June 24 - June 28 Jin-Long Yu
Jun 28
1. arXiv:1306.6473 [pdf, ps, other]
High accuracy photoassociation of $^{40}$Ca near the ${^3P_1}+{^1S_0}$ asymptote and its Zeeman effect
Max Kahmann, Eberhard Tiemann, Oliver Appel, Uwe Sterr, Fritz Riehle
We report on the first measurement of narrow photoassociation lines of $^{40}$Ca near the ${^3P_1}+{^1S_0}$ asymptote related to the molecular states $^3\Pi_u$ and $^3\Sigma^+_u$. The highly accurate binding energies and Zeeman splittings are well described by a coupled channel theoretical model, confirming theoretical predictions of long-range coefficients. Our analysis shows that only the inclusion of both energies and Zeeman splittings provides an accurate description of the long-range interaction potentials.
2. arXiv:1306.6610 [pdf, other]
Rashba Spin-Orbit Coupled Bose-Einstein Condensates with Dipolar Interactions
Ryan M. Wilson, Brandon M. Anderson, Charles W. Clark
We study the effects of dipolar interactions on a Bose-Einstein condensate with synthetically generated Rashba spin-orbit coupling. The dipolar interaction includes long-range direct and spin-exchange terms in addition to terms that couple spin and orbital angular momentum in a way perfectly congruent with the single-particle Rashba coupling. We show that this internal spin-orbit coupling plays a crucial role in the rich ground state phase diagram of the trapped condensate, exhibiting topological spin textures including a meron spin configuration.
3. arXiv:1306.6440 [pdf, ps, other]
Rashbon condensation in a Bose gas with Rashba spin-orbit coupling
Rong Li, Lan Yin
We show that in a two-component Bose gas with Rashba spin-orbit coupling (SOC) two atoms can form bound states (Rashbons) with any intra-species scattering length. At zero center-of-mass momentum there are two degenerate Rashbons due to time-reversal symmetry, but the degeneracy is lifted at finite in-plane momentum with two different effective masses. A stable Rashbon condensation can be created in a dilute system with weakly attractive intra-species and repulsive inter-species interactions. The critical temperature of Rashbon condensation is about six times smaller than the BEC transition temperature of an ideal Bose gas. Due Rashba SOC, excitations in the Rashbon condensation phase are anisotropic in momentum space.
4. arXiv:1306.6563 [pdf, ps, other]
Quantum phase transition between one-channel and two-channel Kondo polarons
Julian Rincon, Daniel J. Garcia, K. Hallberg, Matthias Vojta
For a mobile spin-1/2 impurity, coupled antiferromagnetically to a one-dimensional gas of fermions, perturbative ideas have been used to argue in favor of two-channel Kondo behavior of the impurity spin. Here we combine general considerations and extensive numerical simulations to show that the problem displays a novel quantum phase transition between two-channel and one-channel Kondo screening upon increasing the Kondo coupling. We construct a ground-state phase diagram and discuss the various non-trivial crossovers as well as possible experimental realizations.
Jun 27
1. arXiv:1306.6102 [pdf, ps, other]
Phase Winding a Two-Component BEC in an Elongated Trap: Experimental Observation of Moving Magnetic Orders and Dark-bright Solitons
C. Hamner, Yongping Zhang, J.J. Chang, Chuanwei Zhang, P. Engels
We experimentally investigate the phase winding dynamics of a harmonically trapped two-component BEC subject to microwave induced Rabi oscillations between two pseudospin components. While the single particle dynamics can be explained by mapping the system to a two-component Bose-Hubbard model, nonlinearities due to the interatomic repulsion lead to new effects observed in the experiments: In the presence of a linear magnetic field gradient, a qualitatively stable moving magnetic order that is similar to antiferromagnetic order is observed after critical winding is achieved. We also demonstrate how the phase winding can be used as a new tool to generate copious dark-bright solitons in a two-component BEC, opening the door for new experimental studies of these nonlinear features.
2. arXiv:1306.6229 [pdf, ps, other]
Time crystals: can diamagnetic currents drive a charge density wave into rotation?
Philippe Nozières
It has been argued recently that an inhomogeneous system could rotate spontaneously in its ground state - hence a 'time crystal' which is periodic in time. In this note we present a very simple example: a superfluid ring threaded by a magnetic field which develops a charge density wave (CDW). A naive calculation shows that diamagnetic currents cannot drive rotation of the CDW, with a clear picture of the cancellation mechanism.
