Aug 2016

Aug 1-Aug 5 Bo Liu, Aug 8-Aug 12 Biao Huang, Aug 15-Aug 19 Haiyuan Zou, Aug 22-Aug 26 Ahmet Kel

Aug 19

arXiv:1608.05096 [pdf, other]
Helical Floquet Channels in 1D Lattices
Jan Carl Budich, Ying Hu, Peter Zoller
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We show how dispersionless channels exhibiting perfect spin-momentum locking can arise in a 1D lattice model. While such spectra are forbidden by fermion doubling in static 1D systems, here we demonstrate their appearance in the stroboscopic dynamics of a periodically driven system. Remarkably, this phenomenon does not rely on any adiabatic assumptions, in contrast to the well known Thouless pump and related models of adiabatic spin pumps. The proposed setup is shown to be experimentally feasible with state of the art techniques used to control ultracold alkaline earth atoms in optical lattices.


Aug 18

arXiv:1608.04831 [pdf, ps, other]
Long-range transverse Ising model built with dipolar condensates in two-well arrays
Yongyao Li, Wei Pang, Jun Xu, Chaohong Lee, Boris A. Malomed, Luis Santos
Comments: 6 pages, 5 figures, and 37 references
Subjects: Quantum Gases (cond-mat.quant-gas); Disordered Systems and Neural Networks (cond-mat.dis-nn); Pattern Formation and Solitons (nlin.PS)
Dipolar Bose-Einstein condensates in an array of double-well potentials realize an effective transverse Ising model with peculiar inter-layer interactions, that may result under proper conditions in an anomalous first-order ferromagnetic-antiferromagnetic phase transition, and nontrivial phases due to frustration. The considered setup as well allows the study of Kibble-Zurek defect formation, whose kink statistics follows that expected from the universality class of the mean-field transverse Ising model in 1D. Furthermore, random occupation of each layer of the stack leads to random effective Ising interactions and generation of local transverse fields, thus allowing the study of Anderson-like localization of imbalance perturbations in the two-well stack under controllable conditions.


Aug 17

arXiv:1608.04564 [pdf, ps, other]
Vortex solitons in two-dimensional spin-orbit coupled Bose-Einstein condensates: effects of the Rashba-Dresselhaus coupling and the Zeeman splitting
Hidetsugu Sakaguchi, E. Ya. Sherman, Boris A. Malomed
Comments: Physical Review E, in press
Subjects: Quantum Gases (cond-mat.quant-gas); Pattern Formation and Solitons (nlin.PS); Optics (physics.optics)
We present an analysis of two-dimensional (2D) matter-wave solitons, governed by the pseudo-spinor system of Gross-Pitaevskii equations with self- and cross-attraction, which includes the spin-orbit coupling (SOC) in the general Rashba-Dresselhaus form, and, separately, the Rashba coupling and the Zeeman splitting. Families of semi-vortex (SV) and mixed-mode (MM) solitons are constructed, which exist and are stable in free space, as the SOC terms prevent the onset of the critical collapse and create the otherwise missing ground states in the form of the solitons. The Dresselhaus SOC produces a destructive effect on the vortex solitons, while the Zeeman term tends to convert the MM states into the SV ones, which eventually suffer delocalization. Existence domains and stability boundaries are identified for the soliton families. For physically relevant parameters of the SOC system, the number of atoms in the 2D solitons is limited by ∼1.5×104. The results are obtained by means of combined analytical and numerical methods.


Aug 16

arXiv:1608.03916 [pdf, other]
Impurity driven Brownian motion of solitons in elongated Bose-Einstein Condensates
L. M. Aycock, H. M. Hurst, D. Genkina, H.-I Lu, V. Galitski, I. B. Spielman
Comments: 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
Solitons, spatially-localized, mobile excitations resulting from an interplay between nonlinearity and dispersion, are ubiquitous in physical systems from water channels and oceans to optical fibers and Bose-Einstein condensates (BECs). For the first time, we observed and controlled the Brownian motion of solitons. We launched long-lived dark solitons in highly elongated 87Rb BECs and showed that a dilute background of impurity atoms in a different internal state dramatically affects the soliton. With no impurities and in one-dimension (1-D), these solitons would have an infinite lifetime, a consequence of integrability. In our experiment, the added impurities scatter off the much larger soliton, contributing to its Brownian motion and decreasing its lifetime. We describe the soliton's diffusive behavior using a quasi-1-D scattering theory of impurity atoms interacting with a soliton, giving diffusion coefficients consistent with experiment.


