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 M21: Precision Many-Body Physics II: Model Systems and HamiltoniansFocus Live
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Sponsoring Units: DCOMP DCMP DAMOP Chair: James LeBlanc, Memo Univ of Newfoundland |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M21.00001: The variational and diagrammatic quantum Monte Carlo approach to the many-electron problem Invited Speaker: Kun Chen Two of the most influential ideas developed by Richard Feynman are the Feynman diagram technique and his variational approach. Here we show that combining both and introducing a 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 it to the quintessential problem of solid-state, the uniform electron gas, which is at the heart of the density functional theory success in describing real materials, yet it has not been adequately solved for over 90 years. We precisely calculate the one- and two-electron spectrums. Our results address the long-standing electron mass puzzle. We also establish the full spin-dependent exchange-correlation potential for the first time. Our method can be applied to a number of moderately interacting electron systems, including models of realistic metallic and semiconducting solids. |
Wednesday, March 17, 2021 12:06PM - 12:42PM Live |
M21.00002: Stripes, Antiferromagnetism, and the Pseudogap in the Doped Hubbard Model at Finite Temperature Invited Speaker: Alexander Wietek The phase diagram of the two-dimensional Hubbard model at finite temperature poses one of the most interesting conundrums in contemporary condensed matter physics. Tensor network techniques, such as matrix-product based approaches as well as 2D tensor networks, yield state-of-the-art unbiased simulations of the 2D Hubbard model at zero temperature and are capable of giving unbiased results at finite temperature as well. A promising approach for applying tensor networks to study finite-temperature quantum systems is the minimally entangled typical thermal state (METTS) algorithm, which is a Monte Carlo technique that samples from a family of entangled wavefunctions, and which offers favorable scaling and parallelism. In this talk I will present some of our recent results applying this technique in the strong coupling, low-temperature and finite hole-doping regime. We discover that a novel phase characterized by commensurate short-range antiferromagnetic correlations and no charge ordering occurs at temperatures above the half-filled stripe phase extending to zero temperature. We find the single-particle gap to be smallest close to the nodal point and detect a maximum in the magnetic susceptibility. These features bear a strong resemblance to the pseudogap phase of high-temperature cuprate superconductors. The simulations are verified using a variety of different unbiased numerical methods in the three limiting cases of zero temperature, small lattice sizes, and half-filling. |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M21.00003: Quasiparticle Interaction in the Three-dimensional Uniform Electron Gas Bao-Zong Wang, Pengcheng Hou, Youjin Deng, Kristjan Haule, Kun Chen We establish the quasiparticle interaction in the three-dimensional uniform electron gas using a controlled effective field theory approach---the recently developed variational diagrammatic Monte Carlo method. We accurately determine the angle-resolved Landau interaction function as well as the Landau parameters. Several emergent features of the forward-scattering electron interaction on the Fermi surface are identified. In particular, we find that the different scattering channels of two electrons with parallel momenta and opposite spins almost perfectly cancel out, indicating the electrons are nearly asymptotically free in this limit. |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M21.00004: Towards Strongly Correlated 2D Systems of Ultracold Dipolar Sodium-Cesium Molecules Ian Stevenson, Niccolo' Bigagli, Aden Z Lam, Claire Warner, Sebastian Will Dipolar many-body quantum systems offer an exciting pathway to realizing novel quantum phases. Specifically, in two dimensions, predictions include the formation of supersolid and hexatic phases, as well as dipolar quantum crystals. Our goal is to pursue this physics with quantum gases of ultracold dipolar sodium-cesium molecules, created via atom-by-atom assembly from ultracold mixtures of sodium and cesium atoms. In their absolute internal ground state, sodium-cesium molecules feature a large electric dipole moment of 4.6 Debye, which leads to highly controllable long-range interactions. In this talk, we report on the production of overlapping Bose-Einstein condensates of 2 x 105 sodium atoms and 2 x 104 cesium atoms, the exploration of Feshbach resonances in this novel quantum gas mixture, and our pathway to prepare NaCs molecules in their absolute ground state for the study of many-body physics in two dimensions. |
Wednesday, March 17, 2021 1:06PM - 1:18PM Live |
M21.00005: A cavity-QED simulator of dynamical phases of a BCS superconductor Diego Barberena, Robert J Lewis-Swan, Julia R. K. Cline, Dylan Young, James K Thompson, Ana Maria Rey In this work we describe a way to simulate dynamical phases of a BCS superconductor using an ensemble of cold atoms confined in an optical cavity. The absence or presence of Cooper pairs is encoded using the internal electronic states of the atoms, while cavity mediated atom-atom interactions provide the analogue of the electron-electron attraction. By carefully controlling the relative strength of the interactions with respect to the tunable dispersion relation of the effective Cooper pairs, this cavity-QED simulator provides a realistic way to probe, in real time and without the need of ultrafast probes, the dynamical phase diagram of the BCS model as a function of the system parameters and/or the initial non-equilibrium state. Given the level of control attained in state-of-the-art cavities, our work paves the way for the study of non-equilibrium features of quantum magnetism and superconductivity by harnessing atom-light interactions in cold atomic gases. |
Wednesday, March 17, 2021 1:18PM - 1:30PM Live |
