Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P26: Quantum Gases in Optical Lattices |
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Sponsoring Units: DAMOP Chair: Haiping Hu, University of Texas at Dallas Room: LACC 404A |
Wednesday, March 7, 2018 2:30PM - 2:42PM |
P26.00001: Meson gas theory of the t-J model Fabian Grusdt, Marton Kanasz-Nagy, Annabelle Bohrdt, Christie Chiu, Geoffrey Ji, Markus Greiner, Daniel Greif, Eugene Demler We introduce a semi-variational theory for calculating the properties of holes in moderately doped t-J and related models. Our ansatz provides a microscopic description how holes form a confined pair of a spinon and a holon, connected by a geometric string of displaced spins. This resembles quarks constituting mesons in high-energy physics. We use our method to calculate how spin correlations change as a function of hole doping. Our results are benchmarked by comparison to experimental results from ultracold fermions in a quantum gas microscopes. In particular we show that a weakly correlated meson gas ansatz is sufficient for describing a broad range of dopings even at temperatures below J. Implications for the phase diagram of high-temperature superconductors, and the possibility of spin-charge separation, are also discussed. |
Wednesday, March 7, 2018 2:42PM - 2:54PM |
P26.00002: Quantum information measures for lattice fermions in a harmonic trap Yicheng Zhang, Lev Vidmar, Marcos Rigol We study spinless fermions in a one-dimensional lattice in the presence of a harmonic trap from the perspective of quantum information measures. In the ground state, the system undergoes a local quantum phase transition upon increasing the particle number, which is the result of the formation of a local band insulator in the center of the trap. We show that the bipartite entanglement entropy can be used as an order parameter to characterize the transition. We also study excited eigenstates by calculating the average von Neumann and second Renyi entropies, and compare results to the thermodynamic entropy and the mutual information of mixed states at the same energy density. While the thermodynamic entropy exhibits a peak at the critical point, other quantities decrease with increasing the particle number and, at low temperatures, vanish close to the critical point. Experiments with ultracold atoms on optical lattices may therefore detect the local quantum phase transition by measuring the second Renyi entropy. |
Wednesday, March 7, 2018 2:54PM - 3:06PM |
P26.00003: Quantifying Complexity in Quantum Phase Transitions via Mutual Information Complex Networks Lincoln Carr, Marc Andrew Valdez, Daniel Jaschke, David Vargas, Bhuvanesh Sundar, Kaden Hazzard We quantify the emergent complexity of quantum states near quantum critical points on regular 1D lattices, via complex network measures based on quantum mutual information as the adjacency matrix, in direct analogy to quantifying the complexity of EEG/fMRI measurements of the brain. Using matrix product state methods, we show that network density, clustering, disparity, and Pearson's correlation obtain the critical point for both quantum Ising and Bose-Hubbard models to a high degree of accuracy in finite-size scaling for three classes of quantum phase transitions, $Z_2$, mean field superfluid/Mott insulator, and a BKT crossover. Moreover, they uncover new kinds of structure in the quantum critical region not visible in correlation length and other established measures. For the Ising model we then analytically explore the effect of temperature on the complex network structure, covering mutual information, two-point correlations, and Renyi entropies, and find re-entrant behavior in the quantum critical fan. |
Wednesday, March 7, 2018 3:06PM - 3:18PM |
P26.00004: Particle-hole character of the Higgs and Goldstone modes in strongly-interacting lattice bosons Marco Di Liberto, Alessio Recati, Nandini Trivedi, Iacopo Carusotto, Chiara Menotti We study the low-energy excitations of the Bose-Hubbard model in the strongly-interacting superfluid phase using a Gutzwiller approach and extract the single-particle and single-hole excitation amplitudes for each mode. We report emergent mode-dependent particle-hole symmetry on specific arc-shaped lines in the phase diagram connecting the well-known Lorentz-invariant limits of the Bose-Hubbard model. By tracking the in-phase particle-hole symmetric oscillations of the order parameter, we provide an answer to the long-standing question about the fate of the pure amplitude Higgs mode away from the integer-density critical point. Furthermore, we point out that out-ofphase oscillations are responsible for a full suppression of the condensate density oscillations of the gapless Goldstone mode. Possible detection protocols are also discussed. |
Wednesday, March 7, 2018 3:18PM - 3:30PM |
