Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F19: Precision Many Body Physics VFocus
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Sponsoring Units: DCOMP DCMP Chair: Luca Fausto Tocchio Room: BCEC 156C |
Tuesday, March 5, 2019 11:15AM - 11:51AM |
F19.00001: High-precision data for the unitary Fermi gas from diagrammatic series with zero convergence radius Invited Speaker: Kris Van Houcke In this talk I will mainly focus on the unitary Fermi gas (spin 1/2 fermions with contact interactions in 3D, which describes cold atomic gases at a Feshbach resonance) in the normal phase. Thanks to a diagrammatic Monte Carlo algorithm, we accurately sample all skeleton diagrams (built on dressed single-particle and pair propagators) up to order nine [1]. The diagrammatic series is divergent and there is no small parameter so that a resummation method is needed. We compute the large-order asymptotics of the diagrammatic series, based on a functional integral representation of the skeleton series and the saddle-point method. We show that the radius of convergence is actually zero, but the series is still resummable, by a generalised conformal-Borel transformation that incorporates the large-order asymptotics [2]. This yields new high-precision data, not only for the equation of state, but also for Tan's contact coefficient and for the momentum distribution [3]. I will also highlight some recent developments in (determinant) diagrammatic Monte Carlo and present new high-precision data for the Fermi polaron, which is a single impurity atom immersed in a Fermi sea. |
Tuesday, March 5, 2019 11:51AM - 12:27PM |
F19.00002: Understanding the metal-insulator transition in VO2 from quantum Monte Carlo, DMFT, and experiment Invited Speaker: Jaron Krogel Vanadium dioxide displays the quintessential example of metal-insulator transition (MIT) physics in a strongly correlated material. Despite numerous studies, the nature of the MIT is still controversial and new perspectives are needed. Recent experiments view rutile VO2 as an unconventional metal due to its anomalously low electronic thermal conductivity. Due to strong correlations in VO2, beyond DFT approaches are required and here we study pristine and non-stoichiometric VO2 with quantum Monte Carlo and DMFT. New perspective is provided by the momentum distribution, which contains no discontinuity in the metallic phase, indicating a non-Fermi liquid metal consistent with experimental findings. Quasi-1D back-scattering along the rutile c-axis is reminiscent of a Tomanaga-Luttinger liquid, where the scattering is induced by impurities. In non-stoichiometric VO2 the calculated spectral function indicates a competition between a1g and eπg orbitals which have a role in the formation of the insulating state. DMFT-VCA calculations show that the a1g/eπg orbital dichroism falls below its pristine value at a doping concentration of δ=0.07, in near agreement with the experimentally determined critical doping threshold for the suppression of the insulating state. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F19.00003: Computable formulae for Hall and Nernst Coefficients of Strongly Correlated Metals Assa Auerbach, Ilia Khait Exact formulae for the temperature dependent Hall coefficient (published in Phys. Rev. Lett. 121, 066601 (2018)), and for a modified Nernst coefficient of metals are derived from the Kubo linear response functions. The formuale are valid for a large range of microscopic Hamiltonians of fermions and bosons, subject to arbitrary potentials and interactions. These DC transport coefficients (remarkably) depend solely on equilibrium susceptibilities, which are amenable to well controlled numerical algorithms - including Quantum Monte Carlo (in imaginary time), high temperature series expansions, and variational wavefucntions. Applications of these formulae are demonstrated for band electrons, the Bose Hubbard model, and for the t-J model. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F19.00004: Self Consistent Auxiliary Field Quantum Monte Carlo Method for Realistic Materials Shiwei Zhang, Hao Shi The auxiliary field quantum Monte Carlo (QMC) calculations in interacting fermion systems require a constraint to control the sign and phase problem. The constraint involves an input trial wave function which restricts the random walks. We introduce a systematically improvable constraint for realistic materials. An independent-particle calculation is coupled to the phaseless auxiliary-field QMC calculation and the independent-particle solution is used as the constraint in QMC. The constraint is optimized by the self-consistency between the QMC and independent-particle calculations. We demonstrate this approach in transition metal oxides. Connection with other electronic structure methods will be discussed. