Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session K48: Quantum ManyBody Systems and Methods IIRecordings Available

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Sponsoring Units: DCOMP Chair: Alina Kononov, Sandia National Lab Room: McCormick Place W471A 
Tuesday, March 15, 2022 3:00PM  3:12PM 
K48.00001: Quantification of electron correlation for approximate quantum calculations. Shunyue Yuan, Lucas K Wagner Manybody quantum systems are often divided into “strongly correlated” and “weakly correlated.” In strongly correlated systems, the determinant expansion of eigenstates includes several determinants with large weights, while in weakly correlated systems, the expansion is instead dominated by a single determinant. What sort of correlation is present can strongly affect the efficiency of a given approximate wave function method. For example, according to lore in the field, selected configuration interaction methods are efficient when there are just a few determinants with large weights. On the contrary, SlaterJastrow wave functions are efficient for weakly correlated systems. 
Tuesday, March 15, 2022 3:12PM  3:24PM 
K48.00002: A quantum Monte Carlo study of systems with effective core potentials and node nonlinearities. Haihan Zhou, Anthony Scemama, Guangming Wang, Abdulgani Annaberdiyev, Benjamin E Kincaid, Michel Caffarel, Lubos Mitas We consider the realspace, fixednode diffusion Monte Carlo (DMC) method that involves Hamiltonians with nonlocal operators, which could break the fixednode constraint, and therefore proper algorithmic adjustments are necessary, for example, localization approximation. 
Tuesday, March 15, 2022 3:24PM  3:36PM 
K48.00003: Auxiliary boson diffusion Monte Carlo approach for excitations Fernando A Reboredo, Jaron T Krogel A theoretical method to evaluate the excitation spectra of an interacting system from the ground state of an auxiliary system is presented and tested in a model system. The imaginary time evolution of an arbitrary state can be obtained by sampling the ground state distribution of walkers, and the gradients of the distribution of walkers of an auxiliary bosonic system. The wavefunction of each excitation is expanded into a product of bosonic and fermionic wavefunctions. The bosonic function is treated statistically in DMC while the fermionic part is expanded in a basis and evaluated numerically. A fermionboson coupling term appears that can be sampled on the bosonic ground state. The bosonic factor is expected to reduce the size of the basis required by the fermionic component: we show it incorporates in DMC the correlations missed by standard Jastrow factors. The implementation of this method in current DMC codes will be facilitated by reuse of procedures already developed and tested for the calculation of ground state properties with DMC or excitations with variational Monte Carlo(VMC). The perspectives and potential problems of this method when applied to realistic fermionic systems are discussed. 
Tuesday, March 15, 2022 3:36PM  3:48PM 
K48.00004: Combining branching random walks with Metropolis sampling: constraint release in auxiliaryfield quantum Monte Carlo ZhiYu Xiao We present an approach to combine the branching random walks of auxiliaryfield quantum Monte Carlo (AFQMC) with Markov chain Monte Carlo sampling. The formulation of branching random walks along imaginarytime is required to realize a constraint on the paths to control the sign or phase problem, according to an exact gauge condition which, in practice, is implemented approximately with a trial wave function or trial density matrix. We use the generalized Metropolis algorithm to sample a selected portion of the imaginarytime path after it has been produced by the branching random walk. This allows a constraint release to follow seamlessly from the constrainedpath sampling, which can reduce the systematic error from the latter. It also provides a way to improve the computation of correlation functions and observables which do not commute with the Hamiltonian. We illustrate the method in a number of atoms/molecules, where improvements in accuracy are observed and nearexact results are obtained. 
Tuesday, March 15, 2022 3:48PM  4:00PM 
K48.00005: Electronic temperature effects on the dissociation of diatomic molecules using density matrix quantum Monte Carlo Hayley R Petras, William Z Van Benschoten, Emily J Landgreen, Sai Kumar Ramadugu, James J Shepherd We present data from finite temperature calculations on dissociation curves from density matrix quantum Monte Carlo methods. We describe how strong correlation changes in the diatomics as a function of temperature through an analysis of the population distribution on the density matrix for various bond lengths and the isolated atoms. This is then related to selected applications in surface chemistry. 
