Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session L34: Precision Many Body Physics IIIFocus
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Sponsoring Units: DCOMP DAMOP DCMP Chair: Vito Scarola, Virginia Tech Room: LACC 409A |
Wednesday, March 7, 2018 11:15AM - 11:51AM |
L34.00001: Site-resolved microscopy of ultracold Fermi-Hubbard systems in new regimes Invited Speaker: Waseem Bakr The ability to probe and manipulate ultracold fermions in optical lattices at the atomic level using quantum gas microscopes has enabled quantitative studies of Fermi-Hubbard models in a temperature regime that is challenging for state-of-the-art numerical simulations. Experiments have focused on spin-balanced gases of repulsively interacting atoms with the hope of elucidating phenomena in high-temperature superconductors. In this talk, I will present experiments that explore the Hubbard model in two new regimes: repulsive gases with spin-imbalance and attractive spin-balanced gases. In the first regime, we observe canted antiferromagnetism at half-filling, with stronger correlations in the direction orthogonal to the magnetization. Away from half-filling, the polarization of the gas exhibits non-monotonic behavior with doping, resembling the behavior of the magnetic susceptibility of the cuprates. The attractive Hubbard model studied in the second set of experiments is the simplest theoretical model for studying pairing and superconductivity of fermions in a lattice. Our measurements on the normal state reveal checkerboard charge-density wave correlations close to half-filling. The charge-density-wave correlations are a sensitive thermometer in the low temperature regime relevant for future studies of inhomogeneous superfluid phases in spin-imbalanced attractive gases. |
Wednesday, March 7, 2018 11:51AM - 12:03PM |
L34.00002: Towards quantum simulation with circular Rydberg atoms Guillaume Roux, Thanh Long Nguyen, Jean-Michel Raimond, Clément Sayrin, Rodrigo Cortiñas, Tigrane Cantat-Moltrecht, Frédéric Assemat, Igor Dotsenko, Sébastien Gleizes, Serge Haroche, Michel Brune We propose to realize a quantum simulator of spin arrays, based on laser-trapped circular Rydberg atoms. The atoms are protected from spontaneous emission decay, reaching lifetimes in the minute range. A defect-free chain of 40 atoms can be prepared thanks to an innovative technique, that bears resemblance with evaporative cooling, based on van der Waals interaction between the atoms. |
Wednesday, March 7, 2018 12:03PM - 12:15PM |
L34.00003: Using Single-Particle Content to Distinguish Single-Particle, Collective and Strongly Correlated Atomic-Scale Quantum Systems Emily Townsend, Tomas Neuman, Javier Aizpurua, Garnett Bryant Linear atomic chains, such as atom chains on surfaces, linear arrays of dopant atoms in semiconductors, or linear molecules, provide ideal testbeds for studying single-particle, collective (plasmonic) and strongly correlated excitations in the quantum limit for interacting matter systems. We use exact diagonalization to find the many-body excitations of finite (4-26) atom chains, described by hopping plus long-range electron-electron repulsion and the corresponding electron-core attraction. A combination of criteria involving the many-body state transition dipole moment, balance, dynamical response, induced transition charge density and single-particle content can be used to characterize the excitations of atomic-scale systems as a function of the electron-electron interaction strength. The single-particle content clearly displays distinct transitions from a regime for single-particle excitations to collective then to strongly correlated behavior as the electron-electron interaction increases and shows how this transition takes place. This allows us to define and investigate regimes that support quantum plasmon excitations. The onset of quantum plasmons in small atomic-scale systems and the relation to Luttinger liquid theory is discussed. |
Wednesday, March 7, 2018 12:15PM - 12:27PM |
L34.00004: Precision auxiliary-field quantum Monte Carlo computations of Rashba spin-orbit coupling in interacting many-body systems Peter Rosenberg, Hao Shi, Shiwei Zhang We describe the treatment of Rashba spin-orbit coupling (SOC) in interacting many-fermion systems within the auxiliary-field quantum Monte Carlo (AFQMC) framework, and present a set of illustrative results. We show that this technique can be applied to a wide range of systems, including the Fermi gas in the continuum and the lattice, with attractive or repulsive interactions. In the unpolarized, attractive case our results provide a numerically exact description of the ground-state of the Fermi gas in the continuum [1], and the lattice [2]. For the repulsive case a constraint is applied and we perform a set of benchmark calculations that achieve similar accuracy to calculations without SOC [3]. These developments enable high-precision AFQMC simulations of many of the novel Hamiltonians currently being engineered in ultra-cold atoms, and provide a general approach for predictive computations in models and materials to study the interplay of SOC and strong correlation. In addition to establishing a new set of benchmarks, this technique offers quantitative numerical insight to guide the search for topological phases. |
