Sep 24 - Sep 28 Johannes Schachenmayer
Sep 28
1. arXiv:1209.6343 [pdf, other]
Evaporative cooling of the dipolar radical OH
Benjamin K. Stuhl, Matthew T. Hummon, Mark Yeo, Goulven Quéméner, John L. Bohn, Jun YeAtomic physics was revolutionized by the development of forced evaporative cooling: it led directly to the observation of Bose-Einstein condensation, quantum-degenerate Fermi gases, and ultracold optical lattice simulations of condensed matter phenomena. More recently, great progress has been made in the production of cold molecular gases, whose permanent electric dipole moment is expected to generate rich, novel, and controllable phases, dynamics, and chemistry in these ultracold systems. However, while many strides have been made in both direct cooling and cold-association techniques, evaporative cooling has not yet been achieved due to unfavorable elastic-to-inelastic ratios and impractically slow thermalization rates in the available trapped species. We now report the observation of microwave-forced evaporative cooling of hydroxyl (OH) molecules loaded from a Stark-decelerated beam into an extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at least an order of magnitude in temperature and three orders in phase-space density, limited only by the low-temperature sensitivity of our spectroscopic thermometry technique. With evaporative cooling and sufficiently large initial populations, much colder temperatures are possible, and even a quantum-degenerate gas of this dipolar radical -- or anything else it can sympathetically cool -- may now be in reach.
2. arXiv:1209.6197 [pdf, other]
Topological Wigner Crystal of Half-Solitons in a Spinor BEC
H. Terças, D. D. Solnyshkov, G. Malpuech
We consider a one-dimensional gas of half-solitons in a spinor Bose-Einstein condensate. We calculate the topological interaction potential between the half-solitons. Using a kinetic equation of the Vlasov-Boltzmann type, we model the coupled dynamics of the interacting solitons. We show that the dynamics of the system in the gaseous phase is marginally stable and spontaneously evolves towards a Wigner crystal.
Sep 27
1. arXiv:1209.5795 [pdf, other]
Non-equilibrium dynamics of Ising models with decoherence: an exact solution
Michael Foss-Feig, Kaden R. A. Hazzard, John J. Bollinger, Ana Maria ReyThe interplay between interactions and decoherence in many-body systems is of fundamental importance in quantum physics: Decoherence can degrade correlations, but can also give rise to a variety of rich dynamical and steady-state behaviors. We obtain an exact analytic solution for the non-equilibrium dynamics of Ising models with arbitrary interactions and subject to the most general form of local Markovian decoherence. Our solution shows that decoherence affects the relaxation of observables more than predicted by single-particle considerations. It also reveals a dynamical phase transition, specifically a Hopf bifurcation, which is absent at the single-particle level. These calculations are applicable to ongoing quantum information and emulation efforts using a variety of atomic, molecular, optical, and solid-state systems.
2. arXiv:1209.5935 [pdf, other]
Topological superfluid in a quasi-two-dimensional polarized Fermi gas: When and how the third dimension matters?
Ren Zhang, Fan Wu, Jun-Rong Tang, Guang-Can Guo, Wei Yi, Wei ZhangWe investigate the properties of a spin-orbit coupled quasi-two-dimensional Fermi gas with tunable s-wave interaction between the two spin species. By analyzing the two-body bound state, we find that the population of the excited states in the tightly-confined axial direction can be significant when the two-body binding energy becomes comparable or exceeds the axial confinement. Since the Rashba spin-orbit coupling that we study here tends to enhance the two-body binding energy, this effect can become prominent at unitarity or even on the BCS side of the Feshbach resonance. To study the impact of these excited modes along the third dimension, we adopt an effective two-dimensional Hamiltonian in the form of a two-channel model, where the dressed molecules in the closed channel consist of the conventional Feshbach molecules as well as the excited states occupation in the axial direction. With properly renormalized interactions between atoms and dressed molecules, we find that both the density distribution and the phase structure in the trap can be significantly modified near a wide Feshbach resonance. In particular, the stability region of the topological superfluid phase is increased. Our findings are helpful for the experimental search for the topological superfluid phase in ultra-cold Fermi gases, and have interesting implications for quasi-low-dimensional polarized Fermi gases in general.
3. arXiv:1209.5852 [pdf, ps, other]
Exact solutions to the spin-2 Gross-Pitaevskii equations
Zhi-Hai Zhang, Yong-Kai Liu, Shi-Jie YangWe present several exact solutions to the coupled nonlinear Gross-Pitaevskii equations which describe the motion of the one-dimensional spin-2 Bose-Einstein condensates. The nonlinear density-density interactions are decoupled by making use of the properties of Jacobian elliptical functions. The distinct time factors in each hyperfine state implies a "Lamor" procession in these solutions. Furthermore, exact time-evolving solutions to the time-dependent Gross-Pitaevskii equations are constructed through the spin-rotational symmetry of the Hamiltonian. The spin-polarizations and density distributions in the spin-space are analyzed.
4. arXiv:1209.5824 [pdf, ps, other]
Inverse Energy Cascade in Forced 2D Quantum Turbulence
Matthew T. Reeves, Thomas P. Billam, Brian P. Anderson, Ashton S. BradleyWe demonstrate an inverse energy cascade in a minimal model of forced 2D quantum vortex turbulence. We simulate the Gross-Pitaevskii equation for a moving superfluid subject to forcing by a stationary grid of obstacle potentials, and damping by a stationary thermal cloud. The forcing injects large amounts of vortex energy into the system at the scale of a few healing lengths. A regime of forcing and damping is identified where vortex energy is efficiently transported to large length scales via an inverse energy cascade associated with the growth of clusters of same-circulation vortices, a Kolmogorov scaling law in the kinetic energy spectrum over a substantial inertial range, and spectral condensation of kinetic energy at the scale of the system size. Our results provide clear evidence that the inverse energy cascade phenomenon, previously observed in a diverse range of classical systems, can also occur in quantum fluids.
Sep 26
1. arXiv:1209.5641 [pdf, other]
Tan contact and universal high momentum behavior of the fermion propagator in the BCS-BEC crossover
Igor Boettcher, Sebastian Diehl, Jan M. Pawlowski, Christof WetterichWe derive the universal high momentum factorization of the fermion self-energy in the BCS-BEC crossover of ultracold atoms using nonperturbative quantum field theoretical methods. This property is then employed to compute the Tan contact as a function of interaction strength, temperature, chemical potential and Fermi momentum. We clarify the mechanism of the factorization from an analysis of the self-consistent Schwinger-Dyson equation for the fermion propagator, and compute the perturbative contact on the BCS and BEC sides within this framework. A Functional Renormalization Group approach is then put forward, which allows to determine the contact over the whole crossover and, in particular, for the Unitary Fermi gas. We present numerical results from an implementation of the Renormalization Group equations within a basic truncation scheme.