P26.00005: Periodically modulated interaction of two species Bosons on the optical lattice Shijie Hu, Tao Wang, Axel Pelster, Sebastian Eggert, Xuefeng Zhang Systems far away from non-equilibrium show interesting new phenomena. In this work, we propose to generate a periodically driven system of two species of bosons on a one-dimensional optical lattice by modulating the magnetic field nearby Feshbach resonance. We further investigate properties of this system by various numerical methods. Surprisingly, at zero-temperature we found the driving is not always suppressing the superfluid order; instead it can abnormally enhance the order in a specific parameter region. In other regions, the driving can induce a new kind of superfluid order because of the cooperation of particles and gauge phases. The results are consistent with a rigorous solution at an integrable point. We also discuss the behavior at finite temperatures. |
Wednesday, March 7, 2018 3:30PM - 3:42PM |
P26.00006: Spatial Charge and Spin Correlations in the 2D Fermi-Hubbard Model including a Zeeman field Thereza Paiva, Nandini Trivedi Motivated by the ability to cool and bosonic and fermionic atoms down to ultra cold temperatures in optical lattices and experimental advances in site-resolved imaging, we study spatially resolved charge and spin correlations in the two-dimensional Fermi-Hubbard model using quantum Monte Carlo simulations. We investigate the behavior with and without a Zeeman field at a wide range of temperatures, including temperatures below currently reachable in experiments. Antiferromagnetic spin correlations are maximal at half-filling and weaken monotonically upon reducing the filling or equivalently upon adding holes. At small filling, nearest-neighbor correlations between singly charged sites are negative, revealing the formation of a correlation hole, the suppressed probability of finding two fermions near each other. In the presence of a Zeeman field or equivalently in a spin-imbalanced atomic gas, we observe short-range canted antiferromagnetism at half-filling with stronger spin correlations in the direction orthogonal to the magnetization. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P26.00007: Contour-time approach to the Bose-Hubbard model in the strong coupling regime Matthew Fitzpatrick, Malcolm Kennett We study correlations in space and time after a quantum quench in the Bose Hubbard model. We obtain a two particle irreducible effective action within the contour-time formalism that allows us to study both equilibrium and out of equilibrium phenomena in the superfluid and Mott insulating phase regimes of parameter space. We derive equations of motion for both the superfluid order parameter and two-point correlation functions. To assess the accuracy of this formalism, we study the equilibrium solution of the equations of motion and compare our results to existing strong coupling methods as well as exact methods where possible. We discuss possible applications of this formalism to out of equilibrium scenarios. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P26.00008: Exitonic Excitation of a Superfluid Fermi Gas in a Double-layer Optical Lattice Johnson Chan, Shizhong Zhang We study the phase diagram and the collective excitations of an attractive two-component Fermi gas on a doubl-layer optical lattice at zero temperature. The introduction of one-particle hopping term between the two layers generates many more interesting phenomena, including the superfluid-insulator transition and a new excitonic-like excitations. We characterize the phase transitions and investigate in detail the properties of excitonic excitations, including its gap and effective mass. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P26.00009: Ground State Properties of SU(4) Attractive Hubbard Model on a Honeycomb Lattice Zhichao Zhou, Congjun Wu, Yu Wang We study the competing orders in the SU(4) attractive Hubbard model on a honeycomb lattice, which can be realized in optical lattices with ultracold large-spin alkaline-earth fermions. Employing determinant quantum Monte Carlo (DQMC) simulations, we have explored quantum phase transitions from the gapless Dirac semimetals to the charge density wave order at half filling. Doping away from half filling, which is still sign-problem-free in DQMC, we investigate the competitions between pairing orders and quartetting orders, where the quartetting order is a four-fermion counterpart of the Cooper pairing. |
Wednesday, March 7, 2018 4:18PM - 4:30PM |
P26.00010: Ground State of Hard-Core Lattice Bosons from Algebraic Graph Theory David Feder, Sebastian Garcia The hard-core boson problem consists of finding the ground state (and preferably the excitations) of bosons hopping on a lattice, subject to the constraint that no two particles occupy the same site. The Tonks-Girardeau mapping to non-interacting fermions provides an exact solution in one dimensional lattices. For higher dimensions, the fermionic mapping breaks down and no exact solutions are currently known. In algebraic graph theory, the hard-core boson problem is equivalent to finding the maximal eigenvector of the adjacency matrix associated to the symmetric power of a graph. We analyze the Tonks-Girardeau solution in this context without invoking fermions, and suggest a promising route toward generalizing the results to higher-dimensional lattices. |
Wednesday, March 7, 2018 4:30PM - 4:42PM |