M21.00006: Fourth- and Fifth-Order Virial Coefficients from Weak Coupling to Unitarity Yaqi Hou, Joaquin Drut Due to its simplicity, experimental accessibility, and universality across various fields from condensed matter to nuclear physics, the unitary Fermi gas has been one of the most intensively investigated systems of the last two decades. At finite temperature, one widely used tool to study the thermodynamics of such a system is the virial expansion, whose spirit is to encode the many-body physics into a series of n-body contributions, captured by the virial coefficients bn. Implementing a new nonperturbative analytical method, featuring only systematic uncertainties, we have calculated the bn of a Fermi gas from weak coupling to the unitary point. Our method reproduces the exact b3 and supports a previous conjecture for b4, resolving the long-standing disagreement between theory and experiment. Pushing on to b5 for the first time, we use the Pade resummation and find agreement with experimental measurements of various thermodynamic properties. Applied to polarized matter, we find excellent agreement with Monte Carlo calculations. Preliminary results up to b9 are also presented. Connections to low-energy nuclear physics and generalizations to other systems and observables are discussed. |
Wednesday, March 17, 2021 1:30PM - 1:42PM Live |
M21.00007: Quantum Monte Carlo insights into the properties of the polarized Fermi gas around unitarity Adam Richie-Halford, Joaquin Drut, Aurel Bulgac Strongly correlated Fermi systems with pairing interactions become superfluid below a critical temperature Tc. The extent to which such pairing correlations alter the behavior of the liquid at temperatures T > Tc is a subtle issue that remains an area of debate, in particular regarding the appearance of the so-called pseudogap in the BCS-BEC crossover of unpolarized spin-1/2 nonrelativistic matter. To shed light on this issue, we extracted several quantities of crucial importance at and around the unitary limit, namely the odd-even staggering of the total energy, the spin susceptibility, the pairing correlation function, the condensate fraction, and the critical temperature Tc, using a nonperturbative, constrained-ensemble quantum Monte Carlo algorithm. We looked for signatures of the pseudogap at couplings between 0.0 ≤ 1/(kF a) ≤ 0.3. In particular, at 1/(kF a) = 0.3, we see strong pseudogap signatures, which diminish (but not necessarily vanish) as the coupling is varied towards unitarity. |
Wednesday, March 17, 2021 1:42PM - 1:54PM Live |
M21.00008: Two-particle properties within finite-temperature self-consistent one-particle Green’s function methods: theory and application Pavel Pokhilko, Dominika Zgid Finite-temperature Green’s function methods, such as GF2 and GW, provide a route to model many-body electronic structure of materials at finite temperature. Although these methods give useful thermodynamic quantities, properties are usually limited to the information contained in the one-particle Green’s function, such as one-particle density matrix. We show that two-particle density matrices and two-particle properties can be accessed through application of Hellmann-Feynman theorem, bypassing Bethe-Salpeter equation. The obtained two-particle density matrices provide a useful set of diagnostics for GF2 and GW, closely connecting them with wave-function approaches. |
Wednesday, March 17, 2021 1:54PM - 2:06PM Live |
M21.00009: Coupled one-dimensional chains in two-dimensional dipolar bosons Fabio Cinti, Massimo Boninsegni We present the results of computer simulations at low temperature of a two- dimensional system of dipolar bosons, with dipole moments aligned at an arbitrary angle with respect to the direction perpendicular to the plane. The phase diagram includes a homogeneous superfluid phase, as well as triangular and striped crystalline phases, as the particle density and the tilt angle are varied. In the striped solid, no phase coherence among stripes and consequently no “supersolid” phase are found, in disagreement with recent theoretical predictions. We show how meaningful finite-size scaling is necessary, in order to arrive at reliable predictions concerning superfluid behavior. |
Wednesday, March 17, 2021 2:06PM - 2:18PM Live |
M21.00010: The crossover from BEC to BCS in the interacting 2D Fermi gas Shasta Ramachandran, Scott Jensen, Yoram Alhassid The physics of the two-species Fermi gas with an attractive contact interaction in two spatial dimensions (2D) is different from the 3D system in that there is a two-particle bound state for any strength of the interaction. We investigate the thermodynamics of the strongly interacting 2D Fermi system in the crossover between the Bose-Einstein condensate (BEC) and the Bardeen-Cooper-Schrieffer (BCS) limits using finite-temperature auxiliary-field quantum Monte Carlo (AFMC) methods in the canonical ensemble. In particular, we study the superfluid phase transition and the possible existence and extent of a pseudogap regime, in which signatures of pairing correlations survive above the critical temperature. To this end, we calculate the temperature dependence of the condensate fraction and of a model-independent energy staggering pairing gap. We also calculate the temperature dependence of the contact, a fundamental property of quantum many-body systems with short-range correlations which measures the pair correlation at short distances. Our AFMC simulations are carried out on a discrete lattice and extrapolated to the continuum limit. |
Wednesday, March 17, 2021 2:18PM - 2:30PM Live |
M21.00011: Continuum limit results for the unitary Fermi gas and its pseudogap regime Scott Jensen, Christopher N Gilbreth, Yoram Alhassid The unitary Fermi gas (UFG) is a paradigm for strongly correlated Fermi superfluids with applications to dilute neutron matter and can be realized in the lab with ultracold atomic Fermi gases. The existence and extent of a pseudogap regime in the UFG above the critical temperature for superfluidity has been extensively debated in the literature and remains an open question. We present large-scale simulation results in the continuum limit for the condensate fraction and model-independent pairing gaps of the UFG across the superfluid phase transition. The simulations are performed on a discrete lattice using a highly optimized finite-temperature auxiliary-field quantum Monte Carlo (AFMC) algorithm in the canonical ensemble [1]. This extends our previous results for finite filling factor and effective range parameters [2] to the continuum limit. We discuss possible pseudogap signatures in this limit. |
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