Oct 2013

| Oct 28-Nov 1 Stephan Langer | | Oct 21-Oct 25 Bo Liu | | 1. arXiv:1310.5800 pdf, other] | | | Oct 14-Oct 18 Zhifang Xu | | | Oct 18 | Oct 17 | Oct 16 | Oct 15 | Oct 14 | Oct 7-Oct 11, Zhenyu Zhou, | | | | Oct 4 | Oct 3 | Oct 2

Oct 28-Nov 1 Stephan Langer

Nov 1

1. arXiv:1310.8524 [pdf, ps, other]
Interaction-induced localization of mobile impurities in ultracold systems
Jian Li, Jin An, C. S. Ting

The impurities, introduced intentionally or accidentally into certain materials, can significantly modify their characteristics or reveal their intrinsic physical properties, and thus play an important role in solid-state physics. Different from those static impurities in a solid, the impurities realized in cold atomic systems are naturally mobile. Here we propose an effective theory for treating some unique behaviors exhibited by ultracold mobile impurities. Our theory reveals the interaction-induced transition between the extended and localized impurity states, and also explains the essential features obtained from several previous models in a unified way. Based on our theory, we predict many intriguing phenomena in ultracold systems associated with the extended and localized impurities, including the formation of the impurity-molecules and impurity-lattices. We hope this investigation can open up a new avenue for the future studies on ultracold mobile impurities.


1. arXiv:1310.8354 [pdf]
Thermalization of field driven quantum systems
H. Fotso, K. Mikelsons, J. K. freericks

There is much interest in how quantum systems thermalize after a sudden change, because unitary evolution should preclude thermalization. The eigenstate thermalization hypothesis resolves this because all observables for quantum states in a small energy window have essentially the same value; it is violated for integrable systems due to the infinite number of conserved quantities. Here, we show that when a system is driven by a DC electric field there are five generic behaviors: (i) monotonic or (ii) oscillatory approach to an infinite-temperature steady state; (iii) monotonic or (iv) oscillatory approach to a nonthermal steady state; or (v) evolution to an oscillatory state. Examining the Hubbard model (which thermalizes under a quench) and the Falicov-Kimball model (which does not), we find both exhibit scenarios (i-iv), while only Hubbard shows scenario (v). This shows richer behavior than in interaction quenches and integrability in the absence of a field plays no role.

1. arXiv:1310.8332 [pdf, other]
Particle entanglement in continuum many-body systems via quantum Monte Carlo
C. M. Herdman, P.-N. Roy, R. G. Melko, A. Del Maestro



Entanglement of spatial bipartitions, used to explore lattice models in condensed matter physics, may be insufficient to fully describe itinerant quantum many-body systems in the continuum. We introduce a procedure to measure the R\'enyi entanglement entropies on a particle bipartition, with general applicability to continuum Hamiltonians via Path Integral Monte Carlo methods. Via direct simulations of interacting bosons in one spatial dimension, we confirm a logarithmic scaling of the single-particle entanglement entropy with the number of particles in the system. The coefficient of this logarithmic scaling increases with interaction strength, saturating to unity in the strongly interacting limit. Additionally, we show that the single-particle entanglement entropy is bounded by the condensate fraction, suggesting a practical route towards its measurement in future experiments.

Oct 31

1. arXiv:1310.7959 [pdf, other]
Quantum simulation of correlated-hopping models with fermions in optical lattices
M. Di Liberto, C. E. Creffield, G. I. Japaridze, C. Morais Smith


By using a modulated magnetic field in a Feshbach resonance for ultracold fermionic atoms in optical lattices, we show that it is possible to engineer a class of models usually referred to as correlated-hopping models. These models differ from the Hubbard model in exhibiting additional density-dependent interaction terms that affect the hopping processes. In addition to the spin-SU(2) symmetry, they also possess a charge-SU(2) symmetry, which opens the possibility of investigating the η-pairing mechanism for superconductivity introduced by Yang for the Hubbard model. We discuss the known solution of the model in 1D (where η states have been found in the degenerate manifold of the ground state) and show that, away from the integrable point, quantum Monte Carlo simulations at half filling predict the emergence of a phase with coexisting incommensurate spin and charge order.

2. arXiv:1310.8105 [pdf, ps, other]
Direct observation of Dirac cones and a flatband in a honeycomb lattice for polaritons
Thibaut Jacqmin, Iacopo Carusotto, Isabelle Sagnes, Marco Abbarchi, Dmitry Solnyshkov, Guillaume Malpuech, Elisabeth Galopin, Aristide Lemaître,Jacqueline Bloch, Alberto Amo




Two-dimensional lattices of coupled micropillars etched in a planar semiconductor microcavity offer a workbench to engineer the band structure of polaritons. We report experimental studies of honeycomb lattices where the polariton low energy dispersion is analogous to that of electrons in graphene. Using energy resolved photoluminescence we directly observe Dirac cones, around which the dynamics of polaritons is described by the Dirac equation for massless particles. At higher energies, we observe p orbital bands, one of them with the non-dispersive character of a flat band. The realization of this structure which holds massless, massive and infinitely-massive particles opens the route towards studies of the interplay of dispersion, interactions and frustration in a novel and controlled environment.


Oct 30

1. arXiv:1310.7757 [pdf, other]
Frustration by population trapping with polar molecules
O. Dutta, M. Lewenstein, J. Zakrzewski

One of the most important fundamental problems in contemporary physics is to understand the effects of frustration in quantum many-body systems. Typically, frustration is induced by lattice geometries, disorder and/or strong interactions [1]. The resulting states show strong correlations and have exotic properties with possible applications for condensed matter physics, material science, and quantum information technologies, etc. A paradigmatic system that exhibits frustration is a gas of ultracold polar molecules trapped in an optical lattice [2]. Notably, such system is extremely difficult to model theoretically due to proliferation of dipole induced multi-band excitations. In this article we develop for the first time a consistent theoretical model of the polar molecules in a lattice by applying the concepts and ideas of ionization theory [3]. Our approach is necessary for correct account of strong dipolar interaction. Additionally, by merging concepts from quantum optics (population transfer), we show that one can induce frustration and engineer exotic states, such as Majumdar-Ghosh state, or vector-chiral states in such situation. This forms the first attempt to bridge the seemingly two distant areas of research, and opens up a new direction to engineer different strongly-correlated many-body quantum states.

