Oct 2014

Oct 6-Oct 10, Zhenyu zhou & Jinlong Yu, Oct 13-Oct 17, Jiyao chen & Jianhui zhou, Oct 20-Oct 24, Haiyuan Zou & Ahmet Keles, Oct 27-Oct 31, Zhifang Xu & Xuguang Yue

Oct 31
1. arXiv:1410.8487 [pdf, other]
Critical Dynamics of Spontaneous Symmetry Breaking in a Homogeneous Bose gas
Nir Navon, Alexander L. Gaunt, Robert P. Smith, Zoran Hadzibabic
Comments: 15 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)


We explore the dynamics of spontaneous symmetry breaking in a homogeneous system by thermally quenching an atomic gas with short-range interactions through the Bose-Einstein phase transition. Using homodyne matter-wave interferometry to measure first-order correlation functions, we verify the central quantitative prediction of the Kibble-Zurek theory, namely the homogeneous-system power-law scaling of the coherence length with the quench rate. Moreover, we directly confirm its underlying hypothesis, the freezing of the correlation length near the transition due to critical slowing down. Our measurements agree with beyond mean-field theory, and support the previously unverified expectation that the dynamical critical exponent for this universality class, which includes the λ-transition of liquid 4He, is z=3/2.

2. arXiv:1410.8431 [pdf, other]
Topological superradiance in a degenerate Fermi gas
Jian-Song Pan, Xiong-Jun Liu, Wei Zhang, Wei Yi, Guang-Can Guo
Comments: 9 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We predict the existence of a topological superradiant state in a two-component degenerate Fermi gas in a cavity. The superradiant light generation in the transversely driven cavity mode induces a cavity-assisted spin-orbit coupling in the system and opens a bulk gap at half-filling. This mechanism can simultaneously drive a topological phase transition in the system, yielding a topological superradiant phase. We map out the steady-state phase diagram of the system in the presence of an effective Zeeman field, and identify a critical quadracritical point beyond which the topological and the conventional superraidiant phase boundaries separate. We also propose to detect the topological phase transitions based on the unique signatures in the momentum-space density distribution.

3. arXiv:1410.8425 [pdf, other]
Periodically-driven quantum matter: the case of resonant modulations
N. Goldman, J. Dalibard, M. Aidelsburger, N. R. Cooper
Comments: 14 pages, 3 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); High Energy Physics - Lattice (hep-lat); Quantum Physics (quant-ph)
Quantum systems can show qualitatively new forms of behavior when they are driven by fast time-periodic modulations. In the limit of large driving frequency, the long-time dynamics of such systems can often be described by a time-independent effective Hamiltonian, which is generally identified through a perturbative treatment. Here, we present a general formalism that describes time-modulated physical systems, in which the driving frequency is large, but resonant with respect to energy spacings inherent to the system at rest. Such a situation is currently exploited in optical-lattice setups, where superlattice (or Wannier-Stark-ladder) potentials are resonantly modulated so as to control the tunneling matrix elements between lattice sites, offering a powerful method to generate artificial fluxes for cold-atom systems. The formalism developed in this work identifies the basic ingredients needed to generate interesting flux patterns and band structures using resonant modulations. We also discuss the micro-motion underlying the dynamics, and illustrate its characteristics based on diverse dynamic-lattice configurations. It is shown that the impact of the micro-motion on physical observables strongly depends on the implemented scheme, suggesting that a theoretical description in terms of the effective Hamiltonian alone is generally not sufficient to capture the full time-evolution of the system.

4. arXiv:1410.8518 (cross-list from cond-mat.dis-nn) [pdf, other]
Many-body localization in periodically driven systems
Pedro Ponte, Z. Papić, François Huveneers, Dmitry A. Abanin
Comments: 7 pages, 6 figuresSubjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)

We consider disordered many-body systems with periodic time-dependent Hamiltonians in one spatial dimension. By studying the properties of the Floquet eigenstates, we identify two distinct phases: (i) a many-body localized (MBL) phase, in which almost all eigenstates have area-law entanglement entropy, and the eigenstate thermalization hypothesis (ETH) is violated, and (ii) a delocalized phase, in which eigenstates have volume-law entanglement and obey the ETH. MBL phase exhibits logarithmic in time growth of entanglement entropy for initial product states, which distinguishes it from the delocalized phase. We propose an effective model of the MBL phase in terms of an extensive number of emergent local integrals of motion (LIOM), which naturally explains the spectral and dynamical properties of this phase. Numerical data, obtained by exact diagonalization and time-evolving block decimation methods, suggests a direct transition between the two phases. Our results show that many-body localization is not destroyed by sufficiently weak periodic driving.