Wednesday, March 7, 2018 12:27PM - 12:39PM |
L34.00005: New Probes of the t-J Model in Quantum Gas Microscopes Annabelle Bohrdt, Daniel Greif, Eugene Demler, Michael Knap, Fabian Grusdt In this talk I’m going to present a measurement scheme for the single-particle spectral function that allows quantum gas microscopes to perform experiments similar to angle-resolved photoemission spectroscopy in conventional condensed matter systems. As an example for possible applications, we numerically calculate the spectrum of a single hole excitation in 1D t-J models with isotropic and anisotropic antiferromagnetic couplings. A sharp asymmetry in the distribution of spectral weight appears when a hole is created in an isotropic Heisenberg spin chain. This effect slowly vanishes for anisotropic spin interactions and disappears completely in the case of pure Ising interactions. I will introduce a slave-fermion mean field theory, which provides an intuitive physical picture for this behavior. As an outlook, I will discuss possible measurements in two dimensions. |
Wednesday, March 7, 2018 12:39PM - 12:51PM |
L34.00006: The Halon: A Quasiparticle Featuring Critical Charge Fractionalization Kun Chen, Yuan Huang, Youjin Deng, Boris Svistunov The halon is a special critical state of an impurity in a quantum-critical environment. The hallmark of the halon physics is that a well-defined integer charge gets fractionalized into two parts: a microscopic core with half-integer charge and a critically large halo carrying a complementary charge of ±1/2. The halon phenomenon emerges when the impurity–environment interaction is fine-tuned to the vicinity of a boundary quantum critical point (BQCP), at which the energies of two quasiparticle states with adjacent integer charges approach each other. The universality class of such BQCP is captured by a model of pseudo-spin-1/2 impurity coupled to the quantum-critical environment, in such a way that the rotational symmetry in the pseudo-spin XY-plane is respected, with a small local “magnetic” field along the pseudo-spin z-axis playing the role of control parameter driving the system away from the BQCP. On the approach to BQCP, the half-integer projection of the pseudo-spin on its z-axis gets delocalized into a halo of critically divergent radius, capturing the essence of the phenomenon of charge fractionalization. With large-scale Monte Carlo simulations, we confirm the existence of halons—and quantify their universal features—in O(2) and O(3) quantum critical systems. |
Wednesday, March 7, 2018 12:51PM - 1:03PM |
L34.00007: Dipolar extended Fermi-Hubbard Model in two-dimensions Raimundo Rocha Dos Santos, Tiago Mendes-Santos, Rubem Mondaini, Thereza Paiva The ability to cool bosonic and fermionic atoms down to ultra cold temperatures in optical lattices has enabled the experimental emulation of model Hamiltonians for strongly correlated systems. Unlike in Condensed Matter systems, one has control over the model parameters such as interaction strength, hopping amplitude, and population imbalance. A recent experimental development in cold gases is the ability to create quantum degenerate bosonic and fermionic gases of magnetic atoms, leading to the study of magnetic dipolar interactions. The extended Bose-Hubbard model was recently emulated with 168Er atoms in an optical lattice. The study of fermionic systems with anisotropic interactions beyond on-site is clearly in order. Here we use the Lanczos method to explore the ground state phase diagram of the dipolar extended Fermi-Hubbard Model at half-filling and two-dimensions. The anisotropic character of the dipole-dipole interaction, nearest-neighbour as well as next-nearest-neighbour interactions are taken into account. We observe quantum phase transitions between Antiferromagnetic and different Charge-Density Waves phases. |
Wednesday, March 7, 2018 1:03PM - 1:15PM |
L34.00008: An Auxiliary Field Quantum Monte Carlo study of the Hubbard Kanamori model Hongxia Hao, Brenda Rubenstein, Hao Shi In the physics of strongly-correlated many-electron systems, the Hubbard Kanamori model has been extensively studied as a prototype for transition-metal oxides. The model is multiorbital in nature and contains Hund’s coupling terms. As a result, it may manifest metal-insulator transitions and high-temperature superconductivity. Due to the sign problem, it is mainly studied through the framework of Dynamical Mean-Field Theory (DMFT). We study the model using the ground state Auxiliary Field Quantum Monte Carlo (AFQMC) method. The Hubbard-Stratonovich transformation is applied to the Hund’s coupling and pair-exchange terms. The Constrained Path Approximation is used to control the sign problem. A systematic test is carried out on the accuracy of the approach. The ground state properties of the model will be discussed. |
