Dec 2013

Dec 23-Dec 27, Jinlong yu | Dec 25 | Dec 24 | Dec 23 | Dec 16-Dec 20, Lijun Lang | Dec 20 | Dec 19 | Dec 18 | Dec 17 | Dec 16 | Dec 9-Dec 13, Bo Liu | Dec 2-Dec 6, Zhifang Xu

Dec 23-Dec 27, Jinlong yu
Dec 25
1. arXiv:1312.6772 [pdf, ps, other]
The coherence of superradiance

We have measured the 2nd order coherence, or 2-body correlations, of atoms from a Bose--Einstein condensate participating in a superradiance process. We compare the statistics of the superradiant phenomenon with the ordinary spontaneous emission and with a coherent source obtained via a stimulated Raman transition of a Bose--Einstein condensate. Despite strong stimulated emission the correlation properties of the superradiance are close to those of a thermal sample.

2. arXiv:1312.6704 [pdf, other]
Relaxation dynamics of a closed high-spin Fermi system far from equilibrium
Ulrich Ebling, Jasper Simon Krauser, Nick Fläschner, Klaus Sengstock, Christoph Becker, Maciej Lewenstein, André Eckardt
A fundamental question in many-body physics is how closed quantum systems reach equilibrium. We address this question experimentally and theoretically in an ultracold high-spin Fermi gas where we find a complex interplay between internal and motional degrees of freedom. The fermions are initially prepared far from equilibrium with only a few spin states occupied. The subsequent dynamics leading to redistribution among all spin states is observed experimentally and simulated theoretically using a kinetic Boltzmann equation with full spin coherence. The latter is derived microscopically and provides good agreement with experimental data without any free parameters. We identify several collisional processes, which occur on different time scales. By varying density and magnetic field, we control the relaxation dynamics and are able to continuously tune the character of a subset of spin states from an open to a closed system.

3. arXiv:1312.6765 [pdf, ps, other]
Theory of SU(N) Fermi liquid
S.-K. Yip, Bor-Luen Huang, Jung-Shen Kao
We generalized the Fermi liquid theory to N component systems with SU(N) symmetry. We emphasize the important role of fluctuations when N is large. These fluctuations dramatically modifies the properties for repulsive Fermi gases, in particular the spin susceptibility.

Dec 24
1. arXiv:1312.6401 [pdf, other]
Observation of low-field Fano-Feshbach resonances in ultracold gases of dysprosium
Kristian Baumann, Nathaniel Q. Burdick, Mingwu Lu, Benjamin L. Lev
We report the observation of resonance-like loss in the trap population of ultracold dysprosium as a function of magnetic field, which we attribute to anisotropy-induced Fano-Feshbach resonances arising from Dy's large magnetic dipole moment and nonzero electronic orbital angular momentum. We recorded these resonances for four different isotopes, three bosonic and one fermionic, over a field range of 0-6 G and show that the number of resonances changes significantly as a function of temperature, even in the nK regime. Most of the observed resonances are of very narrow width. The fermionic isotope, unlike its bosonic counterparts, possesses nonzero nuclear spin and exhibits a much higher density of resonances.

2. arXiv:1312.6346 [pdf, ps, other]
Rydberg dressing: Understanding of collective many-body effects and implications for experiments
Jonathan B. Balewski, Alexander T. Krupp, Anita Gaj, Sebastian Hofferberth, Robert Löw, Tilman Pfau
The strong interaction between Rydberg atoms can be used to control the strength and character of the interatomic interaction in ultracold gases by weakly dressing the atoms with a Rydberg state. Elaborate theoretical proposals for the realization of various complex phases and applications in quantum simulation exist. Also a simple model has been already developed that describes the basic idea of Rydberg dressing in a two-atom basis. However, an experimental realization has been elusive so far. We present a model describing the ground state of a Bose-Einstein condensate dressed with a Rydberg level based on the Rydberg blockade. This approach provides an intuitive understanding of the transition from pure twobody interaction to a regime of collective interactions. Furthermore it enables us to calculate the deformation of a three-dimensional sample under realistic experimental conditions in mean-field approximation. We compare full three-dimensional numerical calculations of the ground state to an analytic expression obtained within Thomas-Fermi approximation. Finally we discuss limitations and problems arising in an experimental realization of Rydberg dressing based on our experimental results. Our work enables the reader to straight forwardly estimate the experimental feasibility of Rydberg dressing in realistic three-dimensional atomic samples.