2. arXiv:1209.5534 [pdf, other]
Non-equilibrium dynamics in Bose-Hubbard ladders
Wladimir Tschischik, Masudul Haque, Roderich MoessnerMotivated by a recent experiment on the non-equilibrium dynamics of interacting bosons in ladder-shaped optical lattices, we report exact calculations on the sweep dynamics of Bose-Hubbard systems in finite two-leg ladders. The sweep changes the energy bias between the legs linearly over a finite time. As in the experiment, we study the cases of [a] the bosons initially all in the lower-energy leg (ground state sweep) and [b] the bosons initially all in the higher-energy leg (inverse sweep). The approach to adiabaticity in the inverse sweep is intricate, as the transfer of bosons is non-monotonic as a function of both sweep time and intra-leg tunnel coupling. Our exact study provides explanations for these non-monotonicities based on features of the full spectrum, without appealing to concepts (e.g., gapless excitation spectrum) that are more appropriate for the thermodynamic limit. We also demonstrate and study Stueckelberg oscillations in the finite-size ladders.
3. arXiv:1209.5509 [pdf, ps, other]
Hydrodynamic Description of Spin-1 Bose-Einstein Condensates
Emi Yukawa, Masahito UedaWe establish a complete set of hydrodynamic equations for a spin-1 Bose-Einstein condensate (BEC), which are equivalent to the multi-component Gross-Pitaevskii equations and expressed in terms of only observable physical quantities: the spin density and the nematic (or quadrupolar) tensor in addition to the density and the mass current that appear in the hydrodynamic description of a scalar BEC. The obtained hydrodynamic equations involve a generalized Mermin-Ho relation that is valid regardless of the spatiotemporal dependence of the spin polarization. Low-lying collec- tive modes for phonons and magnons are reproduced by linearizing the hydrodynamic equations. We also apply the single-mode approximation to the hydrodynamic equations and find a complete set of analytic solutions.
4. arXiv:1209.5446 [pdf, other]
Bogoliubov theory on the disordered lattice
Christopher Gaul, Cord A. Müller
Quantum fluctuations of Bose-Einstein condensates trapped in disordered lattices are studied by inhomogeneous Bogoliubov theory. Weak-disorder perturbation theory is applied to compute the elastic scattering rate as well as the renormalized speed of sound in lattices of arbitrary dimensionality. Furthermore, analytical results for the condensate depletion are presented, which are in good agreement with numerical data.
Sep 25
1. arXiv:1209.5369 [pdf, other]
Dynamics of Phase Coherence Onset in Bose Condensates of Photons by Incoherent Phonon Emission
D. W. Snoke, S. M. GirvinRecent experiments with photons equilibrating inside a dye medium in a cavity have raised the question of whether Bose condensation can occur in a system with only incoherent interaction with phonons in a bath but without particle-particle interaction. Analytical calculations analogous to those done for a system with particle-particle interactions indicate that a system of bosons interacting only with incoherent phonons can indeed undergo Bose condensation and furthermore can exhibit spontaneous amplification of quantum coherence. We review the basic theory for these calculations.
2. arXiv:1209.5204 [pdf, ps, other]
Quantum dynamics and macroscopic quantum tunneling of two weakly coupled condensates
René John Kerkdyk, S. SinhaWe study the quantum dynamics of a Bose Josephson junction(BJJ) made up of two coupled Bose-Einstein condensates. Apart from the usual ac Josephson oscillations, two different dynamical states of BJJ can be observed by tuning the inter-particle interaction strength, which are known as '$\pi$-oscillation' with relative phase $\pi$ between the condensates and 'macroscopic self-trapped' (MST) state with finite number imbalance. By choosing appropiate intial state we study above dynamical branches quantum mechanically and compare with classical dynamics. The stability region of the '$\pi$-oscillation' is separated from that of 'MST' state at a critical coupling strength. Also a significant change in the energy spectrum takes place above the critical coupling strength, and pairs of (quasi)-degenerate excited states appear. The original model of BJJ can be mapped on to a simple Hamiltonian describing quantum particle in an 'effective potential' with an effective Planck constant. Different dynamical states and degenerate excited states in the energy spectrum can be understood in this 'effective potential' approach. Also possible novel quantum phenomena like 'macroscopic quantum tunneling'(MQT) become evident from the simple picture of 'effective potential'. We study decay of metastable '$\pi$-oscillation' by MQT through potential barrier. The doubly degenerate excited states in the energy spectrum are associated with the classically degenerate MST states with equal and opposite number imbalance. We calculate the energy splitting between these quasi-degenerate excited states due to MQT of the condensate between classically degenerate MST states.
3. arXiv:1209.4929 [pdf, ps, other]
Two-dimensional Bose gases near resonance: universal three-body effects
Mohammad S. Mashayekhi, Jean-Sebastien Bernier, Dmitry Borzov, Jun-Liang Song, Fei Zhou
We report in this Letter the results of our investigation of 2D Bose gases beyond the dilute limit emphasizing the role played by three-body scattering events. We demonstrate that a competition between three-body attractive interactions and two-body repulsive forces results in the chemical potential of 2D Bose gases to exhibit a maximum at a critical scattering length beyond which these quantum gases possess a negative compressibility. For larger scattering lengths, the increasingly prominent role played by three-body attractive interactions leads to an onset instability at a second critical value. The three-body effects studied here are universal, fully characterized by the effective 2D scattering length $a_{2D}$ (or the size of the 2D bound states) and are, in comparison to the 3D case, independent of three-body ultraviolet physics. We find, within our approach, the ratios of the contribution to the chemical potential due to three-body interactions to the one due to two-body to be 0.27 near the maximum of the chemical potential and 0.73 in the vicinity of the onset instability.
4. arXiv:1209.4912 [pdf, other]
Avalanche Mechanism for the Enhanced Loss of Ultracold Atoms
Christian Langmack, D. Hudson Smith, Eric Braaten
In several experiments with ultracold trapped atoms, a narrow loss feature has been observed near an {\it atom-dimer resonance}, at which there is an Efimov trimer at the atom-dimer threshold. The conventional interpretation of these loss features is that they are produced by the {\it avalanche mechanism}, in which the energetic atom and dimer from 3-body recombination undergo secondary elastic collisions that produce additional atoms with sufficient energy to escape from the trapping potential. We use Monte Carlo methods to calculate the average number of atoms lost and the average heat generated by recombination events in a Bose-Einstein condensate and in a thermal gas. We improve on previous models by taking into account the energy-dependence of the cross sections, the spacial structure of the atom cloud, and the elastic scattering of the atoms. We show that the avalanche mechanism cannot produce a narrow loss feature near the atom-dimer resonance. The number of atoms lost from a recombination event can be more than twice as large as the 3 that would be obtained in the absence of secondary collisions. However the resulting loss feature is broad and its peak is at a scattering length that is larger than the atom-dimer resonance and depends on the trap depth.
Sep 17 - Sep 21 Xiaopeng Li
Sep 21
1. arXiv:1209.4363 [pdf, other]
Dynamic stabilization of a quantum many-body system
T.M. Hoang, C.S. Gerving, B.J. Land, M. Anquez, C.D. Hamley, M.S. Chapman
We demonstrate dynamic stabilization of an unstable strongly interacting quantum many-body system by periodic manipulation of the phase of the collective states. The experiment employs a spin-1 atomic Bose condensate initialized to an unstable (hyperbolic) fixed point of the spin-nematic phase space, where subsequent free evolution gives rise to squeezing and quantum spin mixing. To stabilize the system, periodic microwave pulses are applied that manipulate the spin-nematic many-body fluctuations and limit their growth. The range of pulse periods and phase shifts for which the condensate can be stabilized is measured and the resulting stability diagram compares well with a linear stability analysis of the problem.