Oct 30
1.arXiv:1410.7828 [pdf, ps, other]
$p$ orbitals in 3D lattices; fermions, bosons and (exotic) models of magnetism
Fernanda Pinheiro
Comments: 13 pages, 12 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We demonstrate how different types of $SU(3)$ Heisenberg models can be implemented with the use of the $p$ orbitals of three dimensional optical lattices. By considering a Mott insulator with unit filling, the dynamics is well described by an effective model derived from the perturbative treatment of the tunneling elements relative to the onsite interaction terms. This yields systems with degrees of freedom that are generators of the $SU(3)$ group, which extends the Heisenberg models frequently used to analyze quantum magnetism. Due to the different character of interactions in the bosonic and fermionic cases, the choice of atom determines what type of anisotropies will appear in the couplings of the corresponding effective Hamiltonians. Experimental schemes for detection and manipulation of these systems are presented, and properties of the ground states of selected examples are discussed.

2.arXiv:1410.8008 [pdf, ps, other]
Fluctuations of the order parameter of a mesoscopic Floquet condensate
Bettina Gertjerenken, Martin Holthaus
Comments: 5 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We suggest that nonequilibrium Bose-Einstein condensates may occur in time-periodically driven interacting Bose gases. Employing the model of a periodically forced bosonic Josephson junction, we demonstrate that resonance-induced ground state-like many-particle Floquet states possess an almost perfect degree of coherence, as corresponding to a mesoscopically occupied, explicitly time-dependent single-particle orbital. In marked contrast to the customary time-independent Bose-Einstein condensates, the order parameter of such systems is destroyed by violent fluctuations when the particle number becomes too large, signaling the non-existence of a proper mean field limit.

Oct 29
1.arXiv:1410.7517 [pdf, ps, other]
An inequality for spinor Bose-Einstein condensates
Daisuke A. Takahashi
Comments: 3 pages, 2 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

An inequality for spin-$F$ Bose-Einstein condensates (BECs) $ F^2(\rho^2-|\Theta|^2)-\boldsymbol{M}^2\ge0 $ is reported, where $\rho$, $\Theta$, and $\boldsymbol{M}$ represent the density, singlet pair amplitude, and magnetization vector, respectively. The distribution of high-symmetry spinors in the allowed region by the inequality is elucidated with using the Majorana representation. The result is illustrated by the example of spin-2 BECs.

Oct 28
arXiv:1410.7280 [pdf, ps, other]
Cold bosons in optical lattices: correlations, localization, and fragmentation
David Raventós, Tobias Graß, Bruno Juliá-Díaz
Comments: 7 pages, 4 figuresSubjects: Quantum Gases (cond-mat.quant-gas)

Bosons in optical lattices may exhibit interesting quantum phenomena like fragmentation and coherence. Applying direct diagonalization to a small system, we explore different aspects of such setup: We show that, for a microscopic sample, the superfluid to Mott transition can be characterized by ground state overlaps with analytic trial states for both regimes. We also study a variant of the Mott transition in an inhomogeneous lattice, where one potential minimum is made much deeper than the others. Finally, we turn our attention to attractively interacting systems, and discuss the appearance of strongly correlated phases and the onset of localization for a slightly biased lattice.


arXiv:1410.7285 [pdf, ps, other]

An introduction to the Ginzburg-Landau theory of phase transitions and nonequilibrium patterns
P. C. Hohenberg, A. P. Krekhov
Subjects: Statistical Mechanics (cond-mat.stat-mech)

This paper presents an introduction to phase transitions and critical phenomena on the one hand, and nonequilibrium patterns on the other, using the Ginzburg-Landau theory as a unified language. In the first part, mean-field theory is presented, for both statics and dynamics, and its validity tested self-consistently. As is well known, the mean-field approximation breaks down below four spatial dimensions, where it can be replaced by a scaling phenomenology. The Ginzburg-Landau formalism can then be used to justify the phenomenological theory using the renormalization group, which elucidates the physical and mathematical mechanism for universality. In the second part of the paper it is shown how near pattern forming linear instabilities of dynamical systems, a formally similar Ginzburg-Landau theory can be derived for nonequilibrium macroscopic phenomena. The real and complex Ginzburg-Landau equations thus obtained yield nontrivial solutions of the original dynamical system, valid near the linear instability. Examples of such solutions are plane waves, defects such as dislocations or spirals, and states of temporal or spatiotemporal (extensive) chaos.

