Projector Hamiltonians of Interacting SymmetryProtected Topological Phases of Fermions Nathanan Tantivasadakarn, Ashvin Vishwanath Using the decorated domain wall procedure, we construct Finite Depth Local Unitaries (FDLUs) that realize Fermionic SymmetryProtected Topological (SPT) phases. This results in explicit 'full' commuting projector Hamiltonians, where 'full' implies the fact that the ground state, as well as all excited states, of these Hamiltonians realizes the nontrivial SPT phase. We begin by constructing explicit examples of 1+1D phases protected by symmetry groups G = Z2T×Z2F , which also has a free fermion realization in class BDI, and G = Z4×Z4F , which does not. We then turn to 2+1D, and construct the square roots of the LevinGu bosonic SPT phase, protected by Z2×Z2F symmetry, in a concrete model of fermions and spins on the triangular lattice. Edge states and the anomalous symmetry action on them are explicitly derived. Although this phase has a free fermion representation as two copies of p + ip superconductors combined with their p − ip counterparts with a different symmetry charge, the full set of commuting projectors is only realized in the strongly interacting version, which also implies that it admits a manybody localized realization. 
Monday, March 4, 2019 9:12AM  9:24AM 
A01.00007: Origin of Mott insulating behavior and superconductivity in twisted bilayer graphene Liujun Zou, Hoi Chun Po, Ashvin Vishwanath, Senthil Todadri A remarkable recent experiment has observed Mott insulator and proximate superconductor phases in twisted bilayer graphene when electrons partly fill a nearly flat miniband that arises a magic twist angle. However, the nature of the Mott insulator, origin of superconductivity and an effective low energy model remain to be determined. We will present a phenomenological picture of the Mott insulator with intervalley coherence that spontaneously breaks U(1) valley symmetry, and describe a mechanism that selects this order over the competing magnetically ordered states favored by the Hunds coupling. We will discuss consequences of this picture for superconducting states obtained on doping the valley ordered Mott insulator. We show how important features of the experimental phenomenology may be explained and suggest a number of further experiments for the future. 
Monday, March 4, 2019 9:24AM  9:36AM 
A01.00008: Coupled wire models on compact manifolds and the entanglement entropy for Abelian and nonAbelian topological orders Bo Han, Chi Yan Jeffrey Teo We construct coupled wire models for topologically ordered systems on the 2d torus and study the ground state degeneracy, with explicit ground states. Both Abelian and nonAbelian topological orders will be studied. With the ground states written down, we can study the entanglement entropy for the corresponding systems. Generalizations to 2d closed manifolds with higher genus and 3d closed manifolds will also be discussed. 
Monday, March 4, 2019 9:36AM  9:48AM 
A01.00009: Classification of fermionic symmetryprotected topological phases QingRui Wang, Zhengcheng Gu We give a systematic construction and classification of fermionic symmetryprotected topological states for generic fermionic symmetry group G_f, which is a central extension of bosonic symmetry group G_b (may contain time reversal symmetry) by the fermion parity symmetry group Z_2^f. For each class in the classification (except those with 2D p+ip chiral superconductor decorations), we construct a fixedpoint wave function which admits exactly solvable commutingprojector Hamiltonian. The classification is based on the notion of equivalence class of fermionic symmetric local unitary transformations. 
Monday, March 4, 2019 9:48AM  10:00AM 
A01.00010: Inducing and controlling magnetism in the graphene lattice through a trapping potential Karla Baumann, Angelo Valli, Adriano Amaricci, Massimo Capone We study strongly interacting ultracold spin1/2 fermions in a honeycomb lattice in the presence of a harmonic trap. 