2.arXiv:1209.4399 [pdf, ps, other]
Three dimensional Symmetry Protected Topological Phase close to Antiferromagnetic Neel order
**Cenke Xu**
It is well-known that the Haldane phase of one-dimensional spin-1 chain is a symmetry protected topological (SPT) phase, which is described by a nonlinear Sigma model (NLSM) with a Theta-term at Theta = 2Pi. In this work we study a 3+1 dimensional SPT phase of SU(2N) antiferromagnetic spin system with a self-conjugate representation on every site. The spin ordered Neel phase of this system has a ground state manifold M = U(2N)/[U(N)xU(N)], and this system is described by a NLSM defined with manifold M. Since the fourth homotopy group Pi4[M] = Z for N > 1, this NLSM can naturally have a Theta-term. We will argue that when Theta = 2Pi this NLSM describes a SPT phase. We will also construct a lattice model for this SPT phase, whose long wavelength physics is precisely described by this NLSM with Theta = 2Pi. This SPT phase is protected by the SU(2N) spin symmetry, without assuming any lattice symmetry.
3.arXiv:1209.4573 [pdf, other]
Coherent Flow and Trapping of Polariton Condensates with Long Lifetime
Bryan Nelsen, Gangqiang Liu, Mark Steger, David W. Snoke, Ryan Balili, Ken West, Loren Pfeiffer
We report new results of Bose-Einstein condensation of polaritons in specially designed microcavities with very high quality factor, on the order of 10^6, giving polariton lifetimes of the order of 100 ps, which is much longer than their thermalization time. We see qualitatively different physics from previous experiments with polaritons with much shorter lifetime, in part because the longer lifetime in these new structures allows the polaritons to travel macroscopic distances up to a millimeter. We observe novel effects in three regimes of polariton density. At low density, we see classical ballistic motion over hundreds of microns. From this, we obtain a direct measurement of the mean free scattering time of the polaritons. At moderate density, we see coherent wave-like motion over long distance. This appears as a continuous transition, and can be described in terms of quasicondensation. At high density, we see a very sharp transition to a trapped state with very high coherence indicative of equilibrium BEC. We compare our results to previous experiments on transport of polaritons.
4. arXiv:1204.6574 (replaced) [pdf, other]
Simulating Compact Quantum Electrodynamics with ultracold atoms: Probing confinement and nonperturbative effects
Erez Zohar, J. Ignacio Cirac, Benni Reznik
Recently, there has been much interest in simulating quantum field theory effects of matter and gauge fields. In a recent work [Phys. Rev. Lett. 107, 275301 (2011)] a method for simulating compact Quantum Electrodynamics (cQED) using Bose-Einstein condensates has been suggested. We suggest an alternative approach, which relies on single atoms in an optical lattice, carrying 2l+1 internal levels, which converges rapidly to cQED as l increases. That enables the simulation of cQED in 2+1 dimensions in both the weak and the strong coupling regimes, hence allowing to probe confinement as well as other nonperturbative effects of the theory. We provide an explicit construction for the case l=1 which is sufficient for simulating the effect of confinement between two external static charges.
Sep 20
1. arXiv:1209.4076 [pdf, other]
Far from equilibrium quantum magnetism with ultracold polar molecules
Kaden R. A. Hazzard, Salvatore R. Manmana, Michael Foss-Feig, Ana Maria Rey
Recent theory has indicated how to emulate tunable models of quantum magnetism with ultracold polar molecules. Here we show that present molecule optical lattice experiments can accomplish three crucial goals for quantum emulation, despite currently being well below unit filling and not quantum degenerate. The first is to verify and benchmark the models proposed to describe these systems. The second is to prepare correlated and possibly useful states in well-understood regimes. The third is to explore many-body physics inaccessible to existing theoretical techniques. Our proposal relies on a non-equilibrium protocol that can be viewed either as Ramsey spectroscopy or an interaction quench. It uses only routine experimental tools available in any ultracold molecule experiment.
Sep 19
1.arXiv:1209.3947 [pdf, ps, other]
Self-energy feedback and frequency-dependent interactions in the functional renormalization group flow for the two-dimensional Hubbard model
Stefan Uebelacker, Carsten Honerkamp
We study the impact of including the self-energy feedback and frequency-dependent interactions on functional renormalization group grows for the two-dimensional Hubbard model on the square lattice at weak to moderate coupling strength. Previous studies using the functional renormalization group had ignored these two ingredients to large extent, and the question is how much the flows to strong coupling analyzed by this method depend on these approximations. Here we include the imaginary part of the self-energy on the imaginary axis and the frequency-dependence of the running interactions on a frequency mesh of 10 frequencies on the Matsubara axis. We find that i) the critical scales for the flows to strong coupling are shifted downwards by a factor that is usually of order one but can get larger in specific parameter regions, and ii) that the leading channel in this flow does not depend strongly on whether self-energies and frequency-dependence is included or not. We also discuss the main features of the self-energies developing during the flows.
2.arXiv:1209.3942 [pdf, other]
Probing thermoelectric effects with cold atoms
Charles Grenier, Corinna Kollath, **Antoine Georges**
We propose experimental protocols to reveal thermoelectric and thermal effects in the transport properties of ultracold fermionic atoms, using the two-terminal setup recently realized at ETH. We show in particular that, for two reservoirs having equal particle numbers but different temperatures initially, the observation of a transient particle number imbalance during equilibration is a direct evidence of thermoelectric (off-diagonal) transport coefficients. This is a time-dependent analogue of the Seebeck effect, and a corresponding analogue of the Peltier effect can be proposed. We reveal that in addition to the thermoelectric coupling of the constriction a thermoelectric coupling also arises due to the finite dilatation coefficient of the reservoirs. We present a theoretical analysis of the protocols, and assess their feasibility by estimating the corresponding temperature and particle number imbalances in realistic current experimental conditions.
3. arXiv:1209.3861 [pdf, ps, other]
Momentum-space instantons and maximally localized flat-band topological Hamiltonians
Chao-Ming Jian, Zheng-Cheng Gu, Xiao-Liang Qi
Recently, two-dimensional band insulators with a topologically nontrivial (almost) flat band has been studied extensively, which can realize integer and fractional quantum Hall effect in a system without an orbital magnetic field. Realizing a topological flat band generally requires longer range hoppings in a lattice Hamiltonian. It is natural to ask what is the minimal hopping range required.% for a topological flat-band Hamiltonian. In this paper, we prove that the mean hopping range of the flat-band Hamiltonian with Chern number $C_1$ and total number of bands $N$ has a universal lower bound of $\sqrt{4|C_1|/\pi N}$. Furthermore, for the Hamiltonians that reach this lower bound, the Bloch wavefunctions of the topological flat band are instanton solutions of a $CP(N-1)$ non-linear $\sigma$ model on the Brillouin zone torus, which are elliptic functions up to a normalization factor.