Oct 24

arXiv:1410.6189 [pdf, other]
Time-reversal Invariant SU(2) Hofstadter Problem in Three Dimensional Lattices
Yi Li
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We formulate the lattice version of the three-dimensional SU(2) Landau level problem with time reversal invariance. By taking a Landau-type gauge, the system is reduced into the one-dimensional SU(2) Harper equation characterized by a periodic spin-dependent gauge potential. The surface spectra indicate the spatial separation of helical states with opposite eigenvalues of the lattice helicty operator. The band topology is investigated from both the analysis of the boundary helical Fermi surfaces and the calculation of the Z2-index based on the bulk wavefunctions. The transition between a 3D weak topological insulator to a strong one is studied as varying the anisotropy of hopping parameters.

Oct 23

arXiv:1410.6009 [pdf, ps, other]
Resonating Valence-Bond State in an Orbitally Degenerate Quantum Magnet with Dynamical Jahn-Teller Effect
Joji Nasu, Sumio Ishihara
Comments: 8 pages, 7 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Materials Science (cond-mat.mtrl-sci)

Short-range resonating-valence bond states in an orbitally degenerate magnet on a honeycomb lattice is studied. A quantum-dimer model is derived from the Hamiltonian which represents the superexchange interaction and the dynamical Jahn-Teller (JT) effect. We introduce two local units termed "spin-orbital singlet dimer", where two spins in a nearest-neighbor bond form a singlet state associated with an orbital polarization along the bond, and "local JT singlet", where an orbital polarization is quenched due to the dynamical JT effect. A derived quantum-dimer model consists of the hopping of the spin-orbital singlet dimers and the JT singlets, and the chemical potential of the JT singlets. We analyze the model by the mean-field approximation, and find that a characteristic phase, termed "JT liquid phase", where both the spin-orbital singlet dimers and the JT singlets move quantum mechanically, is realized. Possible scenarios for the recently observed non magnetic-ordered state in Ba$_3$CuSb$_2$O$_9$ are discussed.

arXiv:1410.6105 [pdf, other]
Chiral Mott Insulators, Meissner Effect, and Laughlin States in Quantum Ladders
Alexandru Petrescu, Karyn Le Hur
Comments: 26 pages, 11 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)

We introduce generic bosonic models exemplifying that chiral Meissner currents can persist in insulating phases of matter. We first consider interacting bosons on a two-leg ladder. The total density sector can be gapped in a bosonic Mott insulator at odd-integer filling, while the relative density sector remains superfluid due to interchain hopping. Coupling the relative density to gauge fields yields a pseudospin Meissner effect. We show that the same phase arises if the bosons are replaced by spinful fermions confined in Cooper pairs, and find a dual fermionic Mott insulator with spinon currents. We prove that by tuning the mean density the Mott insulator with Meissner currents turns into a low-dimensional bosonic $\nu$ = 1/2 Laughlin state. We finally discuss extensions to multileg ladders and bilayers in which spinon superfluids with Meissner currents become possible. We propose two experimental realizations, one with ultracold atoms in the setup of Atala et al., Nat. Phys. 10, 588 (2014), and another with Josephson junction arrays. We also address a Bose-Fermi mixture subject to a magnetic field in connection with the pseudo-gap phase of high-Tc cuprates.

arXiv:1410.5841 [pdf, other]
Violation of the Wiedemann-Franz Law for ultracold atomic gases
Michele Filippone, Frank Hekking, Anna Minguzzi
Comments: 5 pages, 3 Figures + Supplemental MaterialSubjects: Quantum Gases (cond-mat.quant-gas)

We study energy and particle transport for one-dimensional strongly interacting bosons through a single channel connecting two atomic reservoirs. We show the emergence of particle- and energy- current separation, leading to the violation of the Wiedemann-Franz law. As a consequence, we predict different time scales for the equilibration of temperature and particle imbalances between the reservoirs. Going beyond the linear spectrum approximation, we show the emergence of ther- moelectric effects, which could be controlled by either tuning interactions or the temperature. Our results describe in a unified picture fermions in condensed matter devices and bosons in ultracold atom setups. We conclude discussing the effects of a controllable disorder.

