Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session M49: Precision ManyBody Physics V: New AlgorithmsFocus Recordings Available

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Sponsoring Units: DCOMP Chair: Hansveer Singh, University of Massachusetts, Amherst Room: McCormick Place W471B 
Wednesday, March 16, 2022 8:00AM  8:36AM 
M49.00001: Highorder diagrammatic expansion around BCS: Polarized superfluid phase of the attractive Hubbard model Invited Speaker: Félix Werner In contrast to conventional QMC methods, expansions of intensive quantities in series of connected Feynman diagrams can be formulated directly in the thermodynamic limit. Over the last decade, diagrammatic Monte Carlo algorithms made it possible to reach large expansion orders and to obtain stateoftheart results for various models of interacting fermions in 2 and 3 dimensions, mostly in the normal phase. 
Wednesday, March 16, 2022 8:36AM  8:48AM 
M49.00002: Isometric Tensor Network States on an Infinite Stripe Yantao Wu, Sajant Anand, ShengHsuan Lin, Michael P Zaletel Contraction of standard 2D tensor network ansatzes relies on approximation schemes, as tensor contractions costs scale exponentially with system size. Recently, the Isometric Tensor Network (isoTNS) ansatz was introduced for 2D finite quantum systems on a square lattice, allowing for exact $\mathcal{O}(1)$ evaluation of expectation values provided we can efficiently move the orthogonality center throughout the network. Here we generalize this isoTNS ansatz to strip geometries, in which the networks are infinite by finite and consist of translationally invariant rows of tensors. We demonstrate several algorithms for the infinite Moses Move (iMM), which moves the orthogonality hypersurface between columns of infinite length in the network. Using these iMM algorithms, we perform imaginary time evolution to identify the ground state of our 2D system, where the cost of optimization scales linearly with strip width rather than exponentially. 
Wednesday, March 16, 2022 8:48AM  9:00AM 
M49.00003: Massively parallel exact diagonalization with symmetries and minimal communication Benedikt Kloss Scalable, distributedmemory approaches to exact diagonalization require that functions of the Hamiltonian can be applied efficiently to the wavefunction. We present a method that requires a single collective (alltoall) communication per Hamiltonian application while making use of conserved quantities. The approach is applicable to a wide class of quantum lattice systems with U(1) symmetries and finiteranged interactions as well as multiorbital impurity problems. It is less communication intensive than existing approaches, and its parallel scaling is entirely determined by that of an alltoall communication on the computing platform. 
Wednesday, March 16, 2022 9:00AM  9:12AM 
M49.00004: New formalism for the exact calculation of total energies and associated electronic states of manybody interactions with complexity n^{4} Thierry Deutsch In this talk, I intend to show a new formalism that should solve the famous manybody problem of quantum mechanics with a complexity varying as a function of n⁴ where n is the finite number of states. There was already an object, the reduced twobody density matrix (2RDM) that had this complexity but for it to be a quantum state with N electrons, a gigantic number of conditions must be taken into account. 
Wednesday, March 16, 2022 9:12AM  9:24AM 
M49.00005: Floating point precision imaginary time response functions in quantum manybody physics Hugo Strand, Xinyang Dong, Emanuel C Gull, Dominika Zgid, Jason Kaye The dynamics of quantum manybody systems in thermal equilibrium is amenable to the imaginary time formalism. The sequent numerical treatment of imaginary time propagators is central to many forms of perturbative and nonperturbative algorithmic approaches, e.g. perturbation theory, diagrammatic Monte Carlo, embedding methods like dynamical meanfield theory, etc. However, the prevailing techniques for numerical representation and computations in imaginary time, based on equidistant grids, are suffering from poor accuracy and poor scaling at low temperature. This talk will give an overview of the recent progress in numerical representation and computation using highly accurate and compact imaginary time representations, as well as an outlook for applications to realtime Green's functions. 
Wednesday, March 16, 2022 9:24AM  9:36AM 
M49.00006: Interaction expansion inchworm Monte Carlo Yang Yu, Jia Li, Guy Cohen, Emanuel C Gull We generalize the inchworm Monte Carlo framework for the interaction expansion. Inchworm Monte Carlo is a method to iteratively group and sample classes of diagrams in diagrammatic perturbation expansions. In the hybridization expansion, the inchworm was shown to drastically delay the onset of the sign problem. Here, we generalize the iterative procedure to the interaction expansion and show application to multiorbital quantum impurity models with general interaction and hybridization. We clarify convergence properties and highlight similarities and differences to the bare and bold Monte Carlo methods. 