4. arXiv:1209.3823 [pdf, ps, other]
Mapping between finite temperature classical and zero temperature quantum systems: quantum critical jamming and quantum dynamical heterogeneities
Zohar Nussinov, Patrick Johnson, Alexander V. Balatsky, Matthias J. Graf
Many electronic systems (e.g., the cuprate superconductors and heavy fermions) exhibit striking features in their dynamical response over a prominent range of experimental parameters. While there are some empirical suggestions of particular increasing length scales that accompany such transitions in some cases, this identification is not universal and in numerous instances no large correlation length is evident. Using a correspondence between dissipative classical and quantum many-body systems, we show that, even in the absence of imposed disorder, many continuum systems (and possible lattice counterparts) may exhibit zero-point "quantum dynamical heterogeneities" wherein the dynamics, at a given instant, is spatially non-uniform. Towards this end, we extend a known mapping between finite temperature classical Fokker-Planck systems and quantum systems at zero temperature to include general non-equilibrium dynamics. While the static length scales accompanying this phenomenon do not seem to exhibit a clear divergence in standard correlation functions, the length scale of the dynamical heterogeneities can increase dramatically. We furthermore show how a hard core bosonic system can undergo a zero temperature quantum critical metal-to-insulator-type transition with an extremely large effective dynamical exponent z>4 that is consistent with length scales that increase far more slowly than the relaxation time as a putative critical transition is approached. We suggest ways to analyze experimental data in order to adduce such phenomena. Our approach may be applied to quenched quantum systems.
5. arXiv:1209.3816 [pdf, ps, other]
Large time dynamics and the generalized Gibbs ensemble
Victor Gurarie
We study the large time dynamics of quantum systems under a sudden quench. We show that, first of all, for a generic system in the thermodynamic limit the Gibbs distribution correctly captures its large time dynamics. For an integrable system, the generalized Gibbs ensemble captures its large time dynamics only if the system can be thought of as a number of noninteracting uncorrelated fermionic degrees of freedom.
Sep 18
1.arXiv:1209.3664 [pdf, ps, other]
Orbital Nematic Instability in Two-Orbital Hubbard Model: A Renormalization-Group Study
Masahisa Tsuchiizu, Seiichiro Onari, **Hiroshi Kontani**
Motivated by the nematic electronic fluid phase in Sr_{3}Ru_{2}O_{7}, we analyze the (d_{xz},d_{yz})-orbital Hubbard model by the one-loop renormalization-group method. It is confirmed that the present model exhibits the ferro-orbital instability near the magnetic or superconducting quantum criticality, due to the Aslamazov-Larkin type vertex corrections. This mechanism of orbital nematic order presents a natural explanation for the nematic order in Sr_{3}Ru_{2}O_{7}, and is expected to be realized in various multiorbital systems, such as Fe-based superconductors.
2.arXiv:1209.3538 [pdf, ps, other]
Surface criticality at a dynamic phase transition
Hyunhang Park, Michel Pleimling
In order to elucidate the role of surfaces at nonequilibrium phase transitions we consider kinetic Ising models with surfaces subjected to a periodic oscillating magnetic field. Whereas the corresponding bulk system undergoes a continuous nonequilibrium phase transition characterized by the exponents of the equilibrium Ising model, we find that the nonequilibrium surface exponents do not coincide with those of the equilibrium critical surface. In addition, in three space dimensions the surface phase diagram of the nonequilibrium system differs markedly from that of the equilibrium system.
3. arXiv:1209.3368 [pdf, ps, other]
Topological Fermi arcs
M.A. Silaev, G.E. Volovik
We consider the Fermi arcs emerging on the domain wall in the Weyl superfluid $^3$He-A and at the interface between $^3$He-A and the fully gapped topological superfluid $^3$He-B.
Sep 17
1. arXiv:1209.3058 [pdf, other]
Physics of three dimensional bosonic topological insulators: Surface Deconfined Criticality and Quantized Magnetoelectric Effect
Ashvin Vishwanath, T. Senthil
We discuss physical properties of `integer' topological phases of bosons in D=3+1 dimensions, protected by internal symmetries like time reversal and/or charge conservation. These phases invoke interactions in a fundamental way but do not possess topological order and are bosonic analogs of free fermion topological insulators and superconductors. Recently, the mathematical classification of such states was discussed in terms of cohomology theory . However, their physical properties remain mysterious. Here we develop a field theoretic description of several of these states and show that they possess unusual surface states, which, if gapped, must either break the underlying symmetry, or develop topological order. While this is the usual fate of the surface states, exotic gapless states can also be realized. For example, tuning parameters can naturally lead to a deconfined quantum critical point or, in other situations, a fully symmetric vortex metal phase. We discuss cases where the topological phases are characterized by quantized magnetoelectric response \theta, which, somewhat surprisingly, is an odd multiple of 2\pi. Two different theories of surface states are shown to capture these phenomena - the first is a nonlinear sigma model with a topological term. The second invokes vortices on the surface with fractional quantum numbers, that transform under a projective representation of the symmetry group. A bulk field theory consistent with these properties is identified, which is a multicomponent BF theory, supplemented, crucially, with a topological term. Bulk sigma model field theories of these phases are also provided. Topological phases that lie beyond the cohomology classification, characterized by the thermal analog of the quantized magnetoelectric effect, are also discussed.
2. arXiv:1209.3268 (cross-list from quant-ph) [pdf, other]
Entanglement enhancement in spatially inhomogeneous many-body systems
Tobias Bruenner, Erich Runge, Andreas Buchleitner, **Vivian V. Franca**
We investigate the effects of spatial inhomogeneities on the entanglement of modes of strongly correlated systems in the framework of small Fermi-Hubbard chains. We find regimes where entanglement is strongly enhanced by the presence of inhomogeneities. This contrasts recent reports of entanglement destruction due to inhomogeneities. We further study this phenomenon using concepts of Density Functional Theory and, thus, provide a general recipe for the prediction of entanglement enhancement in nanostructures. We find enhancement of up to ~24%, as compared to impurity-free chains.
Sep 10 - Sep 14 Saubhik Sarkar
Sep 10
1.arXiv:1104.1402 (replaced) [pdf, other]
Collapse and revival dynamics of superfluids of ultracold atoms in optical lattices
E. Tiesinga, P. R. Johnson
Recent experiments have shown a remarkable number of collapse-and-revival oscillations of the matter-wave coherence of ultracold atoms in optical lattices [Will et al., Nature 465, 197 (2010)]. Using a mean-field approximation to the Bose-Hubbard model, we show that the visibility of collapse-and-revival interference patterns reveal number squeezing of the initial superfluid state. To describe the dynamics, we use an effective Hamiltonian that incorporates the intrinsic two-body and induced three-body interactions, and we analyze in detail the resulting complex pattern of collapse-and-revival frequencies generated by virtual transitions to higher bands, as a function of lattice parameters and mean-atom number. Our work shows that a combined analysis of both the multiband, non-stationary dynamics in the final deep lattice, and the number-squeezing of the initial superfluid state, explains important characteristics of optical lattice collapse-and-revival physics. Finally, by treating the two- and three-body interaction strengths, and the coefficients describing the initial superposition of number states, as free parameters in a fit to the experimental data it should be possible to go beyond some of the limitations of our model and obtain insight into the breakdown of the mean-field theory for the initial state or the role of nonperturbative effects in the final state dynamics.