3. arXiv:1312.6442 [pdf, other]
Light-induced atomic desorption in a compact system for ultracold atoms
Lara Torralbo-Campo, Graham D. Bruce, Giuseppe Smirne, Donatella Cassettari
In recent years, light-induced atomic desorption (LIAD) of alkali atoms from the inner surface of a vacuum chamber has been employed in cold atom experiments for the purpose of modulating the alkali background vapour. This is beneficial because larger trapped atom samples can be loaded from vapour at higher pressure, after which the pressure is reduced to increase the lifetime of the sample. In this paper we present a comprehensive analysis, based on the case of rubidium atoms adsorbed on pyrex, of various aspects of LIAD that are useful for this application. Firstly, we study the intensity dependence of LIAD by fitting the experimental data with a rate-equation model, from which we extract a correct prediction for the increase in trapped atom number. Following this, we introduce a quantitative figure of merit for the utility of LIAD in cold atom experiments. Secondly, we observe a long-time behaviour of the rubidium pressure and its gradual depletion over several experimental cycles, which we attribute to the movement of rubidium atoms between different parts of the chamber. We show that this can be compensated for by continuously running the rubidium source at low level. Finally, we show that it is possible to use LIAD data to gain information on the binding energy of rubidium atoms on the surface.

4. arXiv:1312.6452 [pdf, ps, other]
Reply to Comment on "Quantum quasicrystals of spin-orbit coupled dipolar bosons''
Sarang Gopalakrishnan, Ivar Martin, Eugene A. Demler
In a recent Letter [Phys. Rev. Lett. 111, 185304 (2013)], we proposed a scheme for realizing quantum quasicrystals using spin-orbit coupled dipolar bosons. We remarked that these quantum quasicrystals have additional phason''-like modes compared with their classical counterparts. A recent comment by Lifshitz [[[http://arxiv.org/abs/1312.1388|arXiv:1312.1388]]] contests this claim. We argue here that our enumeration of gapless modes is indeed the physically relevant one; whether the additional modes are phasons'' is, however, a matter of definition.

Dec 23
1. arXiv:1312.5747 [pdf, other]
Cyclotron dynamics of interacting bosons in artificial magnetic fields
Xiaopeng Li, S. Das Sarma
We study theoretically quantum dynamics of interacting bosons in artificial magnetic fields as engineered in recent ultracold atomic experiments, where quantum cyclotron orbital motion has been observed. With exact numerical simulations and perturbative analyses, we find that interactions induce damping in the cyclotron motion. The damping time is found to be dependent on interaction and tunneling strengths monotonically, while its dependence on magnetic flux is non-monotonic. Sufficiently strong interactions would render bosons dynamically localized inhibiting the cyclotron motion. The damping predicted by us can be construed as an interaction-induced quantum decoherence of the cyclotron motion.

2. arXiv:1312.5772 [pdf, other]
Quantum gases in optical lattices
Peter Barmettler, Corinna Kollath
The experimental realization of correlated quantum phases with ultracold gases in optical lattices and their theoretical understanding has witnessed remarkable progress during the last decade. In this review we introduce basic concepts and tools to describe the many-body physics of quantum gases in optical lattices. This includes the derivation of effective lattice Hamiltonians from first principles and an overview of the emerging quantum phases. Additionally, state-of-the-art numerical tools to quantitatively treat bosons or fermions on different lattices are introduced.

3. arXiv:1312.5784 [pdf, other]
Steerable optical tweezers for ultracold atom studies
Kris O. Roberts, Thomas McKellar, Julia Fekete, Ana Rakonjac, Amita B. Deb, Niels Kjærgaard
We report on the implementation of an optical tweezer system for controlled transport of ultracold atoms along a narrow, static confinement channel. The tweezer system is based on high-efficiency acousto-optical deflectors and offers two-dimensional control over beam position. This opens up the possibility for tracking the transport channel when shuttling atomic clouds along the guide, forestalling atom spilling. Multiple clouds can be tracked independently by time-shared tweezer beams addressing individual sites in the channel. The deflectors are controlled using a multichannel direct digital synthesizer, which receives instructions on a sub-microsecond time scale from a field-programmable gate array. Using the tweezer system, we demonstrate sequential binary splitting of an ultracold 87Rb cloud into $2^5$ clouds.