Aug 12


arXiv:1608.03480
Implementing quantum electrodynamics with ultracold atomic systems
V. Kasper, F. Hebenstreit, F. Jendrzejewski, M. K. Oberthaler, J. Berges
(Submitted on 11 Aug 2016)

We discuss the experimental engineering of model systems for the description of QED in one spatial dimension via a mixture of bosonic 23Na and fermionic 6Li atoms. The local gauge symmetry is realized in an optical superlattice, using heteronuclear boson-fermion spin-changing interactions which preserve the total spin in every local collision. We consider a large number of bosons residing in the coherent state of a Bose-Einstein condensate on each link between the fermion lattice sites, such that the behavior of lattice QED in the continuum limit can be recovered. The discussion about the range of possible experimental parameters builds, in particular, upon experiences with related setups of fermions interacting with coherent samples of bosonic atoms. We determine the atomic system's parameters required for the description of fundamental QED processes, such as Schwinger pair production and string breaking. This is achieved by benchmark calculations of the atomic system and of QED itself using functional integral techniques. Our results demonstrate that the dynamics of one-dimensional QED may be realized with ultracold atoms using state-of-the-art experimental resources. The experimental setup proposed may provide a unique access to longstanding open questions for which classical computational methods are no longer applicable.


Aug 10


arXiv:1608.02868

Proximity effects in cold atom artificial graphene
Tobias Grass, Ravindra W. Chhajlany, Leticia Tarruell, Vittorio Pellegrini, Maciej Lewenstein
(Submitted on 9 Aug 2016)

Cold atoms in an optical lattice with brick-wall geometry have been used to mimic graphene, a two-dimensional material with characteristic Dirac excitations. Here we propose to bring such artificial graphene into the proximity of a second atomic layer with a square lattice geometry. For non-interacting fermions, we find that such bilayer system undergoes a phase transition from a graphene-like semi-metal phase, characterized by a band structure with Dirac points, to a gapped band insulator phase. In the presence of attractive interactions between fermions with pseudospin-1/2 degree of freedom, a competition between semi-metal and superfluid behavior is found at the mean-field level. Upon tuning the coupling between the layers, the system exhibits re-entrant superfluid phases. Using the quantum Monte Carlo method, we also investigate the case of strong repulsive interactions. In the Mott phase, each layer exhibits a different amount of long-range magnetic order. Upon coupling both layers, a valence-bond crystal is formed at a critical coupling strength. Finally, we discuss how these bilayer systems could be realized in existing cold atom experiments.

arXiv:1608.02765
Quantum Logarithmic Butterfly in Many Body Localization
Yu Chen
(Submitted on 9 Aug 2016)

Out of time ordered correlator (OTOC) is recently introduced as a powerful diagnose for quantum chaos. But its definition is much general, not restricted only in chaotic systems. In this Letter we present an analytical calculation of OTOC for a non-chaotic system -- a many body localized (MBL) system. A logarithmic light-cone (LLC) boundary is found as ξlnt=x, where x is the minimal distance of between two OTOC operators and ξ is dimensionless localization length, interpreted as "butterfly" velocity. OTOC will not fall outside the LLC and shows an universal power law decay behavior as 2−λLLlogt inside the LLC. The exponent λLL=ξ is independent of disorder distribution, and could be interpreted as an information scrambling rate, that is, the second R\'{e}nyi entropy growth rate against logt. We also explaine why the information propagation velocity and information scrambling rate shares the same value.