Jun 26
1. arXiv:1306.5754 [pdf, other]
A thermoelectric heat engine with ultracold atoms
Jean-Philippe Brantut, Charles Grenier, Jakob Meineke, David Stadler, Sebastian Krinner, Corinna Kollath, Tilman Esslinger, Antoine Georges
Thermoelectric effects, such as the generation of a particle current by a temperature gradient, have their origin in a reversible coupling between heat and particle flows. These effects are fundamental probes for materials and have applications to cooling and power generation. Here we demonstrate thermoelectricity in a fermionic cold atoms channel, ballistic or diffusive, connected to two reservoirs. We show that the magnitude of the effect and the efficiency of energy conversion can be optimized by controlling the geometry or disorder strength. Our observations are in quantitative agreement with a theoretical model based on the Landauer-Bu ttiker formalism. Our device provides a controllable model-system to explore mechanisms of energy conversion and realizes a cold atom based heat engine.
2. arXiv:1306.5785 [pdf, other]
BCS-BEC Crossover and the Unitary Fermi Gas
Mohit Randeria, Edward Taylor
The crossover from weak coupling Bardeen-Cooper-Schrieffer (BCS) pairing to a Bose-Einstein condensate (BEC) of tightly bound pairs, as a function of the attractive interaction in Fermi systems, has long been of interest to theoretical physicists. The past decade has seen a series of remarkable experimental developments in ultracold Fermi gases that has realized the BCS-BEC crossover in the laboratory, bringing with it fresh new insights into the very strongly interacting unitary regime in the middle of this crossover. In this review, we start with a pedagogical introduction to the crossover and then focus on recent progress in the strongly interacting regime. While our focus is on new theoretical developments, we also describe three key experiments that probe the thermodynamics, transport and spectroscopy of the unitary Fermi gas. We discuss connections between the unitary regime and other areas of physics -- quark-gluon plasmas, gauge-gravity duality and high temperature superconductivity -- and conclude with open questions in strongly interacting Fermi gases.
3. arXiv:1306.5986 [pdf, other]
Bosonic Mott Insulator with Meissner Currents
Alexandru Petrescu, Karyn Le Hur
We introduce a generic bosonic model exemplifying that (spin) Meissner currents can persist in insulating phases of matter, such as the Mott insulator. We consider two species of interacting bosons on a lattice. Our model exhibits separation of the charge (total density) and spin (relative density) degrees of freedom. The charge sector is gapped in a bosonic Mott insulator phase with total density one. At the same time, the spin sector remains superfluid, due to interspecies conversion which acts as a Josephson term. Coupling the spin sector to the gauge fields then yields a spin Meissner effect reflecting the long-range spin superfluid coherence. Applying a mixture of analytical and numerical methods, we investigate the resulting phase diagram and describe other possible spin phases of matter in the Mott regime possessing chiral currents as well as a spin-density wave phase. The model presented here is realizable in Josephson junction arrays and in cold atom experiments.
4. arXiv:1306.5934 [pdf, ps, other]
Inhomogeneous Topological Superfluidity in One-Dimensional Spin-Orbit-Coupled Fermi Gases
Chun Chen
We theoretically predict an exotic topological superfluid state with spatially modulated pairing gap in one-dimensional spin-orbit-coupled Fermi gases. The emergence of this inhomogeneous topological superfluidity is induced by appealing to the conspiration of a perpendicular Zeeman magnetic field and an equally weighted Rashba and Dresselhaus spin-orbit coupling in one-dimensional optical lattices. Based on the self-consistent Bogoliubov-de Gennes theory, we confirm that this novel topological phase is a unique condensation of Cooper pairs, which goes beyond the conventional phase separation phenomena in the artificial confinements. The properties of the emergent Majorana bound states are investigated in detail by examining the associated $\mathbb{Z}_{2}$ topological number, the eigenenergy spectrum, and the wave functions of the localized Majorana end modes.