4. arXiv:1312.5948 [pdf, other]
Scaling of phonons and shortcuts to adiabaticity in a one-dimensional quantum system
Wolfgang Rohringer, Dominik Fischer, Florian Steiner, Igor E Mazets, Jörg Schmiedmayer, Michael Trupke
The observation of characteristic scaling laws is an important step towards a systematic understanding and the efficient control of non-equilibrium dynamics in many-body quantum systems. Recent theoretical work indicates that for a general class of quantum many-body systems, initial equilibrium and nonequilibrium states subsequent to a quench are connected by a scale transformation. Here, we present first experimental evidence for such a universal scaling relation for quasi-1d Bose gases in a tunable external trapping potential. We observe that the measured temperature of phononic excitations after a quench exhibits a power law dependence on the spatial extension of the cloud, as predicted by the theory. Using this scaling law we demonstrate that shortcuts to adiabaticity do not induce additional heating.


Dec 16-Dec 20, Lijun Lang
Dec 20
1. arXiv:1312.5723 [pdf, other]
Unconventional topological phase transitions in helical Shiba chains
Falko Pientka, Leonid I. Glazman, Felix von Oppen
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con)
Chains of magnetic impurities placed on a superconducting substrate and forming helical spin order provide a promising venue for realizing a topological superconducting phase. An effective tight-binding description of such helical Shiba chains involves long-range (power-law) hopping and pairing amplitudes which place the physical system near an unconventional topological critical point. At the critical point, we find exponentially localized Majorana bound states with a short localization length unrelated to a topological gap. Away from the critical point, the Majorana states develop a power-law tail. Our analytical results have encouraging implications for experiment.


Dec 19
1. arXiv:1312.5282 [pdf, ps, other]
Comparing the Dynamics of Skyrmions and Superconducting Vortices
C.J. Olson Reichhardt, S.Z. Lin, D. Ray, C. Reichhardt
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
Vortices in type-II superconductors have attracted enormous attention as ideal systems in which to study nonequilibrium collective phenomena, since the self-ordering of the vortices competes with quenched disorder and thermal effects. Dynamic effects found in vortex systems include depinning, nonequilibrium phase transitions, creep, structural order-disorder transitions, and melting. Understanding vortex dynamics is also important for applications of superconductors which require the vortices either to remain pinned or to move in a controlled fashion. Recently, topological defects called skyrmions have been realized experimentally in chiral magnets. Here we highlight similarities and differences between skyrmion dynamics and vortex dynamics. Many of the previous ideas and experimental setups that have been applied to superconducting vortices can also be used to study skyrmions. We also discuss some of the differences between the two systems, such as the potentially large contribution of the Magnus force in the skyrmion system that can dramatically alter the dynamics and transport properties.

2. arXiv:1312.5289 [pdf, ps, other]
Vortex dynamics in spin-orbit coupled Bose-Einstein condensates
Alexander L. Fetter
Subjects: Quantum Gases (cond-mat.quant-gas)
I use a time-dependent Lagrangian formalism and a variational trial function to study the dynamics of a two-component vortex in a spin-orbit coupled Bose-Einstein condensate (BEC). For a single-component BEC, various experiments have validated this theoretical approach, for example a thermal quench that yields a quantized vortex in roughly 25% of trials. To be definite, I assume the specific spin-orbit form used in recent NIST experiments, which introduces a spatial asymmetry because of the external Raman laser beams. I here generalize this formalism to include a two-component order parameter that has quantized circulation in each component but not necessarily with the same circulation. For example a singly quantized vortex in just one component yields a BEC analog of the half-quantum vortex familiar in [Math Processing Error]He-A and in [Math Processing Error]-wave chiral superconductors. This and other unusual two-component vortices have both periodic trajectories and unbounded trajectories that leave the condensate, depending on the initial conditions. The optimized phase of the order parameter induces a term in the particle current that cancels the contribution from the vector potential, leaving pure circulating current around the vortex.

Dec 18
1. arXiv:1312.4604 [pdf, ps, other]
Spin turbulence in spinor Bose-Einstein condensates
Makoto Tsubota, Kazuya Fujimoto
Subjects: Quantum Gases (cond-mat.quant-gas)
We summarize the recent theoretical and numerical works on spin turbulence (ST) in spin-1 spinor Bose-Einstein condensates. When the system is excited from the ground state, it goes through hy- drodynamic instability to ST in which the spin density vector has various disordered direction. The properties of ST depend on whether the spin-dependent interaction is ferromagnetic or antiferro- magnetic. ST has some characteristics different from other kinds of turbulence in quantum fluids. Firstly, the spectrum of the spin-dependent interaction energy exhibits the characteristic power law different from the usual Kolmogorov -5/3 law. Secondly, ST can show the spin-glass-like behavior; the spin density vectors are spatially random but temporally frozen.