Tuesday, March 15, 2022 4:00PM  4:12PM 
K48.00006: Decomposed Functional Renormalization Group Flows for Multiband Hamiltonians Nahom K Yirga, David K Campbell We use a singular value decomposition to decouple the functional Renormalization group (fRG) equations in the 
Tuesday, March 15, 2022 4:12PM  4:24PM 
K48.00007: MultiMethod, MultiMessenger Approaches to Models of Strong Correlations Thomas Schaefer, Nils Wentzell, Fedor Simkovic, YuanYao He, Marcel Klett, Christian J Eckhardt, Behnam Arzhang, Viktor Harkov, Aaram J Kim, Evgeny Kozik, Evgeny A Stepanov, Anna Kauch, Sabine Andergassen, Philipp Hansmann, Daniel Rohe, James LeBlanc, Shiwei Zhang, A.M. S Tremblay, Michel Ferrero, Olivier P Parcollet, Alexander Wietek, Riccardo Rossi, Miles Stoudenmire, Antoine Georges The Hubbard model is the paradigmatic model for electronic correlations. In this talk I present a general framework for the reliable calculation of its properties, which we coined 'multimethod, multimessenger' approach. I will illustrate the power of this approach with two recent studies: (i) an extensive synopsis of arguably all available finitetemperature methods for the halffilled Hubbard model on a simple square lattice in its weakcoupling regime and (ii) a complementary subset of those applied to the Hubbard model on a triangular geometry. While the former example fully clarifies the impact of spin fluctuations and tracks it footprints on the one and twoparticle level, the latter exhibits the intriguing interplay of geometric frustration (magnetism) and strong correlations (Mottness). These examples may work as a blueprint for similar future studies of strongly correlated systems. 
Tuesday, March 15, 2022 4:24PM  4:36PM 
K48.00008: Fluctuation Approach to ManyBody Quantum Dynamics Erik Schroedter, JanPhilip Joost, Michael Bonitz The dynamics of quantum manybody systems following external excitation is of great interest in many areas such as dense plasmas or correlated solids. At present, only the formalism of nonequilibrium Green functions (NEGF) can rigorously describe such processes in more than one dimension. However, NEGF simulations are computationally expensive, among other things, due to their cubic scaling with simulation time T. Only recently, linear scaling with T could be achieved within the G1G2 scheme [1]. Here a new fluctuation based approach to the NEGF formalism is presented. While in theory the resulting equations are fully equivalent to the G1G2 scheme, in practice the new approach has interesting complementary features such as the capability to simulate manybody effects using stochastic methods [2], which further reduce the computational complexity and increase numerical stability for stronger coupling. Additionally, this approach provides direct access to spectral twoparticle quantities such as the density response function or polarizability. 
Tuesday, March 15, 2022 4:36PM  4:48PM 
K48.00009: Electronphonon excitations in the 1D HubbardHolstein model probed by Resonant Inelastic XRay Scattering Jinu Thomas, Alberto Nocera, Steven S Johnston The prospect of accessing electronphonon (ep) coupling strengths using resonant inelastic xray scattering (RIXS) has attracted significant attention in recent years. With current energy resolution, RIXS experiments can now resolve individual phonon modes. Standard analysis for such experiments is done by using a single site Lang Firsov (LF) model that promises to extract quantative information on momentum resolved ep coupling by fitting data. This method, however, computes the phonon RIXS intensities by approximating the electronphonon interaction in the atomic limit, coupling a single electronic excitation to a single phonon mode. It remains unclear if such a drastic approximation remains valid for a manyparticle system with itinerant carriers. We test the validity of this model in the strongly correlated manyparticle limit. By explicitly computing the RIXS intensity using DMRG, we study the phonon excitations in finitesized HubbardHolstein chains and compare our results against the singlesite and the singleband predictions. 