2. arXiv:1310.7618 [pdf, ps, other]
Fulde-Ferrell states and Berezinskii-Kosterlitz-Thouless phase transition in two-dimensional imbalanced Fermi gases
Shaoyu Yin, J.-P. Martikainen, P. Törmä



We study the superfluid properties of two-dimensional spin-population-imbalanced Fermi gases to explore the interplay between the Berezinskii-Kosterlitz-Thouless (BKT) phase transition and the possible instability towards the Fulde-Ferrell (FF) state. By the mean-field approximation together with quantum fluctuations, we obtain phase diagrams as functions of temperature, chemical potential imbalance and binding energy. We find that the fluctuations change the mean-field phase diagram significantly. We also address possible effects of the phase separation and/or the anisotropic FF phase to the BKT mechanism. The superfluid density tensor of the FF state is obtained, and its transverse component is found always vanishing. This causes divergent fluctuations and possibly precludes the existence of the FF state at any non-zero temperature.



3. arXiv:1310.7639 [pdf, ps, other]
Interaction-Tuned Dynamical Transitions in a Rashba Spin-Orbit Coupled Fermi Gas
Juraj Radic, Stefan S. Natu, Victor Galitski



We consider the time evolution of the magnetization in a Rashba spin-orbit-coupled Fermi gas, starting from a fully-polarized initial state. We model the dynamics using a Boltzmann equation, which we solve in the Hartree-Fock approximation. The resulting non-linear system of equations gives rise to three distinct dynamical regimes with qualitatively different asymptotic behaviors of the magnetization at long times. The distinct regimes and the transitions between them are controlled by the interaction strength: for weakly interacting fermions, the magnetization decays to zero. For intermediate interactions, it displays undamped oscillations about zero and for strong interactions, a partially magnetized state is dynamically stabilized. The dynamics we find is a spin analog of interaction induced self-trapping in double-well Bose Einstein condensates. The predicted phenomena can be realized in trapped Fermi gases with synthetic spin-orbit interactions.




Oct 29
1. arXiv:1310.7557 [pdf, ps, other]
Majorana fermions in three dimensional ultracold fermionic optical lattices
Chunlei Qu, Ming Gong, Yong Xu, Sumanta Tewari, Chuanwei Zhang


The realization of spin-orbit coupling (SOC) in ultracold atomic gases has opened the door for observing Majorana fermions (MFs) in cold atom systems. We show that MFs exist in three dimensional (3D) fermionic optical lattices with strictly one dimensional (1D) SOC which has already been realized in experiments. The presence of an in-plane Zeeman field drives the system from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Fulde-Ferrell (FF) superfluid phase. We find that both phases support multiple MFs at each end of quasi-one dimensional (quasi-1D) optical lattices with a weak transverse tunneling. In the generalization to 3D, the multiple MFs form a zero energy flat band. Our results are useful to guide the experimentalists on searching for MFs in the context of ultracold fermionic atoms.

2. arXiv:1310.7283 [pdf, other]
Fragility of the fractional quantum spin Hall effect in quantum gases
O. Fialko (1), J. Brand (1), U. Zuelicke (2) ((1) Massey University, (2) Victoria University of Wellington)




We consider the effect of contact interaction in a prototypical quantum spin Hall system of pseudo-spin-1/2 particles. A strong effective magnetic field with opposite directions for the two spin states restricts two-dimensional particle motion to the lowest Landau level. While interaction between same-spin particles leads to incompressible correlated states at fractional filling factors as known from the fractional quantum Hall effect, these states are destabilized by interactions between opposite spin particles. Exact results for two particles with opposite spin reveal a quasi-continuous spectrum of extended states with a large density of states at low energy. This has implications for the prospects of realizing the fractional quantum spin Hall effect in electronic or ultra-cold atom systems. Numerical diagonalization is used to extend the two-particle results to many bosonic particles and trapped systems. The interplay between an external trapping potential and spin-dependent interactions is shown to open up new possibilities for engineering exotic correlated many-particle states with ultra-cold atoms.

3. arXiv:1310.7086 [pdf, ps, other]
Spatiotemporal Binary Interaction and Designer quasi particle condensates
R.Radha, P.S.Vinayagam, H.J.Shin, K.Porsezian

We introduce a new integrable model to investigate the dynamics of two component quasi particle condensates with spatio temporal interaction strengths. We derive the associated Lax-pair of the coupled GP equation and construct matter wave solitons. We show that the spatio temporal binary interaction strengths not only facilitate the stabilization of the condensates, but also enables one to fabricate condensates with desirable densities, geometries and properties leading to the so called "designer quasi particle condensates".


Oct 28

1. arXiv:1310.6989 [pdf, ps, other]
Goldstone-mode Instability toward Fragmentation in a Spinor Bose-Einstein Condensation
Yuki Kawaguchi

We apply the number-conserving Bogoliubov theory to spinor Bose-Einstein condensates and show that the Goldstone magnon leads instability toward fragmentation. Different from the dynamical instability, where modes with complex eigenfrequencies grow exponentially, the zero-energy mode exhibits algebraic growth. We also point out that a small fraction of thermally excited atoms enhances the fragmentation dynamics.