2. arXiv:1312.4657 [pdf, other]
Nonequilibrium dynamics of one-dimensional hard-core anyons following a quench: Complete relaxation of one-body observables
Tod M. Wright, Marcos Rigol, Matthew J. Davis, Karen V. Kheruntsyan
Subjects: Quantum Gases (cond-mat.quant-gas)
We address the role of interactions in the relaxation dynamics of isolated integrable quantum systems following a sudden quench. Our study focuses on the dynamics of a family of integrable hard-core lattice anyons that interpolates continuously between hard-core bosons and noninteracting spinless fermions. By employing a geometric measure of distance in the space of single-particle density matrices, we demonstrate that for hard-core anyons all one-body observables relax to the predictions of the generalized Gibbs ensemble (GGE). This complete one-body relaxation occurs for all models in the hard-core anyon family except for the singular limit of noninteracting fermions, establishing the fundamental role of interactions in driving relaxation to the GGE.

3. arXiv:1312.4662 [pdf, ps, other]
Virial expansion of a harmonically trapped Fermi gas across a narrow Feshbach resonance
Shi-Guo Peng, Shuo-Han Zhao, Kaijun Jiang
Comments: 6 pages, 7 figures, accepted by Physical Review A (in press)Subjects: Quantum Gases (cond-mat.quant-gas)
We theoretically investigate the high-temperature thermodynamics of a harmonically trapped Fermi gas across a narrow Feshbach resonance, by using the second-order quantum virial expansion, and point out some new features compared to the broad resonance. The interatomic interaction is modeled by the pseudopotential with an additional parameter, i.e., the effective range, to characterize the narrow resonance width. Deeply inside the width of a narrow Feshbach resonance, we find the second virial coefficient evolves with the effective range from the well-known universal value 1/4 in the broad-resonance limit to one another value 1/2 in the narrow-resonance limit. This means the Fermi gas is more strongly interacted at the narrow resonance. In addition, far beyond the resonance width, we find the harmonically trapped Fermi gas still manifests appreciable interaction effect across a narrow Feshbach resonance, which is contrary to our knowledge of the broad Feshbach resonance. All our results can be directly tested in current narrow Feshbach resonance experiments, which are generally carried out in a harmonic trap.

Dec 17
1. arXiv:1312.4011 [pdf, ps, other]
Spin-Orbit Driven Transitions Between Mott Insulators and Finite Momentum Superfluids of Bosons in Optical Lattices
Yinyin Qian, Ming Gong, Vito W. Scarola, Chuanwei Zhang
Subjects: Quantum Gases (cond-mat.quant-gas)
Synthetic spin-orbit coupling in ultracold atomic gases can be taken to extremes rarely found in solids. We study a two dimensional Hubbard model of bosons in an optical lattice in the presence of spin-orbit coupling strong enough to drive direct transitions from Mott insulators to superfluids. Here we find phase-modulated superfluids with finite momentum that are generated entirely by spin-orbit coupling. Transitions between different superfluids are also found to be first-order. We investigate the rich periodic structure of the phases of the superfluids, which may be directly probed using time-of-flight imaging of the spin-dependent momentum distribution.

2. arXiv:1312.4129 [pdf, other]
Vortices in the two-dimensional dipolar Bose gas
B. C. Mulkerin, D. H. J. O'Dell, A. M. Martin, N. G. Parker
Subjects: Quantum Gases (cond-mat.quant-gas); Soft Condensed Matter (cond-mat.soft); Quantum Physics (quant-ph)
We present vortex solutions for the homogeneous two-dimensional Bose-Einstein condensate featuring dipolar atomic interactions, mapped out as a function of the dipolar interaction strength (relative to the contact interactions) and polarization direction. Stable vortex solutions arise in the regimes where the fully homogeneous system is stable to the phonon or roton instabilities. Close to these instabilities, the vortex profile differs significantly from that of a vortex in a nondipolar quantum gas, developing, for example, density ripples and an anisotropic core. Meanwhile, the vortex itself generates a mesoscopic dipolar potential which, at distance, scales as 1/r^2 and has an angular dependence which mimics the microscopic dipolar interaction.