P26.00011: Response of the Higgs amplitude mode of superfluid Bose gases in a three-dimensional optical lattice: effects of fluctuations and a trapping potential Kazuma Nagao, Yoshiro Takahashi, Ippei Danshita The Higgs amplitude mode is a universal quasiparticle excitation on thermodynamic phases with spontaneous breaking of a continuous symmetry and with a particle-hole symmetry. In this work, we address a question of whether the Higgs mode can exist as a resonance peak or not, in particular, in three-dimensional (3D) optical-lattice systems. Specifically, we calculate the response functions of the 3D Bose-Hubbard model to the hopping strength and onsite-interaction strength modulations by means of the mapping to the effective pseudospin-one model near the insulator transition, finite-temperature Green’s function theory, and local density approximation. We show that a sharp resonance peak at the gap in the response functions survives in the presence of quantum and thermal fluctuations, and a parabolic potential [1]. |
Wednesday, March 7, 2018 4:42PM - 4:54PM |
P26.00012: Imbalanced Superfluid Fermi Gas on a Lieb Lattice Marek Tylutki, Päivi Törmä We study the Hubbard model with spin imbalance in multiband systems, particularly the Lieb lattice, which features a flat band. The flat band is known to enhance superfluidity due to a diverging density of states. We obtain a phase diagrams of this system using a BCS (mean field) theory, and we identify a variety of phases. Apart from the standard BCS and normal states, we observe the Fulde-Ferrell-Larkin-Ovchinnikov type phases with space-dependent pairing function. We also find a thermodynamically stable Sarma phase, where the imbalance coexists with a spatially uniform order parameter. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P26.00013: Cooling schemes for ultracold gases in bilayer optical lattices Ippei Danshita, Shimpei Goto Recent observation of long-range magnetic ordering in an optical lattice system loaded with two-component Fermi gases [1] has highly motivated further technological development of the analog quantum simulator of the Hubbard model. Currently, the most severe bottleneck for the Hubbard quantum simulation is to reduce the temperature of the systems to be much lower than the spin-exchange interaction J. For instance, the critical temperature of the d-wave superconducting phases is estimated to be on the order of 0.1J/k_{B} with some approximate theory. Since the lowest temperature achieved in ultracold-atom quantum simulators is 0.45J/k_{B }[1], one needs to develop techniques for further cooling. In this talk, I will discuss a cooling scheme using layered optical lattices [2,3] and show that our proposed scheme can cool the system down to roughly half of the initial temperature. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P26.00014: Cross-Dimensionality and Emergent Superconductivity with Repulsive P-orbital Fermions Xiaopeng Li I study cross dimensionality of p-orbital atomic fermions loaded in an optical square lattice with repulsive interactions. The cross-dimensionality emerges when the transverse tunneling of p-orbital fermions is negligible. With renormalization group analysis, the system is found to support two dimensional charge, orbital, and spin density wave states with incommensurate wavevectors. The transition temperatures of these states are controlled by perturbations near a one dimensional Luttinger liquid fixed point. Considering transverse tunneling, the cross-dimensionality breaks down and the density wave (DW) orders become unstable, and I find an unconventional superconducting state mediated by fluctuation effects. The superconducting gap has an emergent nodal structure determined by the Fermi momentum, which is tunable by controlling atomic density. Taking an effective description of the superconducting state, it is shown that the nodal structure of Cooper pairing can be extracted from momentum-space radio-frequency spectroscopy in atomic experiments. These results imply that p-orbital fermions could enrich the possibilities of studying correlated physics in optical lattice quantum emulators beyond the single-band Fermi Hubbard model. |
Wednesday, March 7, 2018 5:18PM - 5:30PM |
P26.00015: Statistically induced phase transition in the extended Anyon-Hubbard model Shijie Hu, Kevin Jägering, Martin Bonkhoff, Axel Pelster, Sebastian Eggert We investigate the impact of non-trivial exchange statistics on bosonic SPT (Symmetry protected Topological) phases in one-dimensional optical lattices. As the underlying model we study the anyonic version of the extended Bose-Hubbard Hamiltonian with a generalized Pauli principle, restricting the maximal occupation up to two particles per site. Combining numerical DMRG techniques and a Gutzwiller Mean-Field treatment, we present the phase diagram of the system for different values of the statistical parameter. Additionally we employed bosonization to derive a low-energy field theory in order to describe the universal behavior of the system near the critical points, and analyze the mechanism behind statistically induced phase transitions present in the system. |
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