2.arXiv:1112.4457 (replaced) [pdf, other]
Condensation dynamics in a quantum-quenched Bose gas
Robert P. Smith, Scott Beattie, Stuart Moulder, Robert L. D. Campbell, Zoran Hadzibabic
By quenching the strength of interactions in a partially condensed Bose gas we create a "super-saturated" vapor which has more thermal atoms than it can contain in equilibrium. Subsequently, the number of condensed atoms ($N_0$) grows even though the temperature ($T$) rises and the total atom number decays. We show that the non-equilibrium evolution of the system is isoenergetic and for small initial $N_0$ observe a clear separation between $T$ and $N_0$ dynamics, thus explicitly demonstrating the theoretically expected "two-step" picture of condensate growth. For increasing initial $N_0$ values we observe a crossover to classical relaxation dynamics. The size of the observed quench-induced effects can be explained using a simple equation of state for an interacting harmonically-trapped atomic gas.
3.arXiv:1205.4536 (replaced) [pdf, other]
Superfluid behaviour of a two-dimensional Bose gas
Rémi Desbuquois, Lauriane Chomaz, Tarik Yefsah, Julian Léonard, Jérôme Beugnon, Christof Weitenberg, Jean Dalibard
Two-dimensional (2D) systems play a special role in many-body physics. Because of thermal fluctuations, they cannot undergo a conventional phase transition associated to the breaking of a continuous symmetry. Nevertheless they may exhibit a phase transition to a state with quasi-long range order via the Berezinskii-Kosterlitz-Thouless (BKT) mechanism. A paradigm example is the 2D Bose fluid, such as a liquid helium film, which cannot Bose-condense at non-zero temperature although it becomes superfluid above a critical phase space density. Ultracold atomic gases constitute versatile systems in which the 2D quasi-long range coherence and the microscopic nature of the BKT transition were recently explored. However, a direct observation of superfluidity in terms of frictionless flow is still missing for these systems. Here we probe the superfluidity of a 2D trapped Bose gas with a moving obstacle formed by a micron-sized laser beam. We find a dramatic variation of the response of the fluid, depending on its degree of degeneracy at the obstacle location. In particular we do not observe any significant heating in the central, highly degenerate region if the velocity of the obstacle is below a critical value.
Sep 11
1.arXiv:1209.1976 [pdf, ps, other]
Spin susceptibility and fluctuation corrections in the BCS-BEC crossover regime of an ultracold Fermi gas
Takashi Kashimura, Ryota Watanabe, Yoji Ohashi
We investigate magnetic properties and effects of pairing fluctuations in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover regime of an ultracold Fermi gas. Recently, Liu and Hu, and Parish, pointed out that the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink (NSR), which has been extensively used to successfully clarify various physical properties of cold Fermi gases, unphysically gives negative spin susceptibility in the BCS-BEC crossover region. The same problem is found to also exist in the ordinary non-self-consistent T-matrix approximation. In this paper, we clarify that this serious problem comes from incomplete treatment in term of pseudogap phenomena originating from strong pairing fluctuations, as well as effects of spin fluctuations on the spin susceptibility. Including these two key issues, we construct an extended T-matrix theory which can overcome this problem. The resulting positive spin susceptibility agrees well with the recent experiment on a 6Li Fermi gas done by Sanner and co-workers. We also apply our theory to a polarized Fermi gas to examine the superfluid phase transition temperature Tc, as a function of the polarization rate. Since the spin susceptibility is an important physical quantity, especially in singlet Fermi superfluids, our results would be useful in considering how singlet pairs appear above and below Tc in the BCS-BEC crossover regime of cold Fermi gases.
2.arXiv:1209.2101 [pdf, ps, other]
Quantum phase transitions in the honeycomb-lattice Hubbard model
K. Seki, Y. Ohta
Quantum phase transitions in the Hubbard model on the honeycomb lattice are investigated in the variational cluster approximation. The critical interaction for the paramagnetic to antiferromagnetic phase transition is found to be in remarkable agreement with a recent large-scale quantum Monte Carlo simulation. Calculated staggered magnetization increases continuously with $U$ and thus we find the phase transition is of a second order. We also find that the semimetal-insulator transition occurs at infinitesimally small interaction and thus a paramagnetic insulating state appears in a wide interaction range. A crossover behavior of electrons from itinerant to localized character found in the calculated single-particle excitation spectra and short-range spin correlation functions indicates that an effective spin model for the paramagnetic insulating phase is far from a simple Heisenberg model with a nearest-neighbor exchange interaction.
3.arXiv:1206.1062 (replaced) [pdf, ps, other]
Von Neumann Entropy Spectra and Entangled Excitations in Spin-Orbital Models
Wen-Long You, Andrzej M. Oleś, Peter Horsch
We consider the low-energy excitations of one-dimensional spin-orbital models which consist of spin waves, orbital waves, and joint spin-orbital excitations. Among the latter we identify strongly entangled spin-orbital bound states which appear as peaks in the von Neumann entropy (vNE) spectral function introduced in this work. The strong entanglement of bound states is manifested by a universal logarithmic scaling of the vNE with system size, while the vNE of other spin-orbital excitations saturates. We suggest that spin-orbital entanglement can be experimentally explored by the measurement of the dynamical spin-orbital correlations using resonant inelastic x-ray scattering, where strong spin-orbit coupling associated with the core hole plays a role.
Sep 12
1.arXiv:1209.2350 [pdf, ps, other]
Coexistence of superfluid gap and pseudogap in the BCS-BEC crossover regime of a trapped Fermi gas below $T_{\rm c}$
Ryota Watanabe, Shunji Tsuchiya, Yoji Ohashi
We investigate strong pairing fluctuations and effects of a harmonic trap in the superfluid phase of an ultracold Fermi gas. Including amplitude and phase fluctuations of the inhomogeneous superfluid order parameter $\Delta(r)$ in a trap within a combined $T$-matrix theory with the local density approximation, we examine local properties of single-particle excitations and a thermodynamic quantity in the BCS-BEC crossover region. Below the superfluid phase transition temperature $T_{\rm c}$, we show that inhomogeneous pairing fluctuations lead to a shell structure of the gas cloud in which the spatial region where the ordinary BCS-type superfluid density of states appears is surrounded by the region where the pseudogap associated with strong pairing fluctuations dominates single-particle excitations. The former spatial region enlarges to eventually cover the whole gas cloud far below $T_{\rm c}$. We also examine how this shell structure affects the photoemission spectrum, as well as the local pressure. Since a cold Fermi gas is always trapped in a harmonic potential, our results would be useful for the study of strong-coupling superfluid physics, including this realistic situation.
Sep 13
1.arXiv:1209.2671 [pdf, ps, other]
Unconventional Spin Density Waves in Dipolar Fermi Gases
S. G. Bhongale, L. Mathey, Shan-Wen Tsai, Charles W. Clark, Erhai Zhao
The conventional spin density wave (SDW) phase (Overhauser, 1962), as found in antiferromagnetic metal for example (Fawcett 1988), can be described as a condensate of particle-hole pairs with zero angular momentum, $\ell=0$, analogous to a condensate of particle-particle pairs in conventional superconductors. While many unconventional superconductors with Cooper pairs of finite $\ell$ have been discovered, their counterparts, density waves with non-zero angular momenta, have only been hypothesized in two-dimensional electron systems (Nayak, 2000). Using an unbiased functional renormalization group analysis, we here show that spin-triplet particle-hole condensates with $\ell=1$ emerge generically in dipolar Fermi gases of atoms (Lu, Burdick, and Lev, 2012) or molecules (Ospelkaus et al., 2008; Wu et al.) on optical lattice. The order parameter of these exotic SDWs is a vector quantity in spin space, and, moreover, is defined on lattice bonds rather than on lattice sites. We determine the rich quantum phase diagram of dipolar fermions at half-filling as a function of the dipolar orientation, and discuss how these SDWs arise amidst competition with superfluid and charge density wave phases.
