Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session R21: Precision ManyBody Physics IV: Model Systems and HamiltoniansFocus Live

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Sponsoring Units: DCOMP DCMP DAMOP Chair: Félix Werner, Ecole Normale Superieure 
Thursday, March 18, 2021 8:00AM  8:36AM Live 
R21.00001: Diagrammatic Monte Carlo for the Hubbard Model Invited Speaker: Michel Ferrero In this talk, I will present recent developments of the diagrammatic Monte Carlo method applied to the Hubbard model. Diagrammatic Monte Carlo methods are based on the construction of a perturbation series for physical observables. They face two main challenges: The first challenge is the accurate stochastic computation of the series coefficients that suffers from the fermionic sign problem. The second difficulty is the resummation of the series whose success depends on the structure of the physical observable expressed in the complex plane of the expansion parameter, e.g. the onsite repulsion U in the Hubbard model. I will discuss how these two issues can be addressed and, in particular, how the freedom to choose the noninteracting starting point of the theory can be used to construct more efficient perturbation series and directly investigate brokensymmetry phases. I will illustrate these developments with calculations performed in different regimes of the Hubbard model. 
Thursday, March 18, 2021 8:36AM  8:48AM Live 
R21.00002: Microscopic evolution of doped Mott insulators from polaronic to Fermi liquid regime Dominik Bourgund, Joannis Koepsell, Pimonpan Sompet, Sarah Hirthe, Thomas Chalopin, Petar Bojovic, Annabelle Bohrdt, Yao Wang, Fabian Grusdt, Eugene Demler, Guillaume Salomon, Christian Gross, Timon Alexander Hilker, Immanuel Felix Bloch The competition between antiferromagnetism and hole motion in twodimensional Mott insulators lies at the heart of a dopingdependent transition from an anomalous metal to a conventional Fermi liquid. Condensed matter experiments suggest that charge carriers change their nature within this crossover, but a full understanding remains elusive. We study this regime by preparing a cold fermionic gas in an optical lattice at a temperature around the superexchange energy. It is imaged using a quantum gas microscope with full spin and density resolution allowing the extraction of a wide range of correlators. Crucial to deeper understanding is the capability to calculate higher order correlators as well as common observables from solid states systems such as the spin susceptibility, all of which are studied as a function of doping level. While at low doping the system exhibits magnetic polarons, i.e. holes with a dressed cloud of spins, higher doping leads to the metallic Fermi liquid regime characterised by incommensurate magnetic fluctuations and altered correlations. The crossover is completed for hole dopings around 30%. Several theoretical models are benchmarked and their agreement with the experiment in different doping regimes discussed. 
Thursday, March 18, 2021 8:48AM  9:00AM Live 
R21.00003: Tracking the Footprints of Spin Fluctuations: A MultiMethod, MultiMessenger Approach to the Weakcoupling Regime of the TwoDimensional Hubbard Model Thomas Schaefer, Nils Wentzell, Fedor Simkovic, YuanYao He, Cornelia Hille, Marcel Klett, Christian Eckhardt, Behnam Arzhang, Viktor Harkov, FrançoisMarie Le Régent, Yan Wang, Aaram J. Kim, Evgeny Kozik, Evgeny A. Stepanov, Anna Kauch, Sabine Andergassen, Philipp Hansmann, Daniel Rohe, Yuri Vilk, James P. F. LeBlanc, Shiwei Zhang, AndreMarie Tremblay, Michel Ferrero, Olivier Parcollet, Antoine Georges The Hubbard model is the paradigmatic model of this field, similar to what the drosophila is to genetics. Despite its simplicity, the Hubbard model presents a formidable challenge to computational and theoretical methods alike. 
Thursday, March 18, 2021 9:00AM  9:12AM Live 
R21.00004: Pressure, Negative Thermal Expansion and dwave Pair Fluctuations in the 2D tJ Model William Putikka The high temperature series for the entropy of the 2D tJ model has been calculated to 12th order in β. Extrapolating this series below T≤ J has proven difficult in the past due to the presence of two temperature scales. Using a calculation of the Heisenberg AF entropy^{1} a modified series for the entropy per site S of the tJ model can be produced ΔS(n,T) = S_{tJ}(n,T)  nS_{AF}(J^{*}). By adjusting J^{*} the series for ΔS can be extrapolated to lower temperatures using Pade approximants. This can be done for any density and allows the calculation of ∂S/∂n│_{T}. Using standard thermodynamics the temperature derivative of the pressure ∂P/∂T│_{n} = n∂S/∂n│_{T }+ S can be found, along with the full pressure P by integrating ∂P/∂T│_{n} and using the known high temperaure values. There is a range of densities and temperatures where ∂P/∂T│_{n }< 0. This region has a strong overlap with the model parameters where the strongest dwave pair fluctuations^{2} are found. Since ∂P/∂T│_{n} = α/κ, where α is the thermal expansion coefficient and κ ≥ 0 is the isothermal compressibility, the strongest dwave pair fluctuations are found where the tJ model has a negative electronic thermal expansion. 