2. arXiv:1310.6785 [pdf, ps, other]
Dynamics in spinor condensates controlled by a microwave dressing field
L. Zhao, J. Jiang, T. Tang, M. Webb, Y. Liu

We experimentally study spin dynamics in a sodium antiferromagnetic spinor condensate with off-resonant microwave pulses. In contrast to a magnetic field, a microwave dressing field enables us to explore rich spin dynamics under the influence of a negative net quadratic Zeeman shift qnet. We find an experimental signature to determine the sign of qnet, and observe harmonic spin population oscillations at every qnet except near each separatrix in phase space where spin oscillation period diverges. In the negative and positive qnet regions, we also observe a remarkably different relationship between each separatrix and the magnetization. Our data confirms an important prediction derived from the mean-field theory: spin-mixing dynamics in spin-1 condensates substantially depends on the sign of the ratio of qnet and the spin-dependent interaction energy. This work may thus be the first to use only one atomic species to reveal mean-field spin dynamics, especially the separatrix, which are predicted to appear differently in spin-1 antiferromagnetic and ferromagnetic spinor condensates.

3. arXiv:1310.6771 [pdf, other]
Instabilities and "phonons" of optical lattices in hollow optical fibers
N.K. Wilkin, J.M.F. Gunn



Instabilities are predicted for a sufficiently long hollow photonic optical fiber, or "cavity", containing a one dimensional Bose-gas in the presence of a classical, far red-detuned, confined weak electromagnetic mode. We examine both a single beam with Bose gas (a type of Brillouin instability) and the case of a standing wave, or optical lattice. The instabilities of these driven systems have pronounced spatial structure, of combined modulational instabilities in the electromagnetic and Bose density fields. Near the critical wave vectors for the instability the coupled modes of the BEC and light can be interpreted as "phonons" of the optical lattice. We conjecture these spatially-structured instabilities for the optical lattice, which we predict at weak fields, develop into the source of spatially homogeneous heating predicted for strong fields.

Oct 21-Oct 25 Bo Liu
Oct 25

1. arXiv:1310.6722 [pdf, ps, other]
Quantum and thermal fluactuations of three dimensional dipolar Bose gas
Abdelâali Boudjemâa
We investigate quantum and thermal fluctuations in dilute three dimensional dipolar Bose gas employing the generalized Bogoliubov theory. We calculate fluctuation corrections to the ground state energy, equation of state, compressibility, and superfluid fraction. It is shown that the presence of the dipole-dipole (DDI) strongly enhances fluctuations, and we identify the validity criterion of the Bogoliubov approach both at zero and finite temperatures for Bose-condensed dipolar gases. At finite temperatures, the effect of thermal fluctuations is stronger and it may lead to a large depletion and compressibility on one hand, and small superfluid fraction of the gas on the other.

2. arXiv:1310.6350 [pdf, other]
Using an artificial electric field to create the analog of the red spot of Jupiter in light-heavy Fermi-Fermi mixtures of ultracold atoms
H. F. Fotso, J. Vicente, J. K. Freericks
Time-of-flight images are a common tool in ultracold atomic experiments, employed to determine the quasimomentum distribution of the interacting particles. If one introduces a constant artificial electric field, then the quasimomentum distribution evolves in time as Bloch oscillations are generated in the system and then damped showing a complex series of patterns. Surprisingly, in different mass Fermi-Fermi mixtures, these patterns can survive for long times, and resemble the stability of the red spot of Jupiter in classical nonlinear hydrodynamics. In this work, we illustrate the rich phenomena that can be seen in these driven quantum systems.



Oct 24


1.arXiv:1310.6105 [pdf, ps, other]
Singlet Mott State Simulating the Bosonic Laughlin Wave Function
Biao Lian, Shou-Cheng Zhang
We study properties of a class of spin singlet Mott states for arbitrary spin S bosons on a lattice, with particle number per cite n=S/l+1, where l is a positive integer. We show that such a singlet Mott state can be mapped to a bosonic Laughlin wave function on the sphere with a finite number of particles at filling {\nu}=1/2l. Bosonic spinons, particle and hole excitations in the Mott state are discussed, among which the hole excitation can be mapped to the quasi-hole of the bosonic Laughlin wave function. We show that this singlet Mott state can be realized in a cold atom system on optical lattice, and can be identified using Bragg spectroscopy and Stern-Gerlach techniques. This class of singlet Mott states may be generalized to simulate bosonic Laughlin states with filling {\nu}=q/2l.


Oct 23
1. arXiv:1310.5800 [pdf, other]
Fermionic Functional Renormalization Group Approach to Superfluidity
Yuya Tanizaki, Gergely Fejős, Tetsuo Hatsuda
Fermionic functional renormalization group is applied to describe the superfluid phase transition of the two-component fermionic system with attractive contact interaction. Connection between the fermionic FRG approach and the conventional Bardeen-Cooper-Schrieffer (BCS) theory with Gorkov and Melik-Barkhudarov (GMB) correction are clarified in the weak coupling region by using the renormalization group flow of the fermionic four-point vertex with particle-particle and particle-hole scattering contributions. The FRG flow of the fermion self-energy, which is important to go beyond the BCS theory + GMB correction, is also studied. The superfluid transition temperature and the associated chemical potential are calculated in the region of the negative scattering length on the basis of the fermionic FRG approach.