3. arXiv:1312.4470 [pdf, ps, other]
Abnormal Superfluid Fraction of Harmonically Trapped Few-Fermion Systems
Yangqian Yan, D. Blume
Subjects: Quantum Gases (cond-mat.quant-gas)
Superfluidity is a fascinating phenomenon that, at the macroscopic scale, leads to dissipationless flow and the emergence of vortices. While these macroscopic manifestations of superfluidity are well described by theories that have their origin in Landau's two-fluid model, our microscopic understanding of superfluidity is far from complete. Using analytical and numerical \textit{ab initio} approaches, this paper determines the superfluid fraction and local superfluid density of small harmonically trapped two-component Fermi gases as a function of the interaction strength and temperature. At low temperature, we find that the superfluid fraction is, in certain regions of the parameter space, negative. This counterintuitive finding is traced back to the symmetry of the system's ground state wave function, which gives rise to a diverging quantum moment of inertia Iq. Analogous abnormal behavior of Iq has been observed in even-odd nuclei at low temperature. Our predictions can be tested in modern cold atom experiments.

Dec 16
1. arXiv:1312.3888 [pdf, ps, other]
Coulomb interaction effects on the Majorana states in quantum wires
Andrei Manolescu, D. C. Marinescu, Tudor D. Stanescu
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
The stability of the Majorana modes in the presence of a repulsive interaction is studied in the standard semiconductor wire - metallic superconductor configuration. The effects of short-range Coulomb interaction, which is incorporated using a purely repulsive δ-function to model the strong screening effect due to the presence of the superconductor, are determined within a Hartree-Fock approximation of the effective Bogoliubov-De Gennes Hamiltonian that describes the low-energy physics of the wire. Through a numerical diagonalization procedure we obtain interaction corrections to the single particle eigenstates and calculate the extended topological phase diagram in terms of the chemical potential and the Zeeman energy. We find that, for a fixed Zeeman energy, the interaction shifts the phase boundaries to a higher chemical potential, whereas for a fixed chemical potential this shift can occur either to lower or to higher Zeeman energies. This effects can be interpreted as a renormalization of the g-factor due to the interaction. The minimum Zeeman energy needed to realize Majorana fermions decreases with increasing the strength of the Coulomb repulsion. Furthermore, we find that in wires with multi-band occupancy this effect can be enhanced by increasing the chemical potential, i. e. by occupying higher energy bands.

Dec 9-Dec 13, Bo Liu
Dec 13
1.arXiv:1312.3520 [pdf, other]
Fluctuation-induced first-order quantum phase transition in spinor Bose-Einstein condensates
Shun Uchino
It is argued that a continuous quantum phase transition between different ordered phases in spinor Bose-Einstein condensates predicted by the mean-field theory is vulnerable to quantum fluctuations. By analyzing Lee-Huang-Yang corrections in the condensate, we demonstrate that such a transition reduces to a first order, namely, fluctuation-induced first-order phase transition occurs. A jump to be expected in the first-order transition is induced by a correction from density fluctuations despite a transition between different magnetic properties. We exemplify this with an experimentally relevant case and show that a measurement of a condensate depletion can be utilized to confirm the first-order phase transition.

2. arXiv:1312.3484 [pdf, ps, other]
Quantum particle in a parabolic lattice in the presence of a gauge field
Andrey R. Kolovsky, Fabian Grusdt, Michael Fleischhauer
We analyze the eigenstates of a two-dimensional lattice with additional harmonic confinement in the presence of an artificial magnetic field. While the softness of the confinement makes a distinction between bulk and edge states difficult, the interplay of harmonic potential and lattice leads to a different classification of states in three energy regions: In the low-energy regime, where lattice effects are small, all states are transporting topologically non-trivial states. For large energies above a certain critical value, the periodic lattice causes localization of all states through a mechanism similar to Wannier-Stark localization. In the intermediate energy regime transporting, topologically non- trivial states coexist with topologically trivial counter-transporting chaotic states. The character of the eigenstates, in particular their transport properties are studied numerically and are explained using a semiclassical analysis.

Dec 12
1.arXiv:1312.3169 [pdf, ps, other]
Quench-induced breathing mode of one-dimensional Bose gases
Bess Fang (LCF), Giuseppe Carleo (LCF), Isabelle Bouchoule (LCF)
We measure the position- and momentum- space breathing dynamics of trapped one-dimensional Bose gases. The profile in real space reveals sinusoidal width oscillations whose frequency varies continuously through the quasicondensate to ideal Bose gas crossover. A comparison with theoretical models taking into account the effect of finite temperature is provided. In momentum space, we report the first observation of a frequency doubling in the quasicondensate regime, corresponding to a self-reflection mechanism. The disappearance of this mechanism through the quasicondensation crossover is mapped out.