Tuesday, March 15, 2022 4:48PM  5:00PM 
K48.00010: Spin and Charge Orders in the Doped TwoDimensional Hubbard Model at Finite Temperature Bo Xiao, YuanYao He, Shiwei Zhang Competing orders, including inhomogeneous spin and charge orders, are observed in many correlated electron materials, including the hightemperature superconductors. The twodimensional Hubbard model provides a minimal paradigm for studying these orders. Using constrainedpath auxiliaryfield quantum Monte Carlo, We study the interplay between thermal and quantum fluctuations in this model. Reaching large supercell sizes to extract properties in the thermodynamics limit, we obtain an accurate and systematic characterization of the behaviors of the spin and charge orders as a function of temperature. In all three electron densities, we find increasing shortrange antiferromagnetic correlations as temperature is lowered. As the correlation length grows sufficiently large, a modulating wave appears to produce spindensitywave (SDW). In the case of ρ=0.9 and 0.875, this evolves smoothly into the groundstate longrange SDW order. In the case of ρ=0.8, the SDW remains shortranged as temperature is lowered to zero. We study the interplay between spin and charge orders and find that formation of charge order appears to follow that of spin order. This leads to a very low upper bound for the transition temperature for CDW or stripe order. 
Tuesday, March 15, 2022 5:00PM  5:12PM 
K48.00011: A new TimeDomain Approach for Linear Responses and Charge Transport Michel Panhans Linearresponse theory is a powerful theoretical framework to investigate, e.g., electrical and magnetic transport and to compare 
Tuesday, March 15, 2022 5:12PM  5:24PM 
K48.00012: Effects of operator backflow on quantum transport Tibor Rakovszky, Curt von Keyserlingk, Frank Pollmann Tensor product states have proved extremely powerful for simulating the lowtemperature properties of manybody systems. The applicability of such methods to the dynamics of manybody systems is less clear: as entanglement grows under time evolution, memory requirements or truncation errors spiral out of control. In this talk, we present a method that seeks to reduce this memory barrier by selectively discarding highly nonlocal correlations in a controlled manner. We illustrate our method on various model systems and develop a theory to estimate the size of the error from the neglected "backflow" processes from nonlocal to local quantities. Our results suggest that backflow errors are exponentially suppressed in the sizecutoff; based on this result, we conjecture that the numerical resources needed to capture transport coefficients in ergodic diffusive systems scale effectively polynomially with the required precision, significantly better than the exponential scaling of more bruteforce methods. We also discuss how our method performs compared to other approximation schemes. 
Tuesday, March 15, 2022 5:24PM  5:36PM 
K48.00013: Direct solution of multiple excitations in a matrix product state with block Lanczos David Sénéchal, Alexandre Foley, Thomas E Baker Matrix product state methods are known to be efficient for computing ground states of local, gapped Hamiltonians, particularly in one dimension. We introduce the multitargeted method that acts on a bundled matrix product state, holding many excitations. The use of a block or banded Lanczos algorithm allows for the simultaneous, variational optimization of the bundle of excitations. The method is demonstrated on a Heisenberg model and other cases of interest. A large of number of excitations can be obtained at a small bond dimension with highly reliable local observables throughout the chain. 
Tuesday, March 15, 2022 5:36PM  5:48PM 
K48.00014: Relativistic threeboson bound states in the zerorange limit Mohammadreza Hadizadeh, Kamyar Mohseni, Andre J Chaves, Diego Rabelo da Costa, Tobias Frederico The relativistic Faddeev integral equations are solved to calculate threeboson mass and wave function for ground and excited states. The inputs of relativistic Faddeev integral equations are the fullyoffshell boosted tmatrices, calculated from the boosted interactions by solving the relativistic LippmannSchwinger equation. We employ Kamada and Glöcke boosted potentials obtained directly from nonrelativistic shortrange separable potentials by solving a quadratic integral equation using an iterative scheme. By adopting Yamaguchi and Gaussian potentials and driving them towards the zerorange limit, we show that relativistic masses and wave functions are modelindependent, and the Thomas collapse is avoided, while the nonrelativistic limit keeps the Efimov effect. We compare our results for relativistic masses with LightFront and Euclidean calculations. 
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