Oct 22

1. arXiv:1310.5490 [pdf, ps, other]
Pseudogap phenomenon and effects of population imbalance in the normal state of a unitary Fermi gas
Takashi Kashimura, Ryota Watanabe, Yoji Ohashi
We investigate strong-coupling corrections to single-particle excitations in the normal state of a spin-polarized unitary Fermi gas. Within the framework of an extended T-matrix approximation, we calculate the single-particle density of states, as well as the single-particle spectral weight, to show that the so-called pseudogap phenomenon gradually disappears with increasing the magnitude of an effective magnetic field. In the highly spin-polarized regime, the calculated spin-polarization rate as a function of the effective magnetic field agrees well with the recent experiment on a 6Li Fermi gas. Although this experiment has been considered to be incompatible with the existence of the pseudogap in an unpolarized Fermi gas, our result clarifies that the observed spin-polarization rate in the highly spin-polarized regime and the pseudogap in the unpolarized limit can be explained in a consistent manner, when one correctly includes effects of population imbalance on single-particle excitations. Since it is a crucial issue to clarify whether the pseudogap exists or not in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas, our results would be useful for the understanding of this strongly interacting fermion system.


2. arXiv:1310.5180 [pdf, ps, other]
Weyl spin-orbit-coupling-induced interactions in uniform and trapped atomic quantum fluids
Reena Gupta, G. S. Singh, Jürgen Bosse
We establish through analytical and numerical studies of thermodynamic quantities for noninteracting atomic gases that the isotropic three-dimensional spin-orbit coupling, the Weyl coupling, induces interaction which counters "effective" attraction (repulsion) of the exchange symmetry present in zero-coupling Bose (Fermi) gas. The exact analytical expressions for the grand potential and hence for several thermodynamic quantities have been obtained for this purpose in both uniform and trapped cases. It is enunciated that many interesting features of spin-orbit coupled systems revealed theoretically can be understood in terms of coupling-induced modifications in statistical interparticle potential. The temperature-dependence of the chemical potential, specific heat and isothermal compressibility for a uniform Bose gas is found to have signature of the incipient Bose-Einstein condensation in very weak coupling regime although the system does not really go in the Bose-condensed phase. The transition temperature in harmonically trapped case decreases with increase of coupling strength consistent with the weakening of the statistical attractive interaction. Anomalous behavior of some thermodynamic quantities, partly akin to that in dimensions less than two, appears for uniform fermions as soon as the Fermi level goes down the Dirac point on increasing the coupling strength. It is suggested that the fluctuation-dissipation theorem can be utilized to verify anomalous behaviors from studies of long-wavelength fluctuations in bunching and antibunching effects.




Oct 21

1. arXiv:1310.4967 [pdf, other]
Bloch oscillations and quench dynamics of interacting bosons in an optical lattice
K. W. Mahmud, L. Jiang, E. Tiesinga, P. R. Johnson
We study the dynamics of interacting superfluid bosons in a one dimensional vertical optical lattice after a sudden increase of the lattice potential depth. We show that this system can be exploited to investigate the effects of strong interactions on Bloch oscillations. We perform theoretical modelling of this system, identify experimental challenges and explore a new regime of Bloch oscillations characterized by interaction-induced matter-wave collapse and revivals which modify the Bloch oscillations dynamics. In addition, we study three dephasing mechanisms: effective three-body interactions, finite value of tunneling, and a background harmonic potential. We also find that the center of mass motion in the presence of finite tunneling goes through collapse and revivals, giving an example of quantum transport where interaction induced revivals are important. We quantify the effects of residual harmonic trapping on the momentum distribution dynamics and show the occurrence of interaction-modified temporal Talbot effect. Finally, we analyze the prospects and challenges of exploiting Bloch oscillations of cold atoms in the strongly
interacting regime for precision measurement of the gravitational acceleration g.

.


2.arXiv:1310.4983 [pdf, ps, other]
Non-equilibrium topological phase transitions in two-dimensional optical lattices
Masaya Nakagawa, Norio Kawakami
Recently, concepts of topological phases of matter are extended to non-equilibrium systems, especially periodically driven systems. In this paper, we construct a model which shows non-equilibrium topological phase transitions using a simple phenomenon in cold-atomic systems. We show that the celebrated Rabi oscillation has the possibility to tune the band structure in fermionic optical lattices and thereby drives non-equilibrium topological phase transitions. Furthermore we study this non-equilibrium topological phase using time-dependent Schrieffer-Wolff-type perturbation theory, and reveal analytically how the Rabi oscillation changes the effective band structure in optical lattices.


Oct 14-Oct 18 Zhifang Xu
Oct 18
1.arXiv:1310.4551 [pdf, ps, other]
Topological nature of magnetization plateaus in periodically modulated quantum spin chains
Haiping Hu, Chen Cheng, Zhihao Xu, Hong-Gang Luo, Shu Chen
Comments: 5 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)

We unveil nontrivial topological properties of zero-temperature magnetization plateau states in periodically modulated quantum spin chains under a uniform magnetic field. As positions of plateaus are uniquely determined by the modulation period of exchange couplings, we find that the topologically nontrivial plateau state can be characterized by a nonzero integer Chern number and has nontrivial edge excitations. Our study clarifies the topological origin of the well-known phenomena of quantized magnetization plateaus in one-dimensional quantum spin systems and relates the plateau state to the correlated topological insulator.

2. arXiv:1310.4525 (cross-list from cond-mat.mes-hall) [pdf, other]
Time-Reversal Invariant Topological Superconductivity Induced by Repulsive Interactions in Quantum Wires
Arbel Haim, Anna Keselman, Erez Berg, Yuval Oreg
Comments: 4 pages + supplementary material
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)

We consider a model for a one-dimensional quantum wire with Rashba spin-orbit coupling and repulsive interactions, proximity coupled to a conventional s-wave superconductor. Using a combination of Hartree-Fock and density matrix renormalization group calculations, we show that for sufficiently strong interactions in the wire, a time-reversal invariant topological superconducting phase can be stabilized in the absence of an external magnetic field. This phase supports two zero-energy Majorana bound states at each end, which are protected by time-reversal symmetry. The mechanism for the formation of this phase is a reversal of the sign of the effective pair potential in the wire, due to the repulsive interactions. We calculate the differential conductance into the wire and its dependence on an applied magnetic field using the scattering-matrix formalism. The behavior of the zero-bias anomaly as a function of the field direction can serve as a distinct experimental signature of the topological phase.