2.arXiv:1312.3127 [pdf, other]
Tensor networks for Lattice Gauge Theories and Atomic Quantum Simulation
E. Rico, T. Pichler, M. Dalmonte, P. Zoller, S. Montangero
We show that gauge invariant quantum link models, Abelian and non-Abelian, can be exactly described in terms of tensor networks states. Quantum link models represent an ideal bridge between high-energy to cold atom physics, as they can be used in cold-atoms in optical lattices to study lattice gauge theories. In this framework, we characterize the phase diagram of a (1+1)-d quantum link version of the Schwinger model in an external classical background electric field: the quantum phase transition from a charge and parity ordered phase with non-zero electric flux to a disordered one with a net zero electric flux configuration is described by the Ising universality class
.

Dec 11
1. arXiv:1312.2758 [pdf, other]
Observation of many-body long-range tunneling after a quantum quench
Florian Meinert, Manfred J. Mark, Emil Kirilov, Katharina Lauber, Philipp Weinmann, Michael Gröbner, Andrew J. Daley, Hanns-Christoph Nägerl
Quantum tunneling constitutes one of the most fundamental processes in nature. We observe resonantly-enhanced long-range quantum tunneling in one-dimensional Mott-insulating Hubbard chains that are suddenly quenched into a tilted configuration. Higher-order many-body tunneling processes occur over up to five lattice sites when the tilt per site is tuned to integer fractions of the Mott gap. Starting from a one-atom-per-site Mott state the response of the many-body quantum system is observed as resonances in the number of doubly occupied sites and in the emerging coherence in momentum space. Second- and third-order tunneling shows up in the transient response after the tilt, from which we extract the characteristic scaling in accordance with perturbation theory and numerical simulations

2.arXiv:1312.2893 (cross-list from physics.atom-ph) [pdf, other]
Differential Scattering and Rethermalization in Ultracold Dipolar Gases
John L. Bohn, Deborah S. Jin
Analytic expressions for the differential cross sections of ultracold atoms and molecules that scatter primarily due to dipolar interactions are derived within the first Born approximation, and are shown to agree with the partial wave expansion. These cross sections are applied to the problem of cross-dimensional rethermalization. Strikingly, the rate of rethermalization can vary by as much as a factor of two, depending on the orientation of polarization of the dipoles. Thus the anisotropic dipole-dipole interaction can have a profound effect even on the behavior of a nondegenerate ultracold gas.


Dec 10
1.arXiv:1312.2563 [pdf, other]
Vortex Rings in a Trap
Aurel Bulgac, Michael McNeil Forbes
We present a simple Hamiltonian description of the dynamics of a quantized vortex ring in a trapped superfluid, compare this description with dynamical simulations, and characterize the dependence of the dynamics of the shape of the trap

2.arXiv:1312.2436 (cross-list from physics.atom-ph) [pdf, ps, other]
Non equilibrium phase transition with gravitational-like interaction in a cloud of cold atoms
Julien Barré, Bruno Marcos, David Wilkowski
We propose to use a cloud of laser cooled atoms in a quasi two dimensional trap to investigate a non equilibrium collapse phase transition in presence of gravitational-like interaction. Using theoretical arguments and numerical simulations, we show that, like in two dimensional gravity, a transition to a collapsed state occurs below a critical temperature. In addition and as a signature of the non equilibrium nature of the system, persistent particles currents, dramatically increasing close to the phase transition, are observed.
Dec 2-Dec 6, Zhifang Xu
Dec 2
1.arXiv:1311.7644 [pdf, other]
Hydrodynamic long-time tails after a quantum quench
Jonathan Lux, Jan Müller, Aditi Mitra, Achim Rosch
Comments: 5 pages + 4 pages Supplementary Material
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

After a quantum quench, a sudden change of parameters, generic many particle quantum systems are expected to equilibrate. A few collisions of quasiparticles are usually sufficient to establish approximately local equilibrium. Reaching global equilibrium is, however, much more difficult as conserved quantities have to be transported for long distances to build up a pattern of fluctuations characteristic for equilibrium. Here we investigate the quantum quench of the one-dimensional bosonic Hubbard model from infinite to finite interaction strength U using semiclassical methods for weak, and exact diagonalization for strong quenches. Equilibrium is approached only slowly, as t^{-1/2} with subleading corrections proportional to t^{-3/4}, consistent with predictions from hydrodynamics. We show that these long-time tails determine the relaxation of a wide range of physical observables.