Monday, March 4, 2019 10:00AM  10:12AM 
A01.00011: Fracton fusion and statistics Shriya Ramachandran Pai, Michael A Hermele In this work, we describe fusion and statistical processes in Abelian fracton phases in terms of their gapped excitations. The restricted mobility of fractonic excitations implies that statistical processes do not take the form of familiar braiding processes. Also, the number of distinct excitation types in fracton phases is infinite, in contrast to conventional phases with intrinsic topological order. Moreover, if one considers excitations supported in a region with linear size $L$, the number of excitation types supported in the region grows exponentially with $L$. To build a manageable theory that incorporates these features, we consider lattice translation symmetry. Without translation symmetry, the fusion of excitations in an Abelian fracton phase is described by an infinite Abelian group, whose elements correspond to distinct excitation types. Translation symmetry acts on this Abelian group, giving it more structure and making it a more manageable object to work with. Moreover, this action allows us to describe the mobility of excitations at the level of the fusion theory, which then forms the basis for a description of statistical processes. 
Monday, March 4, 2019 10:12AM  10:24AM 
A01.00012: Hall and Faraday effects in interacting multiband systems Reza Nourafkan, AndreMarie Tremblay The Hall conductivity is widely used as a probe of Fermi surface evolution. However, in the case of interacting multiband systems, calculations of the Hall conductivity are challenging because the application of the semiclassical singleband formula to many bands is ambiguous. Here, we derive a formula for the Hall response of interacting multiband systems with arbitrary band topology and spinorbit coupling. The formula is valid beyond the semiclassical approximation and at finite frequency, which is relevant for Faraday rotation, and it takes into account vertex corrections. In addition to the triangular diagrams, the formula includes rectangular diagrams that are absent in the singleband case. We show that these diagrams are necessary to recover a valid semiclassical formula for the Hall effect from the Kubo formula in the DC limit. 
Monday, March 4, 2019 10:24AM  10:36AM 
A01.00013: Interacting Spin3/2 Fermions in threedimensional Luttinger Metal: Phases, Phase Transitions and Global Phase Diagram Andras Szabo, Bitan Roy The notion of quasiparticles constitutes the foundation of condensed matter physics. Recently it became evident that Dirac or Weyl materials support linearly dispersing quasirelativisitc nodal quasiparticles around few isolated points in the Brillouin zone. In this talk, I will focus on a collection of spin3/2 excitations, which in three dimensions display a biquadratic touching of the valence and conduction bands at the $\Gamma$ point. Such peculiar band touching can be realized in the normal state of 227 pyrochlore iridates, halfHeuslers, HgTe and gray tin, for example. Once electronelectron interactions are accounted for, this system can display a rich confluence of a plethora of exotic broken symmetry phases, among which nematic, magnetic, and superconducting (swave and topological dwaves) orders are the most prominent ones. Using a controlled renormalization group analysis at finite temperature and chemical doping, I will demonstrate the competition among these phases, as well as the associated quantum critical phenomena. In addition, I will also present various cuts of the global phase diagram of interacting spin3/2 fermions, and argue that nematic and magnetic interactions are conducive for swave and dwave pairings of Luttinger fermions, respectively. 
Monday, March 4, 2019 10:36AM  10:48AM 
A01.00014: Aspects of Threebody Interactions in Generic Fractional Quantum Hall Systems and Impact of Galilean Invariance Breaking Bo Yang We derive full analytic expressions of threebody interactions from Landau level (LL) mixing in fractional quantum Hall (FQH) systems with SchriefferWolff transformation. The formalism can be applied to any LL, and to very general systems without rotational or Galilean invariance. We illustrate how threebody pseudopotentials (PPs) can be readily computed from the analytical expressions for a wide variety of different systems, and show that for realistic systems, softening the bare Coulomb interactions (e.g. finite thickness or screening) can significantly suppress threebody interactions. More interestingly, for experimental systems without Galilean invariance (which is common for real materials), there is strong evidence that higher orders in band dispersion can drive the MooreRead state from antiPfaffian to Pfaffian phase. Our analysis points to the importance of the realistic band structure details to the nonAbelian topological phases, and the analytical expressions we derived can also be very useful for high fidelity numerical computations. We also discuss about the importance of edge potentials in realistic experiments for determining the topological phases at halffilling, as well as the nature of such phases for systems with and without particlehole symmetry. 
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