2.arXiv:1209.2603 [pdf, other]
Equilibrium density structures in trapped immiscible two-species Bose-Einstein condensates
R. W. Pattinson, T. P. Billam, S. A. Gardiner, D. J. McCarron, H. W. Cho, S. L. Cornish, N. G. Parker, N. P. Proukakis
In a recent experiment [D. J. McCarron et al., Phys. Rev. A 84, 011603(R) (2011)] a two--species Rb-87 - Cs-133 Bose-Einstein condensate was formed and three distinct regimes of density distributions observed depending on relative atom numbers of the two species. To investigate these theoretically, we obtain time-independent solutions of the trapped two-species condensate through zero-temperature mean-field simulations. We find the results to be sensitive to experimentally relevant shifts in the potentials in both longitudinal and transverse directions, and observe a range of structures, including `ball and shell' formations and axially/radially separated states. We find good overall agreement with the experimental results for all regimes.
3.arXiv:1209.2533 [pdf, ps, other]
Phase diagram of spin 1 antiferromagnetic Bose-Einstein condensates
David Jacob, Lingxuan Shao, Vincent Corre, Tilman Zibold, Luigi De Sarlo, Emmanuel Mimoun, Jean Dalibard, Fabrice Gerbier
We study experimentally the equilibrium phase diagram of a spin 1 Bose-Einstein condensate with antiferromagnetic interactions, in a regime where spin and spatial degrees of freedom are decoupled. For a given total magnetization mz, we observe for low magnetic fields an "antiferromagnetic" phase where atoms condense in the m=+/-1 Zeeman states, and occupation of the m=0 state is suppressed. Conversely, for large enough magnetic fields, a phase transition to a "broken axisymmetry" phase takes place: The m=0 component becomes populated and rises sharply above a critical field Bc(mz). This behavior results from the competition between antiferromagnetic spin-dependent interactions (dominant at low fields) and the quadratic Zeeman energy (dominant at large fields). We compare the measured Bc as well as the global shape of the phase diagram with mean-field theory, and find good quantitative agreement.
4.arXiv:1209.2446 [pdf, other]
Superfluid Density of Weakly Interacting Bosons on a Lattice
Yariv Yanay, Erich J Mueller
We use a path integral approach to calculate the superfluid density of a Bose lattice gas in the limit where the number of atoms per site is large. Our analytical expressions agree with numerical results on small systems for low temperatures and relatively weak interactions. We also calculate the superfluid density and drag for two-component lattice bosons. To attain the correct results we develop tools for calculating discrete time path integrals. These tools should be broadly applicable to a range of systems which are naturally described within an overcomplete basis
Sep 14
1.arXiv:1209.2897 [pdf, ps, other]
Ground State Properties of Spin-Orbit Coupled Bose Gases for Arbitrary Interactions
Renyuan Liao, Wu-Ming Liu
We study spin-orbit coupled (SOC) Bose gases with arbitrary interspecies interaction. Besides at a critical interaction our results carry over to the recent publication [PRL 109,025301 (2012)], we identify various new features arising from the interplay of SOC and interspecies interaction, including a roton minimum in the excitation spectrum and dual effects of SOC on ground state energies depending on interspecies interactions. Counterintuitively, we find that at low interspecies interaction the SOC stabilizes the system by suppressing the quantum depletion. We show that the static structure factor is robust with respect to the SOC in the phase space where time-reversal symmetry is preserved.
2.arXiv:1209.2891 [pdf, ps, other]
Two-component few-fermion mixtures in a one-dimensional trap: numerical versus analytical approach
Ioannis Brouzos, Peter Schmelcher
We explore a few-fermion mixture consisting of two components which are repulsively interacting and confined in a one-dimensional harmonic trap. Different scenarios of population imbalance ranging from the completely imbalanced case where the physics of a single impurity in the Fermi-sea is discussed to the partially imbalanced and equal population configurations are investigated. For the numerical calculations the multi-configurational time-dependent Hartree (MCTDH) method is employed, extending its application to few-fermion systems. Apart from numerical calculations we generalize our Ansatz for a correlated pair wave-function proposed in [1] for bosons to mixtures of fermions. From weak to strong coupling between the components the energies, the densities and the correlation properties of one-dimensional systems change vastly with an upper limit set by fermionization where for infinite repulsion all fermions can be mapped to identical ones. The numerical and analytical treatments are in good agreement with respect to the description of this crossover. We show that for equal populations each pair of different component atoms splits into two single peaks in the density while for partial imbalance additional peaks and plateaus arise for very strong interaction strengths. The case of a single impurity atom shows rich behaviour of the energy and density as we approach fermionization, and is directly connected to recent experiments [2-4].
Sep 3 - Sep 7 Johannes Schachenmayer
Sep 7
1. arXiv:1209.1336 [pdf, ps, other]
Detection of Weak Force using a Bose-Einstein Condensate
Sonam Mahajan, Tarun Kumar, Aranya B Bhattacherjee, ManMohan
We investigate the possibility of detecting a weak coherent force by means of a hybrid optomechanical quantum device formed by a Bose Einstein Condensate (BEC) confined in a high quality factor optical cavity with an oscillatory end mirror. We show using the stochastic cooling technique that the atomic two-body interaction can be utilized to cool the mirror and achieve position squeezing essential for making sensitive measurements of weak forces. We further show that the atomic two-body interaction can also increase the signal to noise ratio (SNR) and decrease the noise of the off-resonant stationary spectral measurements.
2. arXiv:1209.1126 [pdf, other]
Measuring topology in a laser-coupled honeycomb lattice: From Chern insulators to topological semi-metals
N. Goldman, E. Anisimovas, F. Gerbier, P. Ohberg, I. B. Spielman, G. Juzeliunas
Ultracold fermions trapped in a honeycomb optical lattice constitute a versatile setup to experimentally realize the Haldane model [Phys. Rev. Lett. 61, 2015 (1988)]. In this system, a non-uniform synthetic magnetic flux can be engineered through laser-induced methods, explicitly breaking time-reversal symmetry. This potentially opens a bulk gap in the energy spectrum, which is associated with a non-trivial topological order, i.e., a non-zero Chern number. In this work, we consider the possibility of producing and identifying such a robust Chern insulator in the laser-coupled honeycomb lattice. We explore a large parameter space spanned by experimentally controllable parameters and obtain a variety of phase diagrams, clearly identifying the accessible topologically non-trivial regimes. We discuss the signatures of Chern insulators in cold-atom systems, considering available detection methods. We also highlight the existence of topological semi-metals in this system, which are gapless phases characterized by non-zero winding numbers, not present in Haldane's original model.