2.arXiv:1311.7684 (cross-list from cond-mat.str-el) [pdf, ps, other]
Optical lattice implementation scheme of a bosonic topological model with fermionic atoms
Anne E. B. Nielsen, Germán Sierra, J. Ignacio Cirac
Comments: 30 pages, 10 figures. This article provides the full analysis of a scheme proposed in Nat. Commun. 4, 2864 (2013)
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We present a scheme to implement a Fermi-Hubbard-like model in ultracold atoms in optical lattices and analyze the topological features of its ground state. In particular, we show that the ground state for appropriate parameters has a large overlap with a lattice version of the bosonic Laughlin state at filling factor one half. The scheme utilizes laser assisted and normal tunnelling in a checkerboard optical lattice. The requirements on temperature, interactions, and hopping strengths are similar to those needed to observe the N\'eel antiferromagnetic ordering in the standard Fermi-Hubbard model in the Mott insulating regime.
Dec 3
1.arXiv:1312.0498 [pdf, other]
In situ imaging of atomic quantum gases
Chen-Lung Hung, Cheng Chin
Comments: 20 pages, 9 figures. This article will be published as Chapter 6 in "Quantum gas experiments - exploring many-body states", edited by P. T\"orm\"a and K. Sengstock, Imperial College Press, London, to be published 2014
Subjects: Quantum Gases (cond-mat.quant-gas)

One exciting progress in recent cold atom experiments is the development of high resolution, in situ imaging techniques for atomic quantum gases [1-3]. These new powerful tools provide detailed information on the distribution of atoms in a trap with resolution approaching the level of single atom and even single lattice site, and complements the well developed time-of-flight method that probes the system in momentum space. In a condensed matter analogy, this technique is equivalent to locating electrons of a material in a snap shot. In situ imaging has offered a new powerful tool to study atomic gases and inspired many new research directions and ideas. In this chapter, we will describe the experimental setup of in situ absorption imaging, observables that can be extracted from the images, and new physics that can be explored with this technique.

2.arXiv:1312.0452 [pdf, ps, other]
The spatial coherence of weakly interacting one-dimensional non-equilibrium Bosonic quantum fluids
Vladimir N. Gladilin, Kai Ji, Michiel Wouters
Comments: 5 pages, 2 figures, supplemental material
Subjects: Quantum Gases (cond-mat.quant-gas)

We present a theoretical analysis of spatial correlations in a one-dimensional driven-dissipative non-equilibrium condensate. Starting from a stochastic generalized Gross-Pitaevskii equation, we derive a noisy Kuramoto-Sivashinsky equation for the phase dynamics. For sufficiently strong interactions, the coherence decays exponentially in close analogy to the equilibrium Bose gas. When interactions are small on a scale set by the nonequilibrium condition, we find through numerical simulations a crossover between a Gaussian and exponential decay with peculiar scaling of the coherence length on the fluid density and noise strength.

3.arXiv:1312.0426 [pdf, ps, other]
Dynamical Instability Induced by Zero Mode Under Symmetry Breaking External Perturbation
J. Takahashi, Y. Nakamura, Y. Yamanaka
Comments: 7 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

The Bogoliubov-de Gennes equation is widely used to judge whether a stationary Bose-Einstein condensate in ultracold atomic system is stable or unstable. Its eigenvalue can be complex, which is interpreted as a sign of the dynamical instability. We also have the zero eigenvalues, called the zero modes, which originate from the spontaneous breakdown of symmetries. Though the zero modes are suppressed in many theoretical analyses, we take account of them in this paper. It is pointed out that the eigenvalues of the zero modes can be pure imaginary under a symmetry breaking external perturbation potential, and this is a new type of the dynamical instability. For illustration, we deal with the one-dimensional homogeneous Bose-Einstein condensate system with a single dark soliton under an perturbation potential, breaking the translational symmetry.
Dec 4
1.arXiv:1312.0869 [pdf, ps, other]
Effect of the symmetry breaking on the stability of persistent currents in a two-component Bose-Einstein condensate
J. Smyrnakis, M. Magiropoulos, G. M. Kavoulakis
Comments: 7 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We study the stability of persistent currents in a two-component Bose-Einstein condensed gas that is confined in a ring potential. The symmetry imposed by the assumption of equal couplings for intra- and interatomic collisions affects the stability of the persistent currents in a dramatic way. We examine the effect of breaking of this symmetry, identifying the major differences in the behavior of the system as compared to the "symmetric" case of equal couplings.