Wednesday, March 16, 2022 9:36AM  10:12AM 
M49.00007: Probingtopologicalspinliquids on a programmable quantum simulator Invited Speaker: Giulia Semeghini Quantum phases with topological order, such as quantum spin liquids, have been the focus of explorations for several decades. Such phases feature a number of remarkable properties including longrange quantum entanglement. Moreover, they can be potentially exploited for the realization of robust quantum computation, as exemplified by the paradigmatic toric code model. In this talk, I will show how a programmable quantum simulator based on Rydberg atom arrays can be used to realize and probe quantum spin liquid states. In our approach, atoms are placed on the links of a kagome lattice and coherent evolution under Rydberg blockade enables the transition into frustrated quantum states with no local order. We detect the onset of a quantum spin liquid phase of the toric code type by measuring topological string operators in two complementary bases. The properties of this state are further revealed using a lattice with nontrivial topology, representing a step towards the realization of a topological qubit. Our observations open the door to the controlled experimental exploration of topological quantum matter and could enable the investigation of new methods for topologically protected quantum information processing. 
Wednesday, March 16, 2022 10:12AM  10:24AM 
M49.00008: Gaussian Fermionic Matrix Product States for generalized HartreeFock in quasi1D systems Alexander H Meiburg, Bela Bauer In many approximate approaches to fermionic quantum manybody systems, such as HartreeFock and density functional theory, solving a system of noninteracting fermions coupled to some effective potential is the computational bottleneck. In this work, we demonstrate that this crucial computational step can be accelerated using recently developed methods for Gaussian fermionic matrix product states (GFMPS). As an example, we studied the generalized HartreeFock method, which unifies HartreeFock and selfconsistent BCS theory, applied to Hubbard models with an inhomogeneous potential. We demonstrate that for quasionedimensional systems with local interactions, our approach scales approximately linear in the length of the system while yielding a similar accuracy to standard approaches that scale cubically in the system size. 
Wednesday, March 16, 2022 10:24AM  10:36AM 
M49.00009: Sparsemodeling solver for dynamical twoparticle response of correlated electrons Hiroshi Shinaoka, Markus Wallerberger, Anna Kauch Computing dynamical twoparticle response functions of correlated materials is a grand challenge in the field of computational material science. The dynamical meanfield theory (DMFT) is one of the most successful theories for computing oneparticle response of correlated electrons. Nevertheless, it is notoriously difficult to compute twoparticle responses as it involves solving the BetheSalpeter equation (BSE), whose computational effort scales unfavourably with inverse temperature and the number of bands. 
Wednesday, March 16, 2022 10:36AM  10:48AM 
M49.00010: Dynamical Variational Monte Carlo as a quantum impurity solver Peter Rosenberg, Maxime Charlebois, AndréMarie Tremblay, David Sénéchal The theoretical and numerical treatment of stronglycorrelated many electron systems remains an outstanding challenge of modern condensed matter physics. The recently developed dynamical Variational Monte Carlo (dVMC) method enables the computation of the Green's function for larger clusters, but has so far been limited to translationally invariant models with periodic boundary conditions. Here we propose a method to relax these constraints. This new approach makes it possible to use dVMC as a quantum impurity solver for embedding techniques such as Cluster dynamical mean field theory (CDMFT). 
Wednesday, March 16, 2022 10:48AM  11:00AM 
M49.00011: SelfConsistent Ab Initio Embedding Results for Real Materials Runxue Yu, ChiaNan Yeh, Yanbing Zhou, Emanuel C Gull, Dominika Zgid We present results for selfconsistent embedding calculations of strongly correlated real materials. The ab initio simulation of periodic solids with strong correlation is an active area of research, since reliable parameterfree methods exist only for weakly correlated solids. We report results from a fully selfconsistent parameterfree ab initio selfenergy embedding theory consisting of a weakly correlated environment (treated at the level of GW) and strongly correlated orbitals (treated with Exact Diagonalization). The method is applied to real materials with transition metal d orbitals and rareearth element f orbitals to obtain information about the spectral function and thermodynamic properties. We also discuss aspects of stability and convergence. 
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