Aug 9


arXiv:1608.02438
Out-of-Time-Order Correlation at a Quantum Phase Transition
Huitao Shen, Pengfei Zhang, Ruihua Fan, Hui Zhai
(Submitted on 8 Aug 2016 (v1), last revised 13 Aug 2016 (this version, v2))

In this Letter we numerically calculate the out-of-time-order correlation functions and extract the Lyapunov exponents in the Bose-Hubbard model. Our study is motivated by a recent conjecture that a system with the Lyapunov exponent saturating the upper bound 2\pi/\beta will have a holographic dual to a black hole at finite temperature. We further conjecture that for a many-body quantum system with a quantum phase transition, the Lyapunov exponent will have a peak in the quantum critical region with the emergent conformal symmetry. Our numerical results on the Bose-Hubbard model support the conjecture. We also compute the butterfly velocity and discuss the measurement of this correlator in the cold atom realizations of the Bose-Hubbard model.


arXiv:1608.02512
Lattice bosons with infinite range checkerboard interactions
Bhuvanesh Sundar, Erich J. Mueller
(Submitted on 8 Aug 2016)

Motivated by experiments performed by Landig et al. [Nature 532, 476-479], we consider a two dimensional Bose gas in an optical lattice, trapped inside a single mode superradiant Fabry Perot cavity. The cavity mediates infinite range checkerboard interactions between the atoms, which produces competition between Mott insulator, charge density wave, superfluid and supersolid phases. We calculate the phase diagram of this Bose gas in a homogeneous system and in the presence of a harmonic trap.

arXiv:1608.02589
Discrete time crystals: rigidity, criticality, and realizations
Norman Y. Yao, Andrew C. Potter, Ionut-Dragos Potirniche, Ashvin Vishwanath
(Submitted on 8 Aug 2016)

Despite being forbidden in equilibrium, spontaneous breaking of time translation symmetry can occur in periodically driven, Floquet systems with discrete time-translation symmetry. The period of the resulting discrete time crystal is quantized to an integer multiple of the drive period, arising from a combination of collective synchronization and many body localization. Here, we consider a simple model for a one dimensional discrete time crystal which explicitly reveals the rigidity of the emergent oscillations as the drive is varied. We numerically map out its phase diagram and compute the properties of the dynamical phase transition where the time crystal melts into a trivial Floquet insulator. Moreover, we demonstrate that the model can be realized with current experimental technologies and propose a blueprint based upon a one dimensional chain of trapped ions. Using experimental parameters (featuring long-range interactions), we identify the phase boundaries of the ion-time-crystal and propose a measurable signature of the symmetry breaking phase transition.


Aug 8
arXiv:1608.01914 [pdf, other]
Out-of-Time-Order Correlation for Many-Body Localization
Ruihua Fan, Pengfei Zhang, Huitao Shen, Hui Zhai
Comments: 5 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Disordered Systems and Neural Networks (cond-mat.dis-nn); Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el)

In this Letter we first compute the out-of-time-order correlators (OTOC) for both a phenomenological model and a random-field XXZ model in the many-body localized phase. We show that, in contrast to the exponential decay in a chaotic system, the OTOC decays in power law in a many-body localized system. We show that the OTOC can also be used to distinguish a many-body localized phase from an Anderson localized phase, while a normal correlator cannot. Furthermore, we prove an exact theorem that relates the growth of the second Renyi entanglement entropy in the quench dynamics to the decay of the OTOC in equilibrium. This theorem works for a generic quantum system. We discuss various implications of this theorem.

arXiv:1608.01829 [pdf, ps, other]
Giant superconducting fluctuations in the compensated semimetal FeSe at the BCS-BEC crossover
S. Kasahara, T. Yamashita, A. Shi, R. Kobayashi, Y. Shimoyama, T. Watashige, K. Ishida, T. Terashima, T. Wolf, F. Hardy, C. Meingast, H. v. Löhneysen, A. Levchenko, T. Shibauchi, Y. Matsuda
Comments: An updated version will appearin Nature Communications
Subjects: Superconductivity (cond-mat.supr-con); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)

The physics of the crossover between weak-coupling Bardeen-Cooper-Schrieffer (BCS) and strong-coupling Bose-Einstein-condensate (BEC) limits gives a unified framework of quantum bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems, but is extremely difficult to be realized for electrons in solids. Recently, the superconducting semimetal FeSe with a transition temperature Tc=8.5 K has been found to be deep inside the BCS-BEC crossover regime. Here we report experimental signatures of preformed Cooper pairing in FeSe below T∗∼20 K, whose energy scale is comparable to the Fermi energies. In stark contrast to usual superconductors, large nonlinear diamagnetism by far exceeding the standard Gaussian superconducting fluctuations is observed below T∗∼20 K, providing thermodynamic evidence for prevailing phase fluctuations of superconductivity. Nuclear magnetic resonance (NMR) and transport data give evidence of pseudogap formation at ∼T∗. The multiband superconductivity along with electron-hole compensation in FeSe may highlight a novel aspect of the BCS-BEC crossover physics.