2.arXiv:1312.0810 [pdf, other]
Moving perturbation in a one-dimensional Fermi gas
A.-M. Visuri, D.-H. Kim, J. J. Kinnunen, F. Massel, P. Törmä
Comments: 5 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)

A balanced two-component Fermi gas in one dimension is perturbed with a moving potential well or barrier that sweeps across the lattice. Different velocities and strengths of the perturbation are studied using the time-evolving block decimation (TEBD) method, and two velocity regimes are distinguished based on features in the particle densities and in the pair correlation function. In the slow velocity regime, the densities deform as particles are either attracted by the potential well or repelled by the barrier, and a wave front of hole or particle excitations propagates at the maximum group velocity. The pair correlations show that the initial superfluid state is broken: coherence over different sites is lost after the potential has passed a point in the chain. In the fast regime, the densities are not considerably deformed and the pair correlations are preserved. This is in contrast with the concept of a superfluid critical velocity in higher dimensions.

3.arXiv:1312.0611 [pdf, ps, other]
Critical Temperature of Interacting Bose Gases in Periodic Potentials
T. T. Nguyen, A. J. Herrmann, M. Troyer, S. Pilati
Comments: 4+epsilon pages, 5 figures and supplemental material: 1.5 pages
Subjects: Quantum Gases (cond-mat.quant-gas)

The superfluid transition of a repulsive Bose gas in the presence of a sinusoidal potential which represents a simple-cubic optical lattice is investigate using quantum Monte Carlo simulations. At the average filling of one particle per well the critical temperature has a nonmonotonic dependence on the interaction strength, with an initial sharp increase and a rapid suppression at strong interactions in the vicinity of the Mott transition. In an optical lattice the positive shift of the transition is strongly enhanced compared to the homogenous gas. By varying the lattice filling we find a crossover from a regime where the optical lattice has the dominant effect to a regime where interactions dominate and the presence of the lattice potential becomes almost irrelevant.
Dec 5
1.arXiv:1312.1235 [pdf, ps, other]
Universal scaling of three-dimensional bosonic gases in a trapping potential
Giacomo Ceccarelli, Jacopo Nespolo
Comments: 8 pages, 9 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)

We investigate the critical properties of cold bosonic gases in three dimensions, confined by an external quadratic potential coupled to the particle density, and realistically described by the Bose-Hubbard (BH) model. The trapping potential is often included in experiments with cold atoms and modifies the critical finite-size scaling of the homogeneous system in a non trivial way. The trap-size scaling (TSS) theory accounts for this effect through the exponent \theta.
We perform extensive simulations of the BH model at the critical temperature, in the presence of harmonic traps. We find that the TSS predictions are universal once we account for the effective way in which the trap locally modifies the chemical potential \mu of the system. The trap exponent for the BH model at \mu=0 is the one corresponding to an effective quartic potential. At positive \mu, evidence suggests that TSS breaks down sufficiently far from the centre of the trap, as the system encounters an effective phase boundary.

Dec 6
1.arXiv:1312.1551 [pdf, ps, other]
Three-body-interacting bosons in free space
D.S. Petrov
Comments: 5 pages, 3 figures + Supplemental Material
Subjects: Quantum Gases (cond-mat.quant-gas)

We propose a method of controlling two- and three-body interactions in an ultracold Bose gas in any dimension. The method requires to have two coupled internal single-particle states split in energy such that the upper state is occupied virtually but amply during collisions. By varying system parameters one can switch off the two-body interaction while maintaining a strong three-body one. The mechanism can be implemented for dipolar bosons in the bilayer configuration with tunnelling or in an atomic system by using radio-frequency fields to couple two hyperfine states. One can then aim to observe a purely three-body-interacting gas, dilute self-trapped droplets, the paired superfluid phase, Pfaffian state, and other exotic phenomena.

2.arXiv:1312.1388 (cross-list from cond-mat.stat-mech) [pdf, ps, other]
Comment on "Quantum Quasicrystals of Spin-Orbit-Coupled Dipolar Bosons"
Ron Lifshitz
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas)

In a recent Letter, Gopalakrishnan, Martin, and Demler [Phys. Rev. Lett. 111 (2013) 185304] show that quasi-two-dimensional dipolar Bose gases, subject to a Rashba spin-orbit coupling, exhibit a variety of spatially ordered, or crystalline, ground states, including a pentagonal quasicrystal. Indeed, as the authors say, realizing quasicrystalline condensates would provide new ways to explore the physics of quasicrystals, and in particular to study the quantum dynamics of their unique collective phason modes. Yet, the authors conclude that "there are typically additional phasons in quantum-mechanical quasicrystals, when compared with their classical equivalents." In this Comment I review the notion of phason modes in quasicrystals, and explain why their number does not depend on whether they are classical or quantum.