Oct 22

arXiv:1410.5526 [pdf, other]
Bose-Einstein condensation with spin-orbit coupling
Yun Li, Giovanni I. Martone, Sandro Stringari
Comments: Book chapter to appear in volume III of the Annual Review of Cold Atoms and Molecules (World Scientific)Subjects: Quantum Gases (cond-mat.quant-gas)

The recent realization of synthetic spin-orbit coupling represents an outstanding achievement in the physics of ultracold quantum gases. In this review we explore the properties of a spin-orbit-coupled Bose-Einstein condensate with equal Rashba and Dresselhaus strengths. This system presents a rich phase diagram, which exhibits a tricritical point separating a zero-momentum phase, a spin-polarized plane-wave phase, and a stripe phase. In the stripe phase translational invariance is spontaneously broken, in analogy with supersolids. Spin-orbit coupling also strongly affects the dynamics of the system. In particular, the excitation spectrum exhibits intriguing features, including the suppression of the sound velocity, the emergence of a roton minimum in the plane-wave phase, and the appearance of a double gapless band structure in the stripe phase. Finally, we discuss a combined procedure to make the stripes visible and stable, thus allowing for a direct experimental detection.

arXiv:1410.5735 [pdf, other]
Nonequilibrium Dicke Quantum Phase Transition of an Ultracold Gas in an Optical Cavity
M.Reza Bakhtiari, Andreas Hemmerich, Helmut Ritsch, Michael Thorwart
Comments: 5 pages, 4 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We investigate the nonlinear light-matter interaction of a Bose-Einstein condensate in an optical cavity with transverse pump. This setup is described by a generalized Bose-Hubbard model which comprises the Dicke model of superradiance and reveals a nonequilibrium quantum phase transition from a superfluid condensate to a self-organized density-wave pattern. Due to the strong mutual dynamical backaction of the cavity light field and the interatomic correlations, we use a nonperturbative bosonic dynamical mean-field theory approach. For a narrow linewidth cavity as used in an experiment, we identify the Dicke quantum phase transition and the superradiant phase along the self-organization order parameter, density distributions and the cavity photon number. Moreover, we find a mixed phase in which the superfluid and the superradiant phases coexist.

Oct 21
arXiv:1410.5364 [pdf, other]
Scattering Theory for Floquet-Bloch States
Thomas Bilitewski, Nigel R. Cooper
Comments: 21 pages, 12 figuresSubjects: Quantum Gases (cond-mat.quant-gas)

Motivated by recent experimental implementations of artificial gauge fields for gases of cold atoms, we study the scattering properties of particles that are subjected to time-periodic Hamiltonians. Making use of Floquet theory, we focus on translationally invariant situations in which the single-particle dynamics can be described in terms of spatially extended Floquet-Bloch waves. We develop a general formalism for the scattering of these Floquet-Bloch waves. An important role is played by the conservation of Floquet quasi-energy, which is defined only up to the addition of integer multiples of $\hbar\omega$ for a Hamiltonian with period $T=2\pi/\omega$. We discuss the consequences of this for the interpretation of "elastic" and "inelastic" scattering in cases of physical interest. We illustrate our general results with applications to: the scattering of a single particle in a Floquet-Bloch state from a static potential; and, the scattering of two particles in Floquet-Bloch states through their interparticle interaction. We analyse examples of these scattering processes that are closely related to the schemes used to general artifical gauge fields in cold-atom experiments, through optical dressing of internal states, or through time-periodic modulations of tight-binding lattices. We show that the effects of scattering cannot, in general, be understood by an effective time-independent Hamiltonian, even in the limit $\omega \to \infty$ of rapid modulation. We discuss the relative sizes of the elastic scattering (required to stablize many-body phases) and of the inelastic scattering (leading to deleterious heating effects). In particular, we describe how inelastic processes that can cause significant heating in current experimental set-up can be switched off by additional confinement of transverse motion.

