Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H19: Precision Many Body Physics VIFocus
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Sponsoring Units: DCOMP DCMP Chair: Shiwei Zhang Room: BCEC 156C |
Tuesday, March 5, 2019 2:30PM - 3:06PM |
H19.00001: Precision spectral densities in correlated systems Invited Speaker: Karen Hallberg We develop an efficient numerical method to calculate spectral densities of complex impurities based on the Density Matrix Renormalization Group (DMRG) and use it as the impurity solver of the Dynamical Mean Field Theory (DMFT). By using a self-consistent bath configuration with very low entanglement, we take full advantage of the DMRG to calculate dynamical response functions paving the way to treat large effective impurities such as those corresponding to multi-orbital interacting models and multi-site or multi-momenta clusters. It also solves for complex impurities using ab-initio input which opens its realm of applications to real materials. This method leads to reliable calculations of non-local self energies on the real frequency axis directly, at zero temperature. arbitrary dopings and interactions and at all energy scales. |
Tuesday, March 5, 2019 3:06PM - 3:42PM |
H19.00002: Approximation to an exchange-correlation functional on the basis of renormalizatio-group theory Invited Speaker: Luiz Oliveira Approximations to the exchange-correlation functional are central to Density Functional Theory. Generalizations of the Local-Density Approximation typically describe electronic systems very well. Nonetheless, many exceptions are known. The molecular junction offers an outstanding example, one in which nonlocal correlations stem from the Kondo effect. The junction comprises two metallic leads bridged by a molecule. At low temperatures, the molecular-orbital spin and the low-energy conduction-electron spins lock into a singlet, a entangled state that renders local approximations inadequate, so large is its diameter. To take a nonlocal approach, we resort to renormalization-group concepts. As the temperature T is reduced, the junction crosses over from the vicinity of a high-T fixed point, in which the molecular spin and lead electrons are decoupled, to a low-T fixed point, in which they are strongly coupled. The high-T fixed point is devoid of entanglement, hence well described by local approximations. We can therefore set up and solve the Kohn-Sham equations for that fixed point. We then combine the resulting Kohn-Sham eigenstates with the molecular spin to define a single-impurity Anderson Hamiltonian. Numerical renormalization-group diagonalization of the latter depicts the crossover to the low-T fixed point and yields the ground-state and temperature-dependent zero-bias transport properties of the junction. As an illustration, results for an inhomogeneous Hubbard model will be presented and compared with experimental data. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H19.00003: Sign-blessed Diagrammatic Monte Carlo Method for Electrons Interacting with the Long-range Coulomb Repulsion Kun Chen, Kristjan Haule We show that combining a variational approach with a new diagrammatic quantum Monte Carlo method results in a powerful and accurate solver to the generic solid state problem, in which a macroscopic number of electrons interact by the long range Coulomb repulsion. We apply the solver to the quintessential problem of solid state, the uniform electron gas (UEG), which is at the heart of the density functional theory (DFT) success in describing real materials, yet it has not been adequately solved for over 90 years. While some wave-function properties, like the ground state energy, have been very accurately calculated by the diffusion Monte Carlo method (DMC), the static and dynamic response functions, which are directly accessed by the experiment, remain poorly understood. Our method allows us to calculate the momentum-frequency resolved spin response functions for the first time, and to improve on the precision of the charge response function. The accuracy of both response functions is sufficiently high, so as to uncover previously missed fine structure in these responses. This method can be straightforwardly applied to a large number of moderately interacting electron systems in the thermodynamic limit, including realistic models of metallic and semiconducting solids. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H19.00004: First principles study of correlation effects in solids Sergei Iskakov, Alexander Rusakov, Emanuel C Gull, Dominika Zgid Designing reliable, predictive, and computationally affordable methods to address electronic correlations in realistic solids is an ongoing |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H19.00005: Theory of Time-Resolved Raman Scattering in Correlated Systems: Ultrafast Engineering of Spin Dynamics and Detection of Thermalization Cheng-Chien Chen, Yao Wang, Thomas Devereaux Ultrafast characterization and control of elementary excitations are critical to understanding and manipulating emergent phenomena in correlated systems. In particular, spin interaction plays an important role in unconventional superconductivity, but efficient tools for probing spin dynamics especially out of equilibrium is still lacking. Here we develop the theory of time-resolved Raman scattering, which can be a powerful tool for nonequilibrium studies. We also simulate a pumped single-band Hubbard model using exact diagonalization. Different ultrafast processes are shown to exist in the time-resolved Raman spectra and dominate under different pump conditions. For high-frequency and off-resonance pumps, the Floquet theory is shown to work well in capturing the bimagnon softening. We also show that effective heating dominates at small pump fluences, while many-body effect takes over at larger pump amplitudes and frequencies resonant to the Mott gap. Time-resolved Raman scattering thereby provides the platform to explore ultrafast processes and design material properties out of equilibrium. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H19.00006: Charge self-consistent DFT + DMFT study on magnetism of transition metals Mancheon Han, Hyoung Joon Choi Density functional theory (DFT) is a static approximation, so it describes system with static magnetic moment quite well. However, if spin fluctuation is much larger than static moment, DFT is not appropriate tool for investigate such material. For example, the high-temperature paramagnetic state of a magnetic material can not be adequately described by DFT alone. Dynamical Mean Field Theory (DMFT), which accounts the local dynamic correlation effects exactly, can be used to account such fluctuation effects. We studied the magnetic properties of period 4 transition metals using our charge self-consistent density function theory + dynamical mean field theory (DFT + DMFT) program. Our calculation shows temperature dependence of magnetism, which is not observed in ordinary DFT calculation. Moreover, paramagnetic phase was investigated by calculating total spin angular momentum and magnetic susceptibility for several temperatures. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H19.00007: Almost exact energies for the G1/G2 set with the semistochastic heat-bath configuration interaction method Yuan Yao, Junhao Li, Cyrus Jehangir Umrigar The recently developed semistochastic heat-bath configuration interaction (SHCI) method is a systematically improvable selected configuration interaction plus perturbation theory method capable of giving essentially exact energies for larger systems than is possible with other such methods.We compute SHCI atomization energies for the 55 molecules in the G1/G2 set, for which accurate experimental data are available. Basis sets from cc-pVDZ to cc-pV5Z are used, totaling up to 500 orbitals and a Hilbert space of $10^53$ determinants for the largest molecules. To speed up convergence, we first optimize orbitals using 1-body and 2-body reduced density matrices constructed from SHCI wavefunctions with a large convergence threshold. For each of the basis sets, the extrapolated energy is within chemical accuracy (1 kcal/mol) of the exact energy for that basis using only a tiny fraction of the entire Hilbert space. The energies are extrapolated to the basis set limit and compared to the experimental atomization energies. We also use our almost exact energies to benchmark coupled cluster theory (CCSD(T)) energies. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H19.00008: One-electron spectral properties of self-assembled structures and defects on semiconductors Jose Carmelo, Tilen Cadez, Yoshiyuki Ohtsubo, Shin-ichi Kimura, David K Campbell Twin grain boundaries in monolayers of transition metal dichalcogenides such as molybdenum diselenide [MoSe(2)] and self-assembled atomic structures on the surface of semiconductors such as a bismuth-induced anisotropic structure on indium antimonide [Bi/InSb(001)] are exceptional candidates for truly one-dimensional metals. The microscopic mechanisms behind their exotic spectral properties involve long-range interactions of electrons confined to one-dimensional channels. We extend the universal theory for the finite-energy spectral properties of a wide class of one-dimensional correlated lattice systems whose microscopic mechanisms involve phase shifts imposed by a mobile quantum impurity to electronic lattice systems with long-range interactions. In contrast to theoretical schemes that do not account for the effects of long- quantitatively range interactions, our theoretical predictions agree quantitatively with the observed one-electron spectral properties of one-dimensional metallic states in MoSe(2) line defects and in Bi/InSb(001). |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H19.00009: On the limitations of cRPA downfolding Carsten Honerkamp, Hiroshi Shinaoka, Fakher F. Assaad, Philipp Werner We check the accuracy of the constrained random phase approximation (cRPA) downfolding scheme by considering one-dimensional two- and three-orbital Hubbard models with a target band at the Fermi level and one or two screening bands away from the Fermi level. Using numerically exact quantum Monte Carlo simulations of the full and downfolded model we demonstrate that depending on filling the effective interaction in the low-energy theory is either barely screened, or antiscreened, in contrast to the cRPA prediction. This observation is explained by a functional renormalization group analysis which shows that the cRPA contribution to the screening is to a large extent cancelled by other diagrams in the direct particle-hole channel. We comment on the implications of this finding for the ab-initio estimation of interaction parameters in low-energy descriptions of solids. |
Tuesday, March 5, 2019 5:06PM - 5:18PM |
H19.00010: Maximum quantum entropy method: the analytic continuation of matrix-valued Green's functions Jae-Hoon Sim, Myung Joon Han Analytic continuation of the quantum Monte Carlo data written in the imaginary-frequency to the real-frequency axis is one of the difficult numeric problems, due to the ill-conditioned nature of the kernel matrix. While the maximum entropy method (MEM) is one of the most suitable choices to gain information from the noisy input data, its applications are limited by the non-negative condition of the output spectral function. Here we have extended the MEM to the matrix-valued function, introducing quantum relative entropy as a regularization function [1]. As a true matrix-valued method, our maximum quantum entropy method (MQEM) is invariant under the arbitrary unitary transformation of the input matrix. Without introducing further ambiguity, Bayesian probabilistic interpretation can be applied to the MQEM. Using our DFT+DMFT package, DMFTpack, the MQEM is applied for real materials, namely Sr2IrO4. The application shows that the generalized method provides a reasonable band structure without introducing a material specific base set. |
Tuesday, March 5, 2019 5:18PM - 5:30PM |
H19.00011: Diagrammatic Quantum Monte Carlo for Molecular Systems Jia Li, Markus Wallerberger, Emanuel C Gull Electron correlations in chemical systems give rise to a wide range of interesting physical properties. Although traditional mean-field quantum chemical algorithms can reliably calculate ground state observables, finite temperature and spectral properties are only accessible with explicit inclusion of electron correlations. Diagrammatic Monte Carlo (DiagMC), which expands the physical observable in terms of connected Feynman diagrams and samples the resulting series stochastically, is a powerful technique for studying electron correlations and does not suffer from numerical sign problem which worsens with increasing system size. Recent developments in DiagMC algorithms have greatly improved their numerical efficiency. In this talk, we aim to introduce our DiagMC implementation for multi-orbital systems, and discuss the difficulties and potentials when it is applied to realistic molecular systems. |
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