3.arXiv:1310.4798 [pdf, ps, other]
Broken discrete and continuous symmetries in two dimensional spiral antiferromagnets
A. Mezio, C. N. Sposetti, L. O. Manuel, A. E. Trumper
Comments: 17 pages, accepted for publication in Journal of Physics: condensed Matter
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study the occurrence of symmetry breakings, at zero and finite temperatures, in the J_1-J_3 antiferromagnetic Heisenberg model on the square lattice using Schwinger boson mean field theory. For spin-1/2 the ground state breaks always the SU(2) symmetry with a continuous quasi-critical transition at J_3/J_1=0.38, from N\'eel to spiral long range order, although local spin fluctuations considerations suggest an intermediate disordered regime around 0.35 < J_3/J_1 < 0.5, in qualitative agreement with recent numerical results. At low temperatures we find a Z_2 broken symmetry region with short range spiral order characterized by an Ising-like nematic order parameter that compares qualitatively well with classical Monte Carlo results. At intermediate temperatures the phase diagram shows regions with collinear short range orders: for J_3/J_1<1 N\'eel (\pi,\pi) correlations and for J_3/J_1>1 a novel phase consisting of four decoupled third neighbour sublattices with N\'eel (\pi,\pi) correlations in each one. We conclude that the effect of quantum and thermal fluctuations is to favour collinear correlations even in the strongly frustrated regime.

Oct 17
1.arXiv:1310.4225 (cross-list from cond-mat.stat-mech) [pdf, ps, other]
Magnetic properties of two-dimensional charged spin-1 Bose gases
Yingxue Chen, Jihong Qin, Qiang Gu
Comments: 6 pages, 6 figures
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)

Within the mean-field theory, we investigate the magnetic properties of a charged spin-1 Bose gas in two dimension. In this system the diamagnetism competes with paramagnetism, where Lande-factor g is introduced to describe the strength of the paramagnetic effect. The system presents a crossover from diamagnetism to paramagnetism with the increasing of Lande-factor. The critical value of the Lande-factor, g_{c}, is discussed as a function of the temperature and magnetic field. We get the same value of g_{c} both in the low temperature and strong magnetic field limit. Our results also show that in very weak magnetic field no condensation happens in the two dimensional charged spin-1 Bose gas.

2. arXiv:1310.4405 [pdf, ps, other]
Terahertz out-of-plane resonances due to spin-orbit coupling
K. Morawetz
Comments: 6 pages 5 pictures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

A microscopic kinetic theory is developed which allows to investigate non-Abelian SU(2) systems interacting with meanfields and spin-orbit coupling under magnetic fields in one, two, and three dimensions. The coupled kinetic equations for the scalar and spin components are presented and linearized with respect to an external electric field. The dynamical classical and quantum Hall effect are described in this way as well as the anomalous Hall effect where a new symmetric dynamical contribution to the conductivity is presented. The coupled density and spin response functions to an electric field are derived including arbitrary magnetic fields. The magnetic field induces a staircase structure at frequencies of the Landau levels. It is found that for linear Dresselhaus and Rashba spin-orbit coupling a dynamical out-of-plane spin response appears at these Landau level frequencies establishing terahertz resonances.

Oct 16
1.arXiv:1310.4100 [pdf, ps, other]
Anisotropic Weyl Fermions from Quasiparticle Excitation Spectrum of a 3D Fulde-Ferrell Superfluid
Yong Xu, Ruilin Chu, Chuanwei Zhang
Comments: 2 pages of supplementary material to present the calculation details of sound speeds. 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Weyl fermions, first proposed for describing massless chiral Dirac fermions in particle physics, have not been observed yet in experiments. Recently, much effort has been devoted to explore Weyl fermions around band touching points of single particle energy dispersions in certain solid state materials (named \textit{Weyl semimetals}), similar as graphene for Dirac fermions. Here we show that such Weyl semimetals also exist in the quasiparticle excitation spectrum of a three-dimensional (3D) spin-orbit coupled Fulde-Ferrell (FF) superfluid. By varying Zeeman fields, the properties of Weyl fermions, such as their creation and annihilation, number and position, as well as anisotropic linear dispersions around band touching points, can be tuned. We study the manifestation of anisotropic Weyl fermions in sound speeds of FF fermionic superfluids, which are detectable in experiments.

2.arXiv:1310.3840 [pdf, ps, other]
Do phase fluctuations influence the Fulde-Ferrell-Larkin-Ovchinnikov state in a 3D Fermi gas?
Jeroen P.A. Devreese, Jacques Tempere
Comments: 23 pages, 2 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

In ultracold Fermi gases, the effect of spin-imbalance on superfluidity has been the subject of intense study. One of the reasons for this is that spin-imbalance frustrates the Bardeen-Cooper-Schrieffer (BCS) superfluid pairing mechanism, in which fermions in different spin states combine into Cooper pairs with zero momentum. In 1964, it was proposed that an exotic superfluid state called the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in which the Cooper pairs have nonzero momentum, could exist in a spin-imbalanced Fermi gas. At the saddle-point (mean field) level, it has been shown that the FFLO state only occupies a very small sliver in the ground state phase diagram of a 3D Fermi gas. However, a question that remains to be investigated is: what is the influence of phase fluctuations around the saddle point on the FFLO state? In this work we show that phase fluctuations only lead to relatively small quantitative corrections to the presence of the FFLO state in the saddle-point phase diagram of a 3D spin-imbalanced Fermi gas. Starting from the partition function of the system, we calculate the effective action within the path-integral adiabatic approximation. The action is then expanded up to second order in the fluctuation field around the saddle point, leading to the fluctuation free energy. Using this free energy, we calculate corrections due to phase fluctuations to the BCS-FFLO transition in the saddle-point phase diagram. At temperatures at which the FFLO state exists, we find only small corrections to the size of the FFLO area. Our results suggest that fluctuations of the phase of the FFLO order parameter, which can be interpreted as an oscillation of its momentum vector, do not cause an instability of the FFLO state with respect to the BCS state.