Jun 25
1. arXiv:1306.5608 [pdf, other]
Density dependent tunneling in the extended Bose-Hubbard model
Michal Maik, Philipp Hauke, Omjyoti Dutta, Jakub Zakrzewski, Maciej Lewenstein
Recently, it has become apparent that, when the interactions between polar molecules in optical lattices becomes strong, the conventional description using the extended Hubbard model has to be modified by additional terms, in particular a density-dependent tunneling term. We investigate here the influence of this term on the ground-state phase diagrams of the two dimensional extended Bose-Hubbard model. Using Quantum Monte Carlo simulations, we investigate the changes of the superfluid, supersolid, and phase-separated parameter regions in the phase diagram of the system. By studying the interplay of the density-dependent hopping with the usual on-site interaction U and nearest-neighbor repulsion V, we show that the ground-state phase diagrams differ significantly from the ones that are expected from the standard extended Bose-Hubbard model. However we find no indication of pair-superfluid behavior in this two dimensional square lattice study in contrast to the one-dimensional case.
2. arXiv:1306.5604 [pdf, other]
Quench dynamics of a Tonks-Girardeau gas released from a harmonic trap
Mario Collura, Spyros Sotiriadis, Pasquale Calabrese
We consider the non-equilibrium dynamics of a gas of impenetrable bosons released from a harmonic trapping potential to a circle. The many body dynamics is solved analytically and the time dependence of all the physically relevant correlations is described. We prove that, for large times and in the thermodynamic limit, the reduced density matrix of any subsystem converges to a generalized Gibbs ensemble as a consequence of the integrability of the model. We discuss the approach to the stationary behavior at late times. We also describe the time-dependence of the entanglement entropy which attains a very simple form in the stationary state.
3. arXiv:1306.5515 [pdf, ps, other]
Interacting spin-1 bosons in a two-dimensional optical lattice
L. de Forges de Parny, F. Hébert, V.G. Rousseau, G.G. Batrouni
We study, using quantum Monte Carlo (QMC) simulations, the ground state properties of spin-1 bosons trapped in a square optical lattice. The phase diagram is characterized by the mobility of the particles (Mott insulating or superfluid phase) and by their magnetic properties. For ferromagnetic on-site interactions, the whole phase diagram is ferromagnetic and the Mott insulators-superfluid phase transitions are second order. For antiferromagnetic on-site interactions, spin nematic order is found in the odd Mott lobes and in the superfluid phase. Furthermore, the superfluid-insulator phase transition is first or second order depending on whether the density in the Mott is even or odd. Inside the even Mott lobes, we observe a singlet-to-nematic transition for certain values of the interactions. This transition appears to be first order.
Jun 24
1. arXiv:1306.5223 [pdf, other]
Detecting Topological Orders in Cold Atoms
Xiong-Jun Liu, K. T. Law, T. K. Ng, Patrick A. Lee
Chern insulators are band insulators which exhibit a gap in the bulk and gapless excitations in the edge. Detection of Chern insulators is a serious challenge in cold atoms since transport measurements are technically unrealistic for neutral atoms. By establishing a natural correspondence between the time-reversal invariant topological insulator and quantum anomalous Hall system, we show for a class of Chern insulators that the topology can be determined by only measuring Bloch eigenstates at highly symmetric points of the Brillouin zone (BZ). Furthermore, we introduce two experimental schemes including the spin-resolved Bloch oscillation to carry out the measurement. These schemes are highly feasible under realistic experimental conditions. Our results provide a powerful tool to detect topological orders in cold atoms.
2. arXiv:1306.5127 [pdf, other]
Tunneling spectra simulation of interacting Majorana wires
Ronny Thomale, Stephan Rachel, Peter Schmitteckert
Recent tunneling experiments on InSb hybrid superconductor-semiconductor devices have provided hope for a stabilization of Majorana edge modes in a spin-orbit quantum wire subject to a magnetic field and superconducting proximity effect. Connecting the experimental scenario with a microscopic description poses challenges of different kind, such as accounting for the effect of interactions on the tunneling properties of the wire. We develop a density matrix renormalization group (DMRG) analysis of the tunneling spectra of interacting Majorana chains, which we explicate for the Kitaev chain model. Our DMRG approach allows us to calculate the spectral function down to zero frequency, where we analyze how the Majorana zero-bias peak is affected by interactions. From the study of topological phase transitions between the topological and trivial superconducting phase in the wire, we argue that the bulk gap closure generically affects both the proximity peaks and the Majorana peak, which should be observable in the transport signal.