Oct 20
arXiv:1410.4790 [pdf, ps, other]
Density-Matrix Renormalization Group Study of Extended Kitaev-Heisenberg Model
Kazuya Shinjo, Shigetoshi Sota, Takami Tohyama
Comments: 10 pages, 10 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el)

We study an extended Kitaev-Heisenberg model including additional anisotropic couplings by using two-dimensional density-matrix renormalization group method. Calculating the gound-state energy, entanglement entropy, and spin-spin correlation functions, we make a phase diagram of the extended Kitaev-Heisenberg model around spin-liquid phase. We find a zigzag antiferromagnetic phase, a ferromagnetic phase, a 120-degree antiferromagnetic phase, and two kinds of incommensurate phases around the Kitaev spin-liquid phase. Furthermore, we study the entanglement spectrum of the model and find that entanglement levels in the Kitaev spin-liquid phase are degenerate forming pairs but those in the magnetically ordered phases are non-degenerate. The Schmidt gap defined as the energy difference between the lowest two levels changes at the phase boundary adjacent to the Kitaev spin-liquid phase. However, we find that phase boundaries between magnetically ordered phases do not necessarily agree with the change of the Schmidt gap.


Oct 17
1. arXiv:1410.4201 [pdf, other]
Composite Dirac liquids: parent states for symmetric surface topological order
David F. Mross, Andrew Essin, Jason Alicea2. arXiv:1410.4283 [pdf, other]
Goos-Hänchen shifts in spin-orbit-coupled cold atoms
Lu Zhou, Jie-Li Qin, Zhihao Lan, Guangjiong Dong, Weiping Zhang
Subjects: Quantum Gases (cond-mat.quant-gas)

Comments: 23 Pages, 5 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Oct 16
1. arXiv:1410.3839 [pdf, other]
Anderson Localization and Quantum Hall Effect: Numerical Observation of Two Parameter Scaling
Miklós Antal Werner, Arne Brataas, Felix von Oppen, Gergely Zaránd
A two dimensional disordered system of non-interacting fermions in a homogeneous magnetic field is investigated numerically. By introducing a new magnetic gauge, we explore the renormalization group (RG) flow of the longitudinal and Hall conductances with higher precision than previously studied, and find that the flow is consistent with the predictions of Pruisken and Khmelnitskii. The extracted critical exponents agree with the results obtained by using transfer matrix methods. The necessity of a second parameter is also reflected in the level curvature distribution. Near the critical point the distribution slightly differs from the prediciton of random matrix theory, in agreement with previous works. Close to the quantum Hall fixed points the distribution is lognormal since here states are strongly localized.

2. arXiv:1410.3995 [pdf, ps, other]

Composite bosons in the 2D BCS-BEC crossover from Gaussian fluctuations
L. Salasnich, F. Toigo

We study Gaussian fluctuations of the zero-temperature attractive Fermi gas in the 2D BCS-BEC crossover showing that they are crucial to get a reliable equation of state in the BEC regime of composite bosons, bound states of fermionic pairs. A low-momentum expansion up to the fourth order of the quadratic action of the fluctuating pairing field gives an ultraviolent divergent contribution of the Gaussian fluctuations to the grand potential. Performing dimensional regularization we evaluate the effective coupling constant in the beyond-mean-field grand potential. Remarkably, in the BEC regime our grand potential gives exactly the Popov's equation of state of 2D interacting bosons, and allows us to identify the scattering length aB of the interaction between composite bosons as aB=aF/(21/2e1/4)=0.551...aF, with aF is the scattering length of fermions. Remarkably, the value from our analytical relationship between the two scattering lengths is in full agreement with that obtained by recent Monte Carlo calculations.


Oct 15
1. arXiv:1410.3724 [pdf, ps, other]
Fractional Quantum Hall States of Dipolar Gases in Chern Bands
Tian-Sheng Zeng, Lan YinWe study fermions and hardcore bosons in topological checkerboard lattices with long range dipolar interactions at fractional fillings. By exact diagonalization of finite two-dimensional systems, we find clear signatures of fractional quantum Hall (FQH) states at filling factors 1/3 and 1/5 for fermions (1/2 and 1/4 for bosons) in the lowest Chern band with a robust spectrum gap at moderate dipolar interaction strength. When the dipolar interaction decreases, the fermionic FQH states turn into normal states, and the bosonic 1/4-FQH state turns into a superfluid state. The bosonic 1/2-FQH state survives even in the absence of the dipolar interaction, but vanishes when the hard core becomes a soft core with a critical onsite repulsion. In the thin torus limit, the static density structure factors indicates that the FQH state turns into a commensurate charge density wave (CDW) state.