Sep 6
1. arXiv:1209.0944 [pdf, other]
Observation of mesoscopic crystalline structures in a two-dimensional Rydberg gas
Peter Schauß, Marc Cheneau, Manuel Endres, Takeshi Fukuhara, Sebastian Hild, Ahmed Omran, Thomas Pohl, Christian Gross, Stefan Kuhr, Immanuel BlochThe ability to control and tune interactions in ultracold atomic gases has paved the way towards the realization of new phases of matter. Whereas experiments have so far achieved a high degree of control over short-ranged interactions, the realization of long-range interactions would open up a whole new realm of many-body physics and has become a central focus of research. Rydberg atoms are very well-suited to achieve this goal, as the van der Waals forces between them are many orders of magnitude larger than for ground state atoms. Consequently, the mere laser excitation of ultracold gases can cause strongly correlated many-body states to emerge directly when atoms are transferred to Rydberg states. A key example are quantum crystals, composed of coherent superpositions of different spatially ordered configurations of collective excitations. Here we report on the direct measurement of strong correlations in a laser excited two-dimensional atomic Mott insulator using high-resolution, in-situ Rydberg atom imaging. The observations reveal the emergence of spatially ordered excitation patterns in the high-density components of the prepared many-body state. They have random orientation, but well defined geometry, forming mesoscopic crystals of collective excitations delocalised throughout the gas. Our experiment demonstrates the potential of Rydberg gases to realise exotic phases of matter, thereby laying the basis for quantum simulations of long-range interacting quantum magnets.
2. arXiv:1209.1056 [pdf, ps, other]
Spin-driven spatial symmetry breaking of spinor condensates in a double-well
Marina Melé-Messeguer, Simone Paganelli, Bruno Juliá-Díaz, Anna Sanpera, Artur PollsThe properties of an F=1 spinor Bose-Einstein condensate trapped in a double-well potential are discussed using both a mean-field two-mode approach and a simplified two-site Bose-Hubbard Hamiltonian. We focus in the region of phase space in which spin effects lead to a symmetry breaking of the system, favoring the spatial localization of the condensate in one well. To model this transition we derive, using perturbation theory, an effective Hamiltonian that describes N/2 spin singlets confined in a double-well potential.
3. arXiv:1209.1053 [pdf, other]
Revealing the Condensate and Non-Condensate Distributions in the Inhomogeneous Bose-Hubbard Model
Ushnish Ray, David M. CeperleyWe calculate the condensate fraction and the condensate and non-condensate spatial and momentum distribution of the Bose-Hubbard model in a trap. From our results, it is evident that using approximate distributions can lead to erroneous experimental estimates of the condensate. Strong interactions cause the condensate to develop pedestal-like structures around the central peak that can be mistaken as non-condensate atoms. Near the transition temperature, the peak itself can include a significant non-condensate component. Using distributions generated from QMC simulations, experiments can map their measurements for higher accuracy in identifying phase transitions and temperature.
4. arXiv:1209.0887 [pdf, ps, other]
All-optical transport and compression of ytterbium atoms into the surface of a solid immersion lens
M. Miranda, A. Nakamoto, Y. Okuyama, A. Noguchi, M. Ueda, M. KozumaWe present an all-optical method to load 174Yb atoms into a single layer of an optical trap near the surface of a solid immersion lens which improves the numerical aperture of a microscope system. Atoms are transported to a region 20 um below the surface using a system comprised by three optical dipole traps. The "optical accordion" technique is used to create a condensate and compress the atoms to a width of 120 nm and a distance of 1.8 um away from the surface. Moreover, we are able to verify that after compression the condensate behaves as a two-dimensional quantum gas.
5. arXiv:1209.0840 [pdf, ps, other]
Dynamical excitations in the collision of 2D Bose-Einstein condensates
T. Yang, B. Xiong, Keith A. BenedictWe carry out simulations of the collision of two components of an adiabatically divided, quasi-2D BEC. We identify under, over and critically damped regimes in the dipole oscillations of the components according to the balance of internal and centre-of-mass (c.m.) energies of the components and investigate the creation of internal excitations. We distinguish the behaviour of this system from previous studies of quasi-1D BEC's. In particular we note that the nature of the internal excitations is only essentially sensitive to an initial phase difference between the components in the overdamped regime.
6. arXiv:1209.1061 (cross-list from quant-ph) [pdf, ps, other]
Superpositions in Atomic Quantum Rings
R. Kanamoto, P. Öhberg, E. M. WrightAn atomic quantum ring involves ultracold atoms that are trapped circumferentially on a ring that is pierced at its center by a flux tube arising from the light-induced gauge potential due to applied Laguerre-Gaussian fields. In this paper we show that by using optical coherent state superpositions to produce the light-induced gauge potentials we can create the situation in which the trapped atoms are simultaneously exposed to two distinct flux tubes thereby creating superpositions in atomic quantum rings. We consider the examples of both a ring geometry and harmonic trapping, and in both cases we show that the ground state of the quantum system is a superposition of counter-rotating states of the atom trapped on the two distinct flux tubes.
7. arXiv:1209.0454 (cross-list from cond-mat.str-el) [pdf, other]
Projected Entangled Pair States at Finite Temperature: Imaginary Time Evolution with Ancillas
Piotr Czarnik, Lukasz Cincio, Jacek DziarmagaA projected entangled pair state (PEPS) with ancillas is evolved in imaginary time. This tensor network represents a thermal state of a 2D lattice quantum system. A finite temperature phase diagram of the 2D quantum Ising model in a transverse field is obtained as a benchmark application.
Sep 5
1. arXiv:1209.0560 [pdf, other]
Bright solitary matter waves: formation, stability and interactions
T. P. Billam, A. L. Marchant, S. L. Cornish, S. A. Gardiner, N. G. Parker
In recent years, bright soliton-like structures composed of gaseous Bose-Einstein condensates have been generated at ultracold temperature. The experimental capacity to precisely engineer the nonlinearity and potential landscape experienced by these solitary waves offers an attractive platform for fundamental study of solitonic structures. The presence of three spatial dimensions and trapping implies that these are strictly distinct objects to the true soliton solutions. Working within the zero-temperature mean-field description, we explore the solutions and stability of bright solitary waves, as well as their interactions. Emphasis is placed on elucidating their similarities and differences to the true bright soliton. The rich behaviour introduced in the bright solitary waves includes the collapse instability and symmetry-breaking collisions. We review the experimental formation and observation of bright solitary matter waves to date, and compare to theoretical predictions. Finally we discuss the current state-of-the-art of this area, including beyond-mean-field descriptions, exotic bright solitary waves, and proposals to exploit bright solitary waves in interferometry and as surface probes.