Oct 15
1. arXiv:1310.3536 [pdf, ps, other]
Symmetry Protected Topological Charge in Symmetry Broken Phase: Spin-Chern, Spin-Valley-Chern and Mirror-Chern Numbers
Motohiko Ezawa
Comments: 5 pages, 1 figure
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

The Chern number is a genuine topological number. On the other hand, a symmetry protected topological (SPT) charge is a topological number only when a symmetry exists. We propose a formula for the SPT charge as a derivative of the Chern number in terms of the Green function in such a way that it is valid even when the symmetry is broken. We estimate the amount of deviation from the quantized value as a function of the strength of the broken symmetry. They are observable by measuring the associated Hall currents. We present two examples. First, we consider Dirac electrons with the spin-orbit interaction on honeycomb lattice, where the SPT charges are given by the spin-Chern, valley-Chern and spin-valley-Chern charges. Though the spin-Chern charge is not quantized in the presence of the Rashba interaction, the deviation is estimated to be $10^{-7}$ for the spin-Chern insulator in the case of silicene, a silicon cousin of graphene. Second, we analyze the effect of the mirror-symmetry breaking of the mirror-Chern number in a thin-film of topological crystalline insulator.

2. arXiv:1310.3559 [pdf, ps, other]
Probing the critical exponent of superfluid fraction in a strongly interacting Fermi gas
Hui Hu, Xia-Ji Liu
Comments: 5 pages,5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We theoretically investigate the critical behavior of second sound mode in a harmonically trapped ultracold atomic Fermi gas with resonant interactions. Near the superfluid phase transition with critical temperature $T_{c}$, the frequency or the sound velocity of second sound mode depends crucially on the critical exponent $\beta$ of superfluid fraction. In an isotropic harmonic trap, we predict that the mode frequency diverges like $(1-T/T_{c})^{\beta-1/2}$ when $\beta<1/2$. In a highly elongated trap, the speed of second sound reduces by a factor $1/\sqrt{2\beta+1}$ from that in a homogeneous three-dimensional superfluid. Our prediction could be readily tested by measurements of second sound wave propagation in a setup such as that exploited by Sidorenkov \textit{et al.} {[}Nature \textbf{498}, 78 (2013){]} for resonantly interacting lithium-6 atoms, once the experimental precision is improved.

3. arXiv:1310.3654 [pdf, ps, other]
Jump of first and second sound velocities at the Berezinskii-Kosterlitz-Thouless transition
Tomoki Ozawa, Sandro Stringari
Comments: 5 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We calculate the temperature dependence of the first and second sound velocities in the superfluid phase of a 2D dilute Bose gas by solving Landau's two fluid hydrodynamic equations. We predict the occurrence of a significant discontinuity in both velocities at the critical temperature, as a consequence of the jump of the superfluid density characterizing the Berezinskii-Kosterlitz-Thouless transition. The key role of the thermal expansion coefficient is discussed. We find that second sound in this dilute Bose gas can be easily excited through a density perturbation, thereby making the perspective of the measurement of the superfluid density particularly favorable.

4. arXiv:1310.3705 [pdf, other]
Quantum magnetism without lattices in strongly-interacting one-dimensional spinor gases
F. Deuretzbacher, D. Becker, J. Bjerlin, S. M. Reimann, L. Santos
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)

We show that strongly-interacting multicomponent gases in one dimension (1D) can be described by an effective spin model. This constitutes a surprisingly simple scenario for the realization of one-dimensional quantum magnetism in cold gases in the absence of an optical lattice. The spin-chain model allows for an intuitive understanding of recent experiments and for a simple calculation of relevant observables. We analyze the adiabatic preparation of antiferromagnetic (AF) and ferromagnetic (F) ground states, and show that many-body spin states may be efficiently probed by means of tunneling experiments. The spin-chain model is valid for more than two components, opening the possibility of realizing SU(N) quantum magnetism in strongly-interacting one-dimensional alkaline-earth or Ytterbium Fermi gases.

5. arXiv:1310.3709 [pdf, ps, other]
Quantum Hall phases of two-component bosons
Tobias Graß, David Raventós, Maciej Lewenstein, Bruno Juliá-Díaz
Comments: 4.5 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)

The recent production of synthetic magnetic fields acting on electroneutral particles, like atoms or photons, has boosted the interest in the quantum Hall physics of bosons. Adding pseudospin-1/2 to the bosons greatly enriches the scenario, as it allows them to form an interacting integer quantum Hall (IQH) phase with no fermionic counterpart. Here we show that, for a small two-component Bose gas on a disk, the complete strongly correlated regime, extending from the integer phase at filling factor $\nu=2$ to the Halperin phase at filling factor $\nu=2/3$, is well described by composite fermionization of the bosons. Moreover we study the edge excitations of the IQH state, which, in agreement with expectations from topological field theory, are found to consist of forward-moving charge excitations and backward-moving spin excitations. Finally, we demonstrate how pair-correlation functions allow one to experimentally distinguish the IQH state from competing states, like non-Abelian spin singlet (NASS) states.