2. arXiv:1410.3634 (cross-list from physics.optics) [pdf, other]

Magnetic-field-driven localization of light in a cold-atom gas
S.E. Skipetrov, I.M. Sokolov

We discover a transition from extended to localized quasi-modes for light in a gas of immobile two-level atoms in a magnetic field. The transition takes place either upon increasing the number density of atoms in a strong field or upon increasing the field at a high enough density. It has many characteristic features of a disorder-driven (Anderson) transition but is strongly influenced by near-field interactions between atoms and the anisotropy of the atomic medium induced by the magnetic field.



Oct 14
1. arXiv:1410.3455 [pdf, other]
The fate of many-body localization under periodic driving
Achilleas Lazarides, Arnab Das, Roderich MoessnerWe study many-body localised quantum systems subject to periodic driving. We find that the presence of a mobility edge anywhere in the spectrum is enough to lead to delocalisation for any driving strength and frequency. By contrast, for a fully localised many-body system, a delocalisation transition occurs at a finite driving frequency. We present numerical studies on a system of interacting one-dimensional bosons and the quantum random energy model, as well as simple physical pictures accounting for those results.

2.arXiv:1410.3093 (cross-list from hep-th) [pdf, other]

Exciton Condensation in a Holographic Double Monolayer Semimetal
Gianluca Grignani, Andrea Marini, Namshik Kim, Gordon W. SemenoffThe formation of intra-layer and inter-layer exciton condensates in a model of a double monolayer Weyl semi-metal is studied in the strong coupling limit using AdS/CFT duality. We find a rich phase diagram which includes phase transitions between inter-layer and intra-layer condensates as the charge densities and the separation of the layers are varied. The tendency to inter-layer condensation is strongest when the charge densities are balanced so that the weak coupling electron and hole Fermi surfaces would be nested. For systems with multiple species of massless Fermions, we find a novel phase transition where the charge balance for nesting occurs spontaneously.




Oct 13
arXiv:1410.2704 [pdf, ps, other]
Chiral Electromagnetic Waves in Weyl Semimetal
A. A. Zyuzin, V. A. Zyuzin

We show that Weyl semimetals with broken time-reversal symmetry can host chiral electromagnetic waves. The magnetization that results in a momentum space separation of a pair of opposite chirality Weyl nodes is also responsible for the non-zero gyration vector in the system. It is then shown that chiral electromagnetic wave can propagate in a region of space where the gyration vector flips its direction. Such waves are analogs of quantum Hall edge states for photons.

Oct 10
1. arXiv:1410.2587 [pdf, ps, other]
Anderson transition of Bogoliubov quasiparticles in the quasiperiodic kicked rotor
Benoit Vermersch , Dominique Delande , Jean Claude Garreau
We study the dynamics of Bogoliubov excitations of a Bose-Einstein condensate in the quasiperiodic kicked rotor. In the weakly interacting regime, the condensate is stable and both the condensate and the excitations undergo a phase transition from a quasilocalized to a diffusive regime. The corresponding critical exponents are identical for the condensate and the excitations, and compare very well with the value ν≈1.6 for non-interacting particles.

Oct 9
1. arXiv:1410.1938 [pdf, ps, other]
Precise measurements on a quantum phase transition in antiferromagnetic spinor Bose-Einstein condensates
A. Vinit, C. Raman
We investigate, both experimentally and theoretically, the quench dynamics of antiferromagnetic spinor Bose-Einstein condensates in the vicinity of a zero temperature quantum phase transition at zero quadratic Zeeman shift q. Both the rate of instability and the associated finite wavevector of the unstable modes - show good agreement with predictions based upon numerical solutions to the Bogoliubov de-Gennes equations. A key feature of this work is inclusion of magnetic field inhomogeneities that smooth the phase transition. Once these were removed, we observed a dramatic sharpening of the transition point, which could then be resolved within a quadratic Zeeman shift of only 1-2 Hz. Our results point to the use of dynamics, rather than equilibrium quantities for high precision measurements of phase transitions in quantum gases.

