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: Highprecision 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 singleparticle 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 largeorder asymptotics of the diagrammatic series, based on a functional integral representation of the skeleton series and the saddlepoint method. We show that the radius of convergence is actually zero, but the series is still resummable, by a generalised conformalBorel transformation that incorporates the largeorder asymptotics [2]. This yields new highprecision 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 highprecision 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 metalinsulator transition in VO2 from quantum Monte Carlo, DMFT, and experiment Invited Speaker: Jaron Krogel Vanadium dioxide displays the quintessential example of metalinsulator 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 nonstoichiometric 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 nonFermi liquid metal consistent with experimental findings. Quasi1D backscattering along the rutile caxis is reminiscent of a TomanagaLuttinger liquid, where the scattering is induced by impurities. In nonstoichiometric VO2 the calculated spectral function indicates a competition between a_{1g} and e^{π}_{g }orbitals which have a role in the formation of the insulating state. DMFTVCA calculations show that the a_{1g/}e^{π}_{g} 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 tJ 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 independentparticle calculation is coupled to the phaseless auxiliaryfield QMC calculation and the independentparticle solution is used as the constraint in QMC. The constraint is optimized by the selfconsistency between the QMC and independentparticle 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 multideterminant trial wavefunctions for Constrained Path Monte Carlo R. Torsten Clay Two of the most successful types of methods for stronglycorrelated 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 LuttingerWard functional Aaram Joo Kim, Evgeny Kozik We systematically study properties of highorder bolddiagrammatic expansions, which may converge to unphysical answers for the Hubbard interaction due to the multivaluedness of the LuttingerWard 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 highorder series. By varying the level of screening in the interaction line; bare U, randomphase approximation W_{rpa}, and fully dressed W_{exact}, we present the convergence properties of the different bold series in connection with multiple branches of the LWF. In particular, we find that the boldGW_{exact} and boldGW_{rpa} 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 TwoParticle 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 singleparticle Green’s function that has been used extensively in direct comparisons with experiments. For example, the singleparticle Green’s Function can be observed with angleresolved photoemission spectroscopy. However, many experimental observables are given by twoparticle correlation functions. We extend CPT to compute these correlation functions and focus on a method to use CPT to calculate the transverse spinsusceptibility, 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 onedimensional FermiHubbard model at halffilling and compare with known results. 
Tuesday, March 5, 2019 1:27PM  1:39PM 
F19.00008: Optimized higherorder LieTrotterSuzuki decompositions for two and more terms Yikang Zhang, Thomas Barthel LieTrotterSuzuki 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 manybody 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 nonAbelian 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 imaginarytime path integral: nonrelativistic U(1) lattice gauge theory in the KohnSham basis Andrei Kryjevski, Thomas Luu In the KohnSham orbital basis imaginarytime 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 imaginarytime electron propagators in several silicon hydrogenpassivated nanocrystals, such as Si_{35}H_{36}, Si_{87}H_{76} and Si_{147}H_{100}, and extract energies of lowlying electron and hole levels. Our qasiparticle gap predictions are in very good agreement with the results of recent G_{0}W_{0} 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 UV 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 highTc superconductivity. However, just as for the doped 2D Hubbard model, the 
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