Thursday, March 18, 2021 9:12AM  9:24AM Live 
R21.00005: Exploration of coupled cluster Green's function in selfenergy embedding theory Avijit Shee, Dominika Zgid We have explored previously developed Coupled Cluster Green’s Function (CCGF) (Shee, Zgid JCTC2019) in two different contexts : 
Thursday, March 18, 2021 9:24AM  9:36AM Live 
R21.00006: The Tensor Network Python (TeNPy) Library Johannes Hauschild, Frank Pollmann, Michael Zaletel We present TeNPy [1], a Python library for the simulation of strongly correlated quantum many body systems with the ansatz of tensor networks, and in particular matrix product states (MPS). The library aims to provide a good balance between code readability, easy implementation of custom models and algorithms, and numerical efficiency for largescale simulations. After a short overview of the features (and limitations) of the library, we demonstrate how to setup the density matrix renormalization group (DMRG) algorithm for a custom model on a long cylinder geometry as a concrete example. Further, we showcase some applications of TeNPy, present benchmarks, and discuss the roadmap for future developments. 
Thursday, March 18, 2021 9:36AM  9:48AM Live 
R21.00007: Impurity in a quantum gas: exact diagonalization meets Bethe Ansatz Evgeni Burovski, Oleksandr Gamayun, Oleg Lychkovskiy We examine stationary state properties of an impurity particle injected into a onedimensional quantum gas. For equal masses of the impurity and host particles the problem reduces to a variant of the GaudinYoung model, which is integrable and admits a formal solution based on the Bethe Ansatz. Obtaining physical observables from the formal solution is still nontrivial: the formfactor summation can be done via a stochastic enumeration based on the Metropolis algorithm. 
Thursday, March 18, 2021 9:48AM  10:24AM Live 
R21.00008: Recent progress in computational studies of the twodimensional Hubbard model Invited Speaker: Shiwei Zhang I will discuss some of the recent developments in manybody computational studies to understand properties of the twodimensional Hubbard model. As a fundamental model in quantum manybody physics, the Hubbard model has presented a tremendous challenge, with multiple competing tendencies and its properties often the outcome of a delicate balance of their competition and coexistence. This places extermely demanding requirements on numerical approaches, including high precision, excellent predictive power and accuracy, and reliable access to the thermynamic limit. Progress in method development and the combined use of complementary methods have led to significant recent progress. I will introduce some of the methological advances in both groundstate and finitetemperature auxiliaryfield quantum Monte Carlo (AFQMC), and discuss joint efforts using AFQMC and other stateoftheart methods to determine the nature of magnetic and pairing orders in the twodimensional Hubbard model. 
Thursday, March 18, 2021 10:24AM  10:36AM Live 
R21.00009: Spin, charge and pairing correlations in a bilayer Hubbard model with an incipient band Seher Karakuzu, Thomas Maier, Steven S. Johnston Understanding the pairing mechanism responsible for hightemperature superconductivity is one of the primary goals of the condensed matter physics community. The bilayer Hubbard model provides a simple testbed, in which one can study unconventional pairing in a multiband system. This model can be tuned through a Lifshitz transition, where one of the bands is pushed below the Fermi energy, similar to the situation in monolayer ironselenide. Here we discuss dynamic cluster approximation (DCA) Quantum Monte Carlo (QMC) calculations of this model for different parameter regimes. In particular, we describe how its pairing correlations evolve as one of the bands becomes incipient, and how this behavior is linked to changes in the dynamical spin and charge fluctuations. 
Thursday, March 18, 2021 10:36AM  10:48AM Live 
R21.00010: Current anomalies, reservoir discretizations, and extendedreservoir quantum transport simulations Justin Elenewski, Gabriela Wojtowicz, Marek Rams, Michael Zwolak Quantum transport simulations are rapidly evolving, including the development of tensor network techniques for manybody calculations. One powerful approach combines matrix product states with extended reservoirs  an open system methodology where relaxation maintains a chemical potential or temperature drop. Due to the presence of external relaxation, this is a simulation analog of Kramers' turnover, with relaxationcontrolled currents at both small and large relaxation strength. Only between these regimes does the natural (Landauer or MeirWingreen) conductance define the simulation. Here, we demonstrate how anomalous transport can appear when employing this methodology, even at moderate relaxation. While certain features are known to arise from band structure and gap states, we demonstrate that small steadystate currents unveil two anomalies, one due to virtual transitions and the other due to unphysical broadening of the occupied density of states. Moreover, we show how to approximate the optimal relaxation strength by exploiting control over the virtual anomalies, and demonstrate how reservoir discretizations impact simulation. Taken together, our results constrain the computational parameters that are needed to properly represent physical behavior in the continuum limit. 
Thursday, March 18, 2021 10:48AM  11:00AM Live 
R21.00011: Theory of a benzene transistor: symmetry, strong correlations and quantum interference Sudeshna Sen, Andrew Mitchell Single molecule transistors offer a fascinatingly diverse range of physics beyond the capabilities of Si transistors. Their ultrasmall size, chemical complexity, and electronic interactions constitute a unique playground for exploring the fundamental physics of correlations on the nanoscale, and their transport signatures. Understanding these systems is an essential prerequisite for possible advanced technological applications utilizing their quantum characteristics. In this talk I examine the interplay of symmetry and Kondo effect in a benzene single electron transistor using a combination of numerical renormalization group and generalised Schrieffer Wolff transformation [1]. Depending on the connectivity of the leads and gate voltages, we uncover spin1/2 and spin1 Kondo effects, a quantum phase transition to a state with robust molecular magnetism, and destructive quantum interference at an emergent SU(4) symmetry point. The interplay between emergent manybody effects and molecular symmetry is discussed in the context of quantumboosted device functionality. 
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