2. arXiv:1410.2203 [pdf, ps, other]
Renormalization group approach to the Fröhlich polaron model: application to impurity-BEC problem
F. Grusdt, Y. E. Shchadilova, A. N. Rubtsov, E. Demler
We develop a renormalization group approach for analyzing Fr\"ohlich polarons and apply it to a problem of impurity atoms immersed in a Bose-Einstein condensate of ultra cold atoms. Polaron energies obtained by our method are in excellent agreement with recent diagrammatic Monte Carlo calculations for a wide range of interaction strengths. We calculate the effective mass of polarons and find a smooth crossover from weak to strong coupling regimes. Possible experimental tests of our results in current experiments with ultra cold atoms are discussed.

Oct 8
1. arXiv:1410.1567 [pdf, other]
Curved spacetimes in the lab
Nikodem Szpak
We present some new ideas on how to design analogue models of quantum fields living in curved spacetimes using ultra-cold atoms in optical lattices. We discuss various types of static and dynamical curved spacetimes achievable by simple manipulations of the optical setup. Examples presented here contain two-dimensional spaces of positive and negative curvature as well as homogeneous cosmological models and metric waves. Most of them are extendable to three spatial dimensions. We mention some interesting phenomena of quantum field theory in curved spacetimes which might be simulated in such optical lattices loaded with bosonic or fermionic ultra-cold atoms. We also argue that methods of differential geometry can be used, as an alternative mathematical approach, for dealing with realistic inhomogeneous optical lattices.

2. arXiv:1410.1595 [pdf, other]
Pairing Superfluidity in Spin-Orbit Coupled Ultracold Fermi Gases
Wei Yi, Wei Zhang, Xiaoling Cui
We review some recent progresses on the study of ultracold Fermi gases with synthetic spin-orbit coupling. In particular, we focus on the pairing superfluidity in these systems at zero temperature. Recent studies have shown that different forms of spin-orbit coupling in various spatial dimensions can lead to a wealth of novel pairing superfluidity. A common theme of these variations is the emergence of new pairing mechanisms which are direct results of spin-orbit-coupling-modified single-particle dispersion spectra. As different configurations can give rise to single-particle dispersion spectra with drastic differences in symmetry, spin dependence and low-energy density of states, spin-orbit coupling is potentially a powerful tool of quantum control, which, when combined with other available control schemes in ultracold atomic gases, will enable us to engineer novel states of matter.


Oct 7
1. arXiv:1410.1513 [pdf, other]
Bloch oscillations of bosonic lattice polarons
Fabian Grusdt, Aditya Shashi, Dmitry Abanin, Eugene Demler
We consider a single impurity atom confined to an optical lattice and immersed in a homogeneous Bose-Einstein condensate (BEC). Interaction of the impurity with the phonon modes of the BEC leads to the formation of a stable quasiparticle, the polaron. We use a variational mean-field approach to study dispersion renormalization and derive equations describing non-equilibrium dynamics of polarons by projecting equations of motion into mean-field (MF) type wavefunctions. As a concrete example, we apply our method to study dynamics of impurity atoms in response to a suddenly applied force and explore the interplay of coherent Bloch oscillations and incoherent drift. We obtain a non-linear dependence of the drift velocity on the applied force, including a sub-Ohmic dependence for small forces for dimensionality d>1 of the BEC. For the case of heavy impurity atoms we derive a closed analytical expression for the drift velocity. Our results show considerable differences with the commonly used phenomenological Esaki-Tsu model.

2. arXiv:1410.1340 [pdf, ps, other]
First and second sound of a unitary Fermi gas in highly oblate harmonic traps
Hui Hu, Paul Dyke, Chris J. Vale, Xia-Ji Liu
We theoretically investigate first and second sound modes of a unitary Fermi gas trapped in a highly oblate harmonic trap at finite temperatures. Following the idea by Stringari and co-workers {[}Phys. Rev. Lett. \textbf{105}, 150402 (2010){]}, we argue that these modes can be described by the simplified two-dimensional two-fluid hydrodynamic equations. Two possible schemes - sound wave propagation and breathing mode excitation - are considered. We calculate the sound wave velocities and discretized sound mode frequencies, as a function of temperature. We find that in both schemes, the coupling between first and second sound modes is large enough to induce significant density fluctuations, suggesting that second sound can be directly observed by measuring \textit{in-situ} density profiles. The frequency of the second sound breathing mode is found to be highly sensitive to the superfluid density.