June 17 - June 21 Stephan Langer
June 21
1. arXiv:1306.4678 [pdf, other]
Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain
M. Mourigal, M. Enderle, A. Klöpperpieper, J.-S. Caux, A. Stunault, H. M. Rønnow
Assemblies of interacting quantum particles often surprise us with properties that are difficult to predict. One of the simplest quantum many-body systems is the spin 1/2 Heisenberg antiferromagnetic chain, a linear array of interacting magnetic moments. Its exact ground state is a macroscopic singlet entangling all spins in the chain. Its elementary excitations, called spinons, are fractional spin 1/2 quasiparticles; they are created and detected in pairs by neutron scattering. Theoretical predictions show that two-spinon states exhaust only 71% of the spectral weight while higher-order spinon states, yet to be experimentally located, are predicted to participate in the remaining. Here, by accurate absolute normalization of our inelastic neutron scattering data on a compound realizing the model, we account for the full spectral weight to within 99(8)%. Our data thus establish and quantify the existence of higher-order spinon states. The observation that within error bars, the entire weight is confined within the boundaries of the two-spinon continuum, and that the lineshape resembles a rescaled two-spinon one, allow us to develop a simple physical picture for understanding multi-spinon excitations.
2. arXiv:1306.4687 [pdf, ps, other]
Discriminating antiferromagnetic signatures in ultracold fermions by tunable geometric frustration
Chia-Chen Chang, Richard T. Scalettar, Elena V. Gorelik, Nils Blümer
We study fermion pairing in a population-imbalanced mixture of $^{6}$Li atomic gas loaded in a three-dimensional lattice at very low temperatures. Using the number equation for each population, the gap equation and the equation for the Helmholtz free energy, we determine the gap, chemical potentials and pair-momentum as functions of polarization. These parameters define the stability regions for: a Fulde-Ferrell-Larkin-Ovchinnikov phase; a phase separation region where BCS and normal phases coexist; a Sarma phase when the pair-momentum vanishes, and the transition to the normal phase when the gap disappears. The collective-mode energies are then calculated using a Bethe-Salpeter approach in the generalized random phase approximation assuming that the system is well described by the single-band Hubbard model. A novel result is that this fermionic gas has a superfluid phase revealed by rotonlike minima in the asymmetric collective-mode energy spectrum.
3. arXiv:1306.4706 [pdf, ps, other]
Superfluidity of a spin-imbalanced Fermi gas in a three-dimensional optical lattice
Rafael Mendoza, Mauricio Fortes, M. A. Solís, Zlatko Koinov
We study fermion pairing in a population-imbalanced mixture of $^{6}$Li atomic gas loaded in a three-dimensional lattice at very low temperatures. Using the number equation for each population, the gap equation and the equation for the Helmholtz free energy, we determine the gap, chemical potentials and pair-momentum as functions of polarization. These parameters define the stability regions for: a Fulde-Ferrell-Larkin-Ovchinnikov phase; a phase separation region where BCS and normal phases coexist; a Sarma phase when the pair-momentum vanishes, and the transition to the normal phase when the gap disappears. The collective-mode energies are then calculated using a Bethe-Salpeter approach in the generalized random phase approximation assuming that the system is well described by the single-band Hubbard model. A novel result is that this fermionic gas has a superfluid phase revealed by rotonlike minima in the asymmetric collective-mode energy spectrum.
June 20
1. arXiv:1306.4611 [pdf, other]
Adiabatic preparation of vortex lattices
Stefan K. Baur, Nigel R. Cooper
By engineering appropriate artificial gauge potentials, a Bose-Einstein condensate can be adiabatically loaded into a current carrying state that resembles a vortex lattice of a rotating uniform Bose gas. We give two explicit, experimentally feasible protocols by which vortex lattices can be smoothly formed from a condensate initially at rest. In the first example we show how this can be achieved by adiabatically loading a uniform BEC into an optical flux lattice, formed from coherent optical coupling of internal states of the atom. In the second example we study a tight binding model that is continuously manipulated in parameter space such that it smoothly transforms into the Harper-Hofstadter model with 1/3 flux per plaquette.
