Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A43: Precision Many-Body Physics I: Ab Initio MethodsFocus
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Sponsoring Units: DCOMP DAMOP DCMP Chair: Moritz Binder, Duke University Room: 702 |
Monday, March 2, 2020 8:00AM - 8:36AM |
A43.00001: Ab initio finite-temperature and excited state computations by auxiliary-field quantum Monte Carlo Invited Speaker: Shiwei Zhang Development in the ground-state auxiliary-field quantum Monte Carlo (AFQMC) approach over the past decade has allowed accurate computations in a broad array of systems ranging from Hubbard-like models, ultracold Fermi gases, to solids and quantum chemistry. I will discuss recent progress in generailizing the approach to non-zero temperatures. Two bottlenecks had to be removed. The first is the sign or phase problem which appears in most cases, similar to ground-state calculations. The second is the unfavorable scaling of finite-temperature, grand-canonical computations as N^3 (N is the size of the lattice or basis set) in contrast with N*M^2 in ground-state computations (M is the number of fermions), a major obstacle in any realistic calculations aiming to describe the continuum limit, where N/M needs to be extrapolated to infinity for convergence. We remove the sign or phase problem by constraining the path-integrals in field space with a gauge condition; a self-consistent procedure is formulated to improve the accuracy of the constraint iteratively [1]. We then introduce a systematically controllable low rank factorization which changes the scaling of the computations to N*M^2 [2]. The method is applicable to both models and real materials. Results will be presented on magnetic and stripe orders in the repulsive Hubbard model, as well as pairing and other properties in the strongly interacting two-dimensional Fermi gas as a function of temperature. |
Monday, March 2, 2020 8:36AM - 8:48AM |
A43.00002: Effects of charge self-consistency in DFT+DMFT calculations for complex transition metal oxides Alexander Hampel, Sophie Beck, Claude Ederer During recent years, the combination of density functional theory (DFT) and dynamical mean-field theory (DMFT) has become a widespread tool to calculate properties in correlated materials. The basic idea of the method is that the electronic degrees of freedom can be separated into a weakly interacting part, for which a standard DFT treatment is adequate, and a correlated subspace, which requires a more elaborate treatment of the electron-electron interaction. The latter leads, in general, to a redistribution of electrons within the correlated subspace compared to the DFT result. This change should then enter, in a self-consistent way, the effective potential felt by the weakly interacting electrons, which is achieved by iterating between DFT and DMFT steps. However, such a charge self-consistent (CSC) DFT+DMFT calculation leads to a higher computational cost compared to simpler one-shot calculations, where this charge rearrangement is neglected. Here, we examine the effect of CSC in DFT+DMFT calculations compared to simpler one-shot calculations for two instructive example materials, CaVO3 and LuNiO3, to clarify in which cases the more complex CSC treatment is necessary and in which cases the simpler one-shot calculation is sufficient. |
Monday, March 2, 2020 8:48AM - 9:00AM |
A43.00003: DFT+eDMFT Study of Finite-temperature Properties of Filled Skutterudite CeGe4Pt12 Khandker Quader, Gheorghe Pascut, Kristjan Haule, Michael Widom Current interest in the filled skutterudites stem from a range of observed physical phenomena, such as superconductivity, Kondo-lattice, heavy fermion, Fermi liquid behavior, etc, and due to thermoelectric properties at room temperature. Following up on our comprehensive density functional theory (DFT) study (PRB 100, 125114 (2019)) of the RPt_4Ge_12 (R=La, Ce, Pr) compounds, we have been performing temperature dependent calculations on these systems using first-principles self-consistent density functional theory with embedded-dynamical mean field theory (DFT+eDMFT). Here we present results for CeGe4Pt12, namely the temperature dependence of spectral function, density of states, hybridization, effective mass, susceptibility, and resistivity, and compare with experiments. Based on our calculations, we suggest that CeGe4Pt12 exhibits Curie-Weiss behavior at high temperature (indicating presence of local moments), a Kondo-lattice like behavior at intermediate temperatures (~80-50K), and a Fermi liquid state of screened local moments at low temperature (~10-5K). |
Monday, March 2, 2020 9:00AM - 9:12AM |
A43.00004: Efficient implementation of ab initio dynamical mean-field theory for periodic systems Tianyu Zhu, Zhi-Hao Cui, Garnet Chan We present an ab initio quantum chemical framework for dynamical mean-field theory (DMFT) in periodic systems. Our DMFT scheme employs ab initio Hamiltonians defined for impurities comprising the full unit cell or a supercell of atoms and for realistic quantum chemical basis sets. We avoid double counting errors by using Hartree-Fock as the low-level theory. Intrinsic and projected atomic orbitals (IAO+PAO) are chosen as the local embedding basis, facilitating numerical bath truncation. Using an efficient integral transformation and coupled-cluster Green's function (CCGF) impurity solvers, we are able to handle large embedded impurity problems with several hundred orbitals. We apply our ab initio DMFT approach to study a hexagonal boron nitride monolayer, crystalline silicon, and nickel oxide in the antiferromagnetic phase, with up to 104 and 78 impurity orbitals in spin-restricted and unrestricted cluster DMFT calculations and over 100 bath orbitals. We show that our scheme produces accurate spectral functions compared to both benchmark periodic coupled-cluster computations and experimental spectra. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A43.00005: Efficient Hybridization Fitting for Dynamical Mean-Field Theory via Semi-Definite Relaxation Carlos Mejuto Zaera, Leonardo Zepeda-Núñez, Michael Lindsey, Norm Tubman, Birgitta K Whaley, Lin Lin Hamiltonian-based solvers for dynamical mean-field theory (DMFT) can compute spectral properties directly in the real axis and are applicable to impurity Hamiltonians presenting general interactions. This flexibility comes at the prize of having to truncate the formally infinite bath, transforming the DMFT self-consistent condition into a non-linear optimization problem. Fulfilling the self-consistency condition exactly with a finite bath is impossible. Furthermore, the large number of degrees of freedom in the optimization problem makes it likely to fall into local minima, which may result in the DMFT calculation converging to the wrong physical solution. As a consequence, the optimization step in Hamiltonian-based DMFT can become the most difficult part of the calculation. In this work1, we propose a nested optimization procedure using semi-definite relaxation which addresses and improves many of the issues that plague the optimization step in Hamiltonian-based DMFT. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A43.00006: Electronic Structure of Strongly Correlated f-electron System: DFT+DMFT Approach Vijay Singh, Uthpala Herath, Benny Wah, Aldo H Romero, Hyowon Park Computational materials design of strongly correlated materials (SCM) has been challenging in modern condensed matter physics since it requires the development of more accurate methodologies beyond density functional theory (DFT). In the present talk, I will discuss our recent development of an efficient computational method so called DMFTwDFT to treat dynamical correlations in SCM accurately. I use dynamical mean-field theory (DMFT) in combination with DFT to compute the electronic structure of strongly correlated f-electron systems, specifically, rare-earth metals. The main point of the debate in f-electron system is related to the understanding of the role played by f electrons — they are localized or itinerant, or more exactly how many f electrons are localized or itinerant. For this reason, the theoretical and experimental investigations of the electronic structure of rare-earth metals have always occupied an important position in rare-earth research. Here, I use the DMFT+DFT method implemented using the maximally localized Wannier function as the local basis set and combining various DFT codes to study electronic properties of these materials. Our results will be also compared to other DMFT+DFT codes employing different local basis sets and DFT implementations. |
Monday, March 2, 2020 9:36AM - 9:48AM |
A43.00007: Kondo route to quantum interference in prototypical single molecule transistors Sudeshna Sen, Andrew Mitchell Single molecule transistors offer a fascinatingly diverse range of physics due to their ultrasmall size, chemical complexity, and strong electronic interactions. They constitute a playground for exploring the fundamental physics of correlations on the nanoscale, and their signatures in transport. Understanding these systems is also an essential prerequisite for possible advanced technological applications utilizing their quantum characteristics. In this talk I examine prototypical molecular junctions π-conjugated hydrocarbon molecular junctions using a combination of perturbative scaling, numerical renormalization group, and machine learning methods [1] after reducing them to effective multi-orbital impurity systems. The interplay of Kondo effect and emergent many body quantum interference effects are explored in the context of quantum boosted functionalities. |
Monday, March 2, 2020 9:48AM - 10:00AM |
A43.00008: Density Matrix Embedding Theory: From Lattice Models to Realistic Materials Zhi-Hao Cui, Tianyu Zhu, Garnet Chan In the past few years, density matrix embedding theory (DMET) [Phys. Rev. Lett. 109, 186404] has emerged as a successful wavefunction-based embedding scheme for both lattice models and molecules, but with few applications to ab initio periodic Hamiltonians. In this work, we will discuss a unified formalsim for both lattice models and realistic solids. We will highlight some practical considerations in the simulation of realistic materials with DMET, including the choice of orbitals and mapping to a lattice, treatment of the virtual space and bath truncation, and the lattice-to-embedded integral transformation. We apply our DMET framework to both Hubbard-like lattice models and several realitic materials, e.g. hexagonal boron nitride monolayer, crystalline silicon, and nickel monoxide in the antiferromagnetic phase, using large embedded clusters with up to 300 embedding orbitals. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A43.00009: Out of equilibrium thermometry with pump-probe x-ray photoemission spectroscopy Oleh Matvyeyev, Andrij Shvaika, James Freericks We calculate the spectral function of a deep core-level hole in a pump-probe x-ray photoemission spectroscopy experiment. Here, an intense light pulse pumps electrons to higher energies, and a second high-energy x-ray probe pulse is used to knock out an electron from a deep core-level. Electrons from the conduction band feel the effects of the core-hole potential and react by screening it. This creates particle-hole excitations within the conduction band until the core-hole has been filled. We examine the spinless Falicov-Kimball model, which possesses a metal-Mott-insulator transition, and has an exact solution within dynamical mean-field theory. In linear response, it is well known that the shape of the core-hole spectra depends strongly on temperature in the high-temperature regime. We employ this effect in nonequilibrium, and describe an ultrafast "thermometer," which can determine the energy content of the conduction electrons with a nondestructive in situ measurement on an ultrafast time scale. |
Monday, March 2, 2020 10:12AM - 10:24AM |
A43.00010: Electronic structure of bulk manganese oxide and nickel oxide from coupled cluster theory Yang Gao, Qiming Sun, Jason Yu, Mario Motta, James McClain, Alec F White, Austin Minnich, Garnet Chan We describe the ground- and excited-state electronic structure of bulk MnO and NiO using coupled cluster theory with single and double excitations (CCSD). Starting from a Hartree-Fock reference, we find fundamental gaps of 3.46 eV and 4.83 eV for MnO and NiO respectively for the 16 unit supercell, slightly overestimated compared to experiment, although finite-size scaling suggests that the gap is more severely overestimated in the thermodynamic limit. From the character of the correlated electronic bands we find both MnO and NiO to lie in the intermediate Mott/charge-transfer insulator regime, although NiO appears as a charge transfer insulator when only the fundamental gap is considered. While the lowest quasiparticle excitations are of metal 3d and O 2p character in most of the Brillouin zone, near the Γ point, the lowest conduction band quasiparticles are of s character. Our study supports the potential of coupled cluster theory to provide high level many-body insights into correlated solids. |
Monday, March 2, 2020 10:24AM - 10:36AM |
A43.00011: The electronic structure of n-doped ABO3 perovskite metals from quantum Monte Carlo. Michael Bennett, Guoxiang Hu, Panchapakesan Ganesh, Jaron Krogel Some perovskites (PVs) are known to undergo metal-to-insulator transitions (MITs) when n-doped. In particular, the PV ferromagnet strontium cobaltite (SrCoO3), undergoes an MIT when a critical level of ordered oxygen vacancies are present in the system. Concomitant topotactic and magnetic transitions can also occur, e.g., the oxygen-deficient SrCoO2.5 phase is an anti-ferromagnet with a brownmillerite crystal structure. The cause of this nonintuitive MIT in these oxygen-deficient systems is not well understood. Density functional theory (DFT) calculations suggest that charge disproprtionation is often associated with the transition, but these systems have strong correlations that are beyond DFT. Furthermore, counting formal oxidation states in the oxygen-rich systems hints at the presence of ligand holes which would conceivably lead to passivation after n-doping. Here, we hypothesize that these systems are indeed self-hole doped and use diffusion Monte Carlo methods that include electron correlations exactly to gain clarity on the electronic/magnetic/structural transitions. |
Monday, March 2, 2020 10:36AM - 10:48AM |
A43.00012: ThetaPhi - a new program for calculation of Bardeen-Cooper-Schrieffer and Magnetic Superstructure Electronic States Evgeny Plekhanov, Andrei Tchougreeff We propose the Theta-Phi package [1] which addresses two of the most important extensions of the essentially single-particle mean-field paradigm of the computational solid state physics: the admission of the Bardeen-Cooper-Schrieffer electronic ground state and allowance of the magnetically ordered states with an arbitrary superstructure (pitch) wave vector. Both features are implemented in the context of multi-band systems which paves the way to an interplay with the solid state quantum physics packages eventually providing access to the first-principles estimates of the relevant matrix elements of the model Hamiltonians derived from the standard DFT calculations. Several examples showing the workability of the proposed code are given. |
Monday, March 2, 2020 10:48AM - 11:00AM |
A43.00013: A Multiorbital Quantum Impurity Solver for General Interactions and Hybridizations Eitan Eidelstein, Emanuel Gull, Guy Cohen We present a numerically exact Inchworm Monte Carlo method for equilibrium multiorbital quantum impurity problems with general interactions and hybridizations. We show that the method, originally developed to overcome the dynamical sign problem in certain real-time propagation problems, can also overcome the sign problem as a function of temperature for equilibrium quantum impurity models. This is shown in several cases where the current method of choice, the continuous-time hybridization expansion, fails due to the sign problem. Our method therefore enables simulations of impurity problems as they appear in embedding theories without further approximations, such as the truncation of the hybridization or interaction structure or a discretization of the impurity bath with a set of discrete energy levels, and eliminates a crucial bottleneck in the simulation of ab initio embedding problems. |
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