3. arXiv:1410.1336 [pdf, ps, other]
First and second sound of a unitary Fermi gas in highly elongated harmonic traps
Xia-Ji Liu, Hui Hu
Using a variational approach, we present the full solutions of the simplified one-dimensional two-fluid hydrodynamic equations for a unitary Fermi gas trapped in a highly elongated harmonic potential, which is recently derived by Stringari and co-workers {[}Phys. Rev. Lett. \textbf{105}, 150402 (2010){]}. We calculate the discretized mode frequencies of first and second sound along the weak axial trapping potential, as a function of temperature and the form of superfluid density. We show that the density fluctuations in second sound modes, due to their coupling to first sound modes, are large enough to be measured in current experimental setups such as that exploited by Tey \textit{et al}. at the University of Innsbruck {[}Phys. Rev. Lett. \textbf{110}, 055303 (2013){]}. Owing to the sensitivity of second sounds on the form of superfluid density, the high precision of the measured second sound frequencies may provide us a promising way to accurately determine the superfluid density of a unitary Fermi gas, which so far remains elusive.

4. arXiv:1410.1034 [pdf, other]
Bose-Hubbard model with random impurities: Multiband and nonlinear hopping effects
Julia Stasinska, Mateusz Lacki, Omjyoti Dutta, Jakub Zakrzewski, Maciej Lewenstein
We investigate the phase diagrams of theoretical models describing bosonic atoms in a lattice in the presence of randomly localized impurities. By including multiband and nonlinear hopping effects we enrich the standard model containing only the chemical-potential disorder with the site-dependent hopping and interaction terms. We compare the extension of the MI and the BG phase in both models using a combination of the local mean-field method and a Hartree-Fock-like procedure, as well as, the Gutzwiller-ansatz approach. We show analytical argument for the presence of triple points in the phase diagram of the model with chemical-potential disorder. These triple points however, cease to exists by the addition of hopping disorder.

5. arXiv:1410.0987 [pdf, ps, other]
Superfluid density and Berezinskii-Kosterlitz-Thouless transition of a spin-orbit coupled Fulde-Ferrell superfluid
Ye Cao, Xia-Ji Liu, Lianyi He, Gui-Lu Long, Hui Hu
We theoretically investigate the superfluid density and Berezinskii-Kosterlitz-Thouless (BKT) transition of a two-dimensional Rashba spin-orbit coupled atomic Fermi gas with both in-plane and out-of-plane Zeeman fields. It was recently predicted that, by tuning the two Zeeman fields, the system may exhibit different exotic Fulde-Ferrell (FF) superfluid phases, including the gapped FF, gapless FF, gapless topological FF and gapped topological FF states. Due to the FF paring, we show that the superfluid density (tensor) of the system becomes anisotropic. When an in-plane Zeeman field is applied along the \textit{x}-direction, the tensor component along the \textit{y}-direction ns,yy is generally larger than ns,xx in most parameter space. At zero temperature, there is always a discontinuity jump in ns,xx as the system evolves from a gapped FF into a gapless FF state. With increasing temperature, such a jump is gradually washed out. The critical BKT temperature has been calculated as functions of the spin-orbit coupling strength, interatomic interaction strength, in-plane and out-of-plane Zeeman fields. We predict that the novel FF superfluid phases have a significant critical BKT temperature, typically at the order of 0.1TF, where TF is the Fermi degenerate temperature. Therefore, their observation is within the reach of current experimental techniques in cold-atom laboratories.

Oct 6
1. arXiv:1410.0743 [pdf, ps, other]
Dynamics of a vortex dipole across a magnetic phase boundary in a spinor Bose-Einstein condensate
Tomoya Kaneda, Hiroki Saito
Dynamics of a vortex dipole in a spin-1 Bose-Einstein condensate in which magnetic phases are spatially distributed is investigated. When a vortex dipole travels from the ferromagnetic phase to the polar phase, or vice versa, it penetrates the phase boundary and transforms into one of the various spin vortex dipoles, such as a leapfrogging ferromagnetic-core vortex dipole and a half-quantum vortex dipole. Topological connections of spin wave functions across the phase boundary are discussed.