Oct 14
1.arXiv:1310.3159 [pdf, other]
Quantum phase transitions of topological insulators with and without gap closing
Stephan Rachel
Comments: 6 pages, 3 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We consider two-dimensional topological insulators as prototypes of symmetry protected topological phases where spin-orbit coupling gives rise to the topological insulator phase. The appearance of additional non-topological terms might cause quantum phase transitions into trivial phases when this topologically trivial term overweights the spin-orbit coupling. Usually such a phase transition is associated with closing of the bulk gap. In contrast, time-reversal symmetry breaking terms immediately destroy the topological phase without closing of the bulk gap unless the axial spin symmetry remains preserved. These findings are discussed in the context of topological insulators in the presence of interactions.
Oct 7-Oct 11, Zhenyu Zhou,
Sep 30-Oct 4, Li-Jun Lang
Oct 4
1. arXiv:1310.0846 [pdf, other]
Stationary entanglement entropies following an interaction quench in 1D Bose gas
Mario Collura, Márton Kormos, Pasquale Calabrese
Comments: 23 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)

We analyze the entanglement properties of the asymptotic steady state after a quench from free to hard-core bosons in one dimension. The R\'enyi and von Neumann entanglement entropies are found to be extensive, and the latter coincides with the thermodynamic entropy of the Generalized Gibbs Ensemble (GGE). Computing the spectrum of the two-point function, we provide exact analytical results both for the leading extensive parts and the subleading terms for the entropies as well as for the cumulants of the particle number fluctuations. We also compare the extensive part of the entanglement entropy with the thermodynamic ones, showing that the GGE entropy equal the entanglement one and it is the double of the diagonal entropy.

Oct 3
1. arXiv:1310.0600 [pdf, other]
Tuning superfluid phases of spin-1 bosons in cubic optical lattice with linear Zeeman effect
Mohamed Mobarak, Axel Pelster
Subjects: Quantum Gases (cond-mat.quant-gas)

We analyze theoretically a spinor Bose gas loaded into a three-dimensional cubic optical lattice. In order to account for different superfluid phases of spin-1 bosons in the presence of an external magnetic field, we work out a Ginzburg-Landau theory for the underlying spin-1 Bose-Hubbard model. In particular at zero temperature, we determine both the Mott and the superfluid phases for the competition between the anti-ferromagnetic interaction and the linear Zeeman effect within the validity range of the Ginzburg-Landau theory. Moreover, we find that the phase transition between the superfluid and Mott insulator phases is of second order and that the transitions between the respective superfluid phases for anti-ferromagnetic interaction can be both of first and second order.

Oct 2

1. arXiv:1310.0391 [pdf]
Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi
Z. K. Liu, B. Zhou, Z. J. Wang, H. M. Weng, D. Prabhakaran, S. -K. Mo, Y. Zhang, Z. X. Shen, Z. Fang, X. Dai, Z. Hussain, Y. L. Chen
Comments: 4 figures in main text; 6 figures in supporting materials
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Other Condensed Matter (cond-mat.other)

Three-dimensional (3D) topological Dirac semimetals (TDSs) represent a novel state of quantum matter that can be viewed as '3D graphene'. In contrast to two-dimensional (2D) Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. The TDS is also an important boundary state mediating numerous novel quantum states, such as topological insulators, Weyl semi-metals, Axion insulators and topological superconductors. By investigating the electronic structure of Na3Bi with angle resolved photoemission spectroscopy, we discovered 3D Dirac fermions with linear dispersions along all momentum directions for the first time. Furthermore, we demonstrated that the 3D Dirac fermions in Na3Bi were protected by the bulk crystal symmetry. Our results establish that Na3Bi is the first model system of 3D TDSs, which can also serve as an ideal platform for the systematic study of quantum phase transitions between rich novel topological quantum states.

2. arXiv:1310.0010 [pdf]
Macroscopic coherence between quantum condensates formed at different times
Alex Hayat, Christoph Lange, Lee A. Rozema, Rockson Chang, Shreyas Potnis, Henry M. van Driel, Aephraim M. Steinberg, Mark Steger, David W. Snoke, Loren N. Pfeiffer,Kenneth W. West
Subjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
We demonstrate macroscopic coherence between quantum condensates generated at different times, separated by more than the particle dephasing time. This is possible due to the dressed light-matter nature of exciton-polaritons, which can be injected resonantly by optical excitation at well-defined momenta. We show that the build-up of coherence between condensates depends on the interaction between the particles, particle density, as well as temperature despite the non-equilibrium nature of the condensate, whereas the mass of the particles plays no role in the condensation of resonantly injected polaritons. This experiment also makes it possible for us to measure directly the large nonlinear phase shift resulting from the polariton-polariton interaction energy. Our results provide direct evidence for coherence between different condensates and demonstrate a new approach for probing their ultrafast dynamics, opening new directions in the study of matter coherence as well as in practical applications such as quantum information and ultrafast logic.

3. arXiv:1310.0134 [pdf, other]
Observation of a Geometric Hall Effect in a Spinor Bose-Einstein Condensate
Jae-yoon Choi, Seji Kang, Sang Won Seo, Woo Jin Kwon, Yong-il Shin
Comments: 8 pages, 9 figuresSubjects: Quantum Gases (cond-mat.quant-gas)
For a spin-carrying particle moving in a spatially varying magnetic field, effective electromagnetic forces can arise from the geometric phase associated with its spin's adiabatic following of the magnetic field. We report the observation of a geometric Hall effect in a spinor Bose-Einstein condensate possessing a skyrmion spin texture. Under translational oscillations of the spin texture, the condensate resonantly develops a circular motion in a harmonic trap, demonstrating the existence of an effective Lorentz force. When the condensate circulates in the inhomogeneous effective magnetic field, quantized vortices are nucleated in the boundary region of the condensate and the vortex number reaches over 100 without significant heating.

Oct 1

1. arXiv:1309.7797 [pdf, ps, other]
Perfect screening of the inter-polaronic interaction
Shimpei Endo, Masahito Ueda
Comments: 5 pages, 1 figure
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)

We consider heavy particles immersed in a Fermi sea of light fermions, and study the interaction between the heavy particles induced by the surrounding light fermions. With the Born-Oppenheimer method, we analytically show that the induced interaction between N heavy particles vanishes for any N in the limit of high light-fermion density. The induced interaction vanishes even in the unitarity regime. This suggests that the formation of N-body bound states associated with the Efimov effect is suppressed in the presence of the dense Fermi sea. We ascribe the vanishing induced interaction to the screening effect in the neutral Fermi system.