Wednesday, March 7, 2018 1:15PM - 1:27PM |
L34.00009: Dynamical Mean-Field Theory of Superconductivity in the Hubbard-Holstein Model Tae-Ho Park, Han-Yong Choi We present a study of the half-filled Hubbard-Holstein model with superconducting order parameter employing the dynamical mean-field theory in combination with the numerical renormalization group. The Hubbard-Holstein model is a prototype model for understanding the interplay between the local Coulomb repulsion U and the electron-phonon coupling g. The ground state is metallic when both U and g are small, but is a Mott-Hubbard insulator (MHI) when U is larger than the critical value of the Coulomb repulsion Uc and a bipolaron insulator (BPI) when g is larger than the critical value of the electron-phonon coupling gc. Here, we investigate the interplay between the electron-electron and electron-phonon interactions in superconducting state emerging around the phase boundary between metallic and insulating states. In particular, the effect of phonon softening on supercondcutivity is investigated in the strong correlation regime. The variation of the superconductivity is also probed by changing the phonon frequency from adiabatic to nonadiabatic regimes. |
Wednesday, March 7, 2018 1:27PM - 1:39PM |
L34.00010: Abstract Withdrawn
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Wednesday, March 7, 2018 1:39PM - 1:51PM |
L34.00011: An exactly solvable BCS-Hubbard Model in arbitrary dimensions Zewei Chen, Xiaohui Li, Tai Kai Ng In this talk, we present an exact solvable BCS-Hubbard model in arbitrary dimensions. The model describes a p-wave BCS superconductor with equal spin pairing moving on a bipartite (cubic, square etc.) lattice with onsite Hubbard interaction U. We show that the model becomes exactly solvable for arbitrary U when the BCS pairing amplitude Delta equals the hopping amplitude t. In this limit, the model is non-magnetized without interaction while it becomes an anti-ferromagnet for arbitrary small interaction. The solitonic excitation of this model shows a transition from spin 1/2 fermionic excitation to spin 1 bosonic excitation. The construction of the exact solution is parallel to the exactly solvable Kitaev honeycomb model for S=1/2 quantum spins and can be viewed as a generalization of Kitaev's construction to S=1/2 interacting lattice fermions [1-2]. The BCS-Hubbard model discussed is just an example of a large class of exactly solvable lattice fermion models that can be constructed similarly. Our arxiv preprint is available at Ref.[3]. |
Wednesday, March 7, 2018 1:51PM - 2:03PM |
L34.00012: Random Matrix Theory for Non-Hermitian Hamiltonians S. Vijay, Liang Fu We study the random matrix theory for non-Hermitian Hamiltonians for dissipative systems. We derive the universal behavior of the level repulsion of the complex eigenvalues of such a non-Hermitian Hamiltonian -- which deviates significantly from the level statistics of a random, Hermitian Hamiltonian -- in the absence of symmetries, or in the presence of time-reversal symmetry with T 2 = ±1. We compute the density of states, which develops tails ρ(ε) ~ 1/ε2 and a loss of spectral weight at low energies, proportional to the strength of non-Hermiticity. We conclude by discussing scattering experiments in which these effects can be measured. |
Wednesday, March 7, 2018 2:03PM - 2:15PM |
L34.00013: Quantum Phase Diagram of the Hamiltonian Mean Field Model Ryan Plestid, James Lambert, Duncan O'Dell, Erik Sorensen Quantum many body systems with long range interactions (LRIs) are increasingly both being realized in laboratory experiments, and recieving theoretical interest. The Hamiltonian Mean Field model (HMFm) is a 1-D model that has been extremely successful at helping to shed light on classical features of LRI systems such as self-gravitating fluids, and neutral plasmas. In this talk we investigate the bosonic HMFm's quantum phase diagram. Ignoring density and phase fluctuations, Chavanis (EPJ 2011) argued that a quantum phase transition, driven by a competition between LRIs and quantum pressure, is present. The ordered phase breaks translational symmetry, and is naively forbidden by the Mermin-Wagner theorem, however this allowed for LRI systems (e.g. Maghrebi PRL 119). In this talk we will discuss how to go beyond the mean-field approximation used by Chavanis, both numerically and analytically, and investigate the role of fluctuations in modifying the model's phase diagram, and the ultimate fate of the ordered phase. |
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