Sep 4
1. arXiv:1209.0346 [pdf, ps, other]
Thermodynamics of spin-orbit-coupled Bose-Einstein condensates
Jinling Lian, Yuanwei Zhang, J. -Q. Liang, Jie Ma, Gang Chen, Suotang Jia
In this paper we develop a quantum field approach to reveal the thermodynamic properties of the trapped BEC with the equal Rashba and Dresselhaus spin-orbit couplings. In the experimentally-feasible regime, the phase transition from the separate phase to the single minimum phase can be well driven by the tunable temperature. Moreover, the critical temperature, which is independent of the trapped potential, can be derived exactly. At the critical point, the specific heat has a large jump and can be thus regarded as a promising candidate to detect this temperature-driven phase transition. In addition, we obtain the analytical expressions for the specific heat and the entropy in the different phases. In the single minimum phase, the specific heat as well as the entropy are governed only by the Rabi frequency. However, in the separate phase with lower temperature, we find that they are determined only by the strength of spin-orbit coupling. Finally, the effect of the effective atom interaction is also addressed. In the separate phase, this effective atom interaction affects dramatically on the critical temperature and the corresponding thermodynamic properties.
2. arXiv:1209.0232 [pdf, ps, other]
BCS pairing in fully repulsive fermion mixtures
T. Espinosa-Ortega, O. Kyriienko, I. A. ShelykhWe consider a mixture of two neutral cold Fermi gases with repulsive interactions. We show that in some region of the parameter space of the system the effective attraction between fermions of the same type can appear due to the exchange of collective excitations. This leads to the formation of BCS pairing in the case where bare inter-atomic interactions are repulsive.
3. arXiv:1209.0186 [pdf, other]
Finite-Temperature Auxiliary-Field Quantum Monte Carlo for Bose-Fermi Mixtures
Brenda M. Rubenstein, Shiwei Zhang, David R. ReichmanWe present a quantum Monte Carlo (QMC) technique for calculating the exact finite-temperature properties of Bose-Fermi mixtures. The Bose-Fermi Auxiliary-Field Quantum Monte Carlo (BF-AFQMC) algorithm combines two methods, a finite-temperature AFQMC algorithm for bosons and a variant of the standard AFQMC algorithm for fermions, into one algorithm for mixtures. We demonstrate the accuracy of our method by comparing its results for the Bose-Hubbard and Bose-Fermi-Hubbard models against those produced using exact diagonalization for small systems. Comparisons are also made with mean-field theory and the worm algorithm for larger systems. As is the case with most fermion Hamiltonians, a sign or phase problem is present in BF-AFQMC. We discuss the nature of these problems in this framework and describe how they can be controlled with well-studied approximations to expand BF-AFQMC's reach. The new algorithm can serve as an essential tool for answering many unresolved questions about many-body physics in mixed Bose-Fermi systems.
4. arXiv:1209.0162 [pdf, ps, other]
Non-perturbative theoretical description of two atoms in an optical lattice with time-dependent perturbations
Philipp-Immanuel Schneider, Sergey Grishkevich, Alejandro SaenzA theoretical approach for a non-perturbative dynamical description of two interacting atoms in an optical lattice potential is introduced. The approach builds upon the stationary eigenstates found by a procedure described in Grishkevich et al. [Phys. Rev. A 84, 062710 (2011)]. It allows presently to treat any time-dependent external perturbation of the lattice potential up to quadratic order. Example calculations of the experimentally relevant cases of an acceleration of the lattice and the turning-on of an additional harmonic confinement are presented.
Sep 3
1. arXiv:1208.6293 [pdf, other]
Nanoplasmonic Lattices for Ultracold atoms
M. Gullans, T. Tiecke, D. E. Chang, J. Feist, J. D. Thompson, J. I. Cirac, P. Zoller, M. D. Lukin
We propose to use sub-wavelength confinement of light associated with the near field of plasmonic systems to create nanoscale optical lattices for ultracold atoms. Our approach combines the unique coherence properties of isolated atoms with the sub-wavelength manipulation and strong light-matter interaction associated with nano-plasmonic systems. It allows one to considerably increase the energy scales in the realization of Hubbard models and to engineer effective long-range interactions in coherent and dissipative many-body dynamics. Realistic imperfections and potential applications are discussed.
2. arXiv:1208.6580 [pdf, other]
Heat and spin transport in a cold atomic Fermi gas
Hyungwon Kim, David A. Huse
Motivated by recent experiments measuring the spin transport in ultracold unitary atomic Fermi gases (Sommer et al., 2011; Sommer et al., 2011), we explore the theory of spin and heat transport in a three-dimensional spin-polarized atomic Fermi gas. We develop estimates of spin and thermal diffusivities and discuss magnetocaloric effects, namely the the spin Seebeck and spin Peltier effects. We estimate these transport coefficients using a Boltzmann kinetic equation in the classical regime and present experimentally accessible signatures of the spin Seebeck effect. We study an exactly solvable model that illustrates the role of momentum-dependent scattering in the magnetocaloric effects.
3. arXiv:1208.6540 [pdf, other]
Ferromagnetism and Nematicity of Fermions in an Optical Flux Lattice
Stefan K. Baur, Nigel R. CooperUltracold atoms in Raman-dressed optical lattices allow for effective momentum-dependent interactions among single-species fermions originating from short-range s-wave interactions. These dressed-state interactions combined with very flat bands encountered in the recently introduced optical flux lattices push the Stoner instability towards weaker repulsive interactions, making it accessible with current experiments. As a consequence of the coupling between spin and orbital degrees of freedom, the magnetic phase features Ising nematic order.
4. arXiv:1208.6506 [pdf, ps, other]
Pair supersolid of the extended Bose-Hubbard model with atom-pair hopping on the triangular Lattice
Wanzhou Zhang, Yancheng Wang, Wenan GuoWe systematically study an extended Bose-Hubbard model with atom hopping and atom-pair hopping in the presence of a three-body constraint on the triangular lattice. By means of large-scale Quantum Monte Carlo simulations, the ground-state phase diagram are studied. We find a continuous transition between the atomic superfluid phase and the pair superfluid when the ratio of the atomic hopping and the atom-pair hopping is adapted. We then focus on the interplay among the atom-pair hopping, the on-site repulsion and the nearest-neighbor repulsion. With on-site repulsion present, we observe first order transitions between the Mott Insulators and pair superfluid driven by the pair hopping. With the nearest-neighbor repulsion turning on, three typical solid phases with 2/3, 1 and 4/3-filling emerge at small atom-pair hopping region. A stable pair supersolid phase is found at small on-site repulsion. This is due to the three-body constraint and the pair hopping, which essentially make the model a quasi hardcore boson system. Thus the pair supersolid state emerges basing on the order-by-disorder mechanism, by which hardcore bosons avoid classical frustration on the triangular lattice. The transition between the pair supersolid and the pair superfluid is first order, except for the particle-hole symmetric point. We compare the results with those obtained by means of mean-field analysis.
5. arXiv:1208.6317 [pdf, ps, other]
Diffusion quantum Monte Carlo calculation of the quasiparticle effective mass of the two-dimensional homogeneous electron gas
N. D. Drummond, R. J. NeedsThe quasiparticle effective mass is a key quantity in the physics of electron gases, describing the renormalization of the electron mass due to electron-electron interactions. Two-dimensional electron gases are of fundamental importance in semiconductor physics, and there have been numerous experimental and theoretical attempts to determine the quasiparticle effective mass in these systems. In this work we report quantum Monte Carlo results for the quasiparticle effective mass of a two-dimensional homogeneous electron gas. Our calculations differ from previous quantum Monte Carlo work in that much smaller statistical error bars have been achieved, allowing for an improved treatment of finite-size effects. In some cases we have also been able to use larger system sizes than previous calculations.