2. arXiv:1306.4610 [pdf, ps, other]
Strongly-interacting fermions in one dimension and microscopic magnetism
A. G. Volosniev, D. V. Fedorov, A. S. Jensen, M. Valiente, N. T. Zinner
Strongly interacting fermions are ubiquitous in nature and play a vital role in magnetism and superconductivity. In one dimension such systems are among the most studied in physics, since many of them belong to the rare case of exactly solvable models. We introduce a new technique that combines topological classification and the variational principle to obtain the full spectrum of one-dimensional fermionic systems with strong short-range interactions in arbitrary confining geometries which, in spite of much effort, had not been solved in general. This allows us to address the spatial correlations of few-body systems analytically and show that both ferro- and antiferromagnetic states can be prepared and observed in experiments. Our method paves the way for quantum manipulation of magnetic correlations at the microscopic scale.
June 19
1. arXiv:1306.4343 [pdf, other]
Heat, molecular vibrations, and adiabatic driving in non-equilibrium transport through interacting quantum dots
F. Haupt, M. Leijnse, H. L. Calvo, L. Classen, J. Splettstoesser, M. R. Wegewijs
In this article we review aspects of charge and heat transport in interacting quantum dots and molecular junctions under stationary and time-dependent non-equilibrium conditions due to finite electrical and thermal bias. In particular, we discuss how a discrete level spectrum can be beneficial for thermoelectric applications, and investigate the detrimental effects of molecular vibrations on the efficiency of a molecular quantum dot as an energy converter. In addition, we consider the effects of a slow time-dependent modulation of applied voltages on the transport properties of a quantum dot and show how this can be used as a spectroscopic tool complementary to standard dc-measurements. Finally, we combine time-dependent driving with thermoelectrics in a double-quantum dot system - a nanoscale analogue of a cyclic heat engine - and discuss its operation and the main limitations to its performance.
2. arXiv:1306.4300 [pdf, ps, other]
Magnetic Dynamics of a Multiferroic with an Antiferromagnetic Layer
Svitlana V. Kondovych, Helen V. Gomonay, Vadim M. Loktev
Shape effects in magnetic particles are widely studied, because of the ability of the shape and the size to control the parameters of a sample during its production. Experiments with nano-sized samples show that the shape can affect also the properties of antiferromagnetic (AFM) materials. However, the theoretical interpretation of these effects is under discussion. We propose a model to study the shape-induced effects in AFM particles at the AFM resonance frequency. The Lagrange function method is used to calculate the spectrum of resonance oscillations of the AFM vector in a synthetic multiferroic (piezoelectric + antiferromagnet). The influence of the specimen shape on the AFM resonance frequency in the presence of an external magnetic field is studied. Conditions for a resonance under the action of an external force or for a parametric resonance to arise in the magnetic subsystem are considered.
June 18
1. arXiv:1306.4306 [pdf, other]
Ballistic spreading of entanglement in a diffusive nonintegrable system
Hyungwon Kim, David A. Huse
We study the time evolution of the entanglement entropy of a one-dimensional nonintegrable spin chain, starting from random nonentangled initial pure states. We use exact diagonalization of a nonintegrable quantum Ising chain with transverse and longitudinal fields to obtain the exact quantum dynamics. We show that the entanglement entropy increases linearly with time before finite-size saturation begins, demonstrating a ballistic spreading of the entanglement, while the energy transport in the same system is diffusive. Thus we explicitly demonstrate that the spreading of entanglement is much faster than the energy diffusion in this nonintegrable system.
2. arXiv:1306.4018 [pdf, other]
Anomalous thermoelectric transport in two-dimensional Bose gas
Eric L. Hazlett, Li-Chung Ha, Cheng Chin
In condensed matter physics, transport measurements are essential not only for the characterization of materials, but also to discern between quantum phases and identify new ones. The extension of these measurements into atomic quantum gases is emerging and will expand the scope of quantum simulation and atomtronics. To push this frontier, we demonstrate an innovative approach to extract transport properties from the time-resolved redistribution of the particles and energy of a trapped atomic gas. Based on the two-dimensional (2D) Bose gas subject to weak three-body recombination we find clear evidence of both conductive and thermoelectric currents. We then identify the contributions to the currents from thermoelectric forces and determine the Seebeck coefficient (a.k.a. thermopower) and Lorenz number, both showing anomalous behavior in the fluctuation and superfluid regimes. Our results call for further exploration of the transport properties, particularly thermoelectric properties, of atomic quantum gases.
3. arXiv:1306.4003 [pdf, other]