2. arXiv:1309.7711 [pdf, ps, other]
Valley-dependent gauge fields for ultracold atoms in square optical superlattices
Dan-Wei Zhang, Chuan-Jia Shan, Feng Mei, Mou Yang, Rui-Qiang Wang, Shi-Liang Zhu
Comments: 5 pages, 2 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

We propose an experimental scheme to realize valley-dependent gauge fields in a spin-dependent square optical lattice trapping ultracold fermonic atoms by means of the laser-assisted tunneling (LAT) method. A couple of Raman beams are used to engineer LAT for cold atoms with a staggered \pi-flux configuration, leading to an effective relativistic Hamiltonian near two inequivalent Dirac points. Another couple or triple of Raman beams with proper Rabi frequencies are added to modulate the atomic hopping parameters. Within smooth perturbations, the hopping-modulations give rise to effective gauge potentials with opposite signs near two valleys, mimicking the interesting strain-induced pseudo-gauge fields in graphene. The tunable valley-dependent gauge fields in this system provide potential applications for quantum vally Hall effects and atomic valleytronics.

3. arXiv:1309.7449 [pdf, other]
Superfluid phases of spin-1 bosons in cubic optical lattice
Mohamed Mobarak, Axel Pelster
Comments: Laser Physics Letters (in press)Subjects: Quantum Gases (cond-mat.quant-gas)

We analyze theoretically the emergence of different superfluid phases of spin-1 bosons in a three-dimensional cubic optical lattice by generalizing the recently developed Ginzburg-Landau theory for the Bose-Hubbard model to a spinor Bose gas. In particular at zero temperature, our theory distinguishes within its validity range between various superfluid phases for an anti-ferromagnetic interaction with an external magnetic field. In addition, we determine that the superfluid-Mott insulator phase transition is of second order and that the transitions between the respective superfluid phases with anti-ferromagnetic interaction can be both of first and second order.

4. arXiv:1309.7978 [pdf]

Experimental Realization of a Three-Dimensional Dirac Semimetal
Sergey Borisenko, Quinn Gibson, Danil Evtushinsky, Volodymyr Zabolotnyy, Bernd Buechner, Robert J. Cava
Comments: Submitted on the 27th of September 2013Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el)
The three dimensional (3D) Dirac semimetal, which has been predicted theoretically, is a new electronic state of matter. It can be viewed as 3D generalization of graphene, with a unique electronic structure in which conduction and valence band energies touch each other only at isolated points in momentum space (i.e. the 3D Dirac points), and thus it cannot be classified either as a metal or a semiconductor. In contrast to graphene, the Dirac points of such a semimetal are not gapped by the spin-orbit interaction and the crossing of the linear dispersions is protected by crystal symmetry. In combination with broken time-reversal or inversion symmetries, 3D Dirac points may result in a variety of topologically non-trivial phases with unique physical properties. They have, however, escaped detection in real solids so far. Here we report the direct observation of such an exotic electronic structure in cadmium arsenide (Cd3As2) by means of angle-resolved photoemission spectroscopy (ARPES). We identify two momentum regions where electronic states that strongly disperse in all directions form narrow cone-like structures, and thus prove the existence of the long sought 3D Dirac points. This electronic structure naturally explains why Cd3As2 has one of the highest known bulk electron mobilities. This realization of a 3D Dirac semimetal in Cd3As2 not only opens a direct path to a wide spectrum of applications, but also offers a robust platform for engineering topologically-nontrivial phases including Weyl semimetals and Quantum Spin Hall systems.

Sep 30

1. arXiv:1309.7219 [pdf, ps, other]
Fractional energy states of strongly-interacting bosons in one dimension
N. T. Zinner, A. G. Volosniev, D. V. Fedorov, A. S. Jensen, M. Valiente
Comments: 9 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Mathematical Physics (math-ph); Quantum Physics (quant-ph)

In one dimension bosons with strong repulsive short-range interaction have quantum mechanical behavior akin to fermions as shown in the famous works of Tonks and Girardeau (TG). In the limit of infinite short-range repulsion, the system is said to fermionize and the ground state wave function can be obtained from that of spinless fermions. Here we study two-component bosonic systems and show that in the regime of strong inter-component interactions, eigenstates exist at energies that are not sums of the single-particle energies with wave functions that are not related to that of spinless fermions. This is first demonstrated in an analytically solvable model for three equal mass particles, two of which are identical bosons, which is exact in the strongly-interacting limit. We verify our analytical results by presenting the first application of the stochastic variational method to this kind of system. The analytical model also demonstrates that the limit where both inter- and intra-component interactions become strong must be treated with extreme care as these limits do not commute. Moreover, we argue that general two-component systems with more than three particles will have eigenstates that are not related to the wave functions of spinless fermions. These new states can be probed using the same techniques that have recently been used for fermionic few-body systems in quasi-1D.

2. arXiv:1309.7269 [pdf, ps, other]
Tight-binding models in a quasiperiodic optical lattice
Anuradha Jagannathan, Michel Duneau
Comments: 5 pages, article submitted for the Proceedings of the International Conference on Quasicrystals CQ12 (Kracow, Poland)Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other)

We report on a two-dimensional quasiperiodic structure with eight-fold symmetry obtained by trapping atoms in an optical potential, for appropriately tuned experimental parameters. We briefly describe the geometrical properties of the structure, and comment on the tight-binding models for particles moving in this potential, closely related to models which have been studied on the well-known quasiperiodic octagonal (or Ammann-Beenker) tiling.