Aug 5

1. arXiv:1608.01326 [pdf, other]
Floquet engineering from long-range to short-range interactions
Tony E. Lee
Quantum simulators based on atoms or molecules often have long-range interactions due to dipolar or Coulomb interactions. We present a method based on Floquet engineering to turn a long-range interaction into a short-range one. By modulating a magnetic-field gradient with one or a few frequencies, one renormalizes the interaction profile, such that the system behaves as if it only had nearest-neighbor interactions. Our approach works in both one and two dimensions and for both spin-1/2 and spin-1 systems. Our approach does not require individual addressing, and is applicable to all experimental systems with long-range interactions: trapped ions, polar molecules, Rydberg atoms, NV centers, and cavity QED.


Aug 4
1. arXiv:1608.01070 [pdf, other]
Anomalous Transport in a Superfluid Fluctuation Regime
Shun Uchino, Masahito Ueda
Motivated by a recent experiment in ultracold atoms [ S. Krinner et al., PNAS 201601812 (2016)], we analyze mass and spin transport of attractively interacting fermions through a one-dimensional wire near the superfluid transition. We show that in a ballistic regime where the quantization of conductance emerges in the absence of an interaction, the contact resistance is strongly renormalized by superfluid fluctuations; the conductance on the mass current is enhanced by pair fluctuations, whereas the spin conductance is suppressed. We point out that the spin current is also affected by a spin gap in a one-dimensional wire that leads to a further suppression of the measured spin conductance. Our results are qualitatively consistent with the experimental observations.

Aug 2

1. arXiv:1608.00457 [pdf, other]
Strongly Interacting Fermi Gases
W. Zwerger
The experimental realization of stable, ultracold Fermi gases near a Feshbach resonance allows to study gases with attractive interactions of essentially arbitrary strength. They extend the classic paradigm of BCS into a regime which has never been accessible before. We review the theoretical concepts which have been developed in this context, including the Tan relations and the notion of fixed points at zero density, which are at the origin of universality. We discuss in detail the universal thermodynamics of the unitary Fermi gas which allows a fit free comparison between theory and experiment for this strongly interacting system. In addition, we adress the consequences of scale invariance at infinite scattering length and the subtle violation of scale invariance in two dimensions. Finally we discuss the Fermionic excitation spectrum accessible in momentum resolved RF-spectroscopy and the origin of universal lower bounds for the shear viscosity and the spin diffusion constant.

2. arXiv:1608.00074 [pdf, ps, other]
Fusing Quantum Hall States in Cold Atoms
Tin-Lun Ho
Realizing quantum Hall states in a fast rotating Bose gas is a long sought goal in cold atom research. The effort is very challenging because Bose statistics fights against quantum Hall correlations. In contrast, Fermi statistics does not cause such conflict. Here, we show that by sweeping the integer quantum Hall states of a spin-1/2 Fermi gas across the Feshbach resonance from the BCS side to the BEC side at a "projection" rate similar to that in the "projection" experiment of fermion superfluid, these states can be "fused" into a bosonic quantum Hall states. A projection sweep means the pair association is sufficiently fast so that the center of mass of the pair remains unchanged in the process. We show that the fusion of integer fermion states with filling factor ν↑=ν↓=n will result in a bosonic Laughlin state and Pfaffian state for n=1 and 2. The is due to a hidden property of the fermionic integer quantum Hall states -- for any grouping of opposite spin into pairs, their centers of mass automatically assume a bosonic quantum Hall structure.


Aug 1
[24] arXiv:1607.08846 [pdf, other]
Supersolid phases of dipolar fermions in a 2D lattices bilayer array
A. Camacho-Guardian, R. Paredes
Supersolid phases as a result of a novel coexistence of superfluid and density ordered checkerboard phases are predicted to appear in ultracold Fermi molecules confined in a bilayer array of 2D square optical lattices. We demonstrate the existence of these phases within the inhomogeneous mean field approach. In particular, we show that tuning the interlayer separation distance at a fixed value of the chemical potential produces different fractions of superfluid, density ordered and supersolid phases.