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F19.00005: Optimized multi-determinant trial wavefunctions for Constrained Path Monte Carlo R. Torsten Clay Two of the most successful types of methods for strongly-correlated models are quantum Monte Carlo and renormalization group methods. Both however suffer from limitations that make large calculations difficult except in special cases, for example one dimension (1D). The Density Matrix Renormalization Group method suffers from poor scaling beyond 1D. The Path Integral Renormalization Group (PIRG) method expands the wavefunction in Slater Determinants and is not limited by dimension, but by the strength of interactions. Quantum Monte Carlo calculations are severely limited by the Fermion sign problem. The Constrained Path Monte Carlo (CPMC) method prevents the exponential loss of precision from the sign problem through the use of a trial wavefunction. However, the trial wavefunction is an uncontrolled approximation with an unknown error. We demonstrate a way to combine the advantages of a renormalization method (PIRG) with those of a quantum Monte Carlo (CPMC), by using PIRG wavefunctions as CPMC trial wavefunctions. The advantage of PIRG wavefunctions as trial |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F19.00006: Skeleton diagrammatic expansions with screened Hubbard interaction versus multivaluedness of the Luttinger-Ward functional Aaram Joo Kim, Evgeny Kozik We systematically study properties of high-order bold-diagrammatic expansions, which may converge to unphysical answers for the Hubbard interaction due to the multivaluedness of the Luttinger-Ward functional (LWF), by the prototypical example of the Hubbard atom. The diagrammatic Monte Carlo method with fully dressed propagator G is adopted to generate the high-order series. By varying the level of screening in the interaction line; bare U, random-phase approximation Wrpa, and fully dressed Wexact, we present the convergence properties of the different bold series in connection with multiple branches of the LWF. In particular, we find that the bold-GWexact and bold-GWrpa series diverge well below the branching point of the LWF, but admit the analytic continuation beyond their convergence radius by standard techniques. We further explore the possibility of using the bold diagrammatic series in the strongly correlated regime to obtain precise results with controlled accuracy. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F19.00007: Cluster Perturbation Theory Applied to Two-Particle Correlation Functions Peter Raum, Vito Scarola, Thomas Maier Developing techniques to solve Hubbard models is an active area of research due to their ability to capture the essential properties of many strongly correlated systems. Cluster Perturbation Theory (CPT) is an economic method to calculate the momentum and energy resolved single-particle Green’s function that has been used extensively in direct comparisons with experiments. For example, the single-particle Green’s Function can be observed with angle-resolved photoemission spectroscopy. However, many experimental observables are given by two-particle correlation functions. We extend CPT to compute these correlation functions and focus on a method to use CPT to calculate the transverse spin-susceptibility, measurable via inelastic neutron scattering on strongly correlated materials or with optical probes of atomic gases in optical lattices. We benchmark our method with the one-dimensional Fermi-Hubbard model at half-filling and compare with known results. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F19.00008: Optimized higher-order Lie-Trotter-Suzuki decompositions for two and more terms Yikang Zhang, Thomas Barthel Lie-Trotter-Suzuki decompositions of operator exponentials have a lot of applications in physics. For example, they are employed to sample equilibrium states in quantum Monte Carlo and to simulate the dynamics of quantum systems on quantum computers or on classical computers using tensor network state techniques. They also provide symplectic integrators for classical physics. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F19.00009: Diagrammatic Monte Carlo approach to angular momentum in quantum many-body systems Giacomo Bighin, Timur V Tscherbul, Mikhail Lemeshko We introduce a Diagrammatic Monte Carlo (DiagMC) approach to molecular impurities, possessing rotational degrees of freedom [1]. The technique is based on a diagrammatic expansion [2] that merges the usual Feynman diagrams with the angular momentum diagrams known from atomic and nuclear structure theory, thereby incorporating the non-Abelian algebra inherent to quantum rotations. Due to the peculiar way in which angular momenta couple, the configuration space is larger with respect to most DiagMC applications, and a new class of updates is needed in order to span it completely. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F19.00010: Electronic structure of semiconductor nanoparticles from stochastic evaluation of imaginary-time path integral: nonrelativistic U(1) lattice gauge theory in the Kohn-Sham basis Andrei Kryjevski, Thomas Luu In the Kohn-Sham orbital basis imaginary-time path integral for electrons in a semiconductor nanoparticle has a mild fermion sign problem and is, therefore, amenable to evaluation by the standard stochastic methods. Utilizing output from the density functional theory simulations we compute imaginary-time electron propagators in several silicon hydrogen-passivated nanocrystals, such as Si35H36, Si87H76 and Si147H100, and extract energies of low-lying electron and hole levels. Our qasiparticle gap predictions are in very good agreement with the results of recent G0W0 calculations. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F19.00011: Insulating states from increased kinetic energy: counterintuitive physics in the basic model of organic conductors and superconductors Adrian Kantian, Thierry Giamarchi The U-V model at quarter filling is the canonical model of the organic Bechgaard and Fabre salts, the first materials to exhibit a superconducting phase based on repulsive electron interactions, which is accompanied by competing magnetic phases just as for the cuprates of high-Tc superconductivity. However, just as for the doped 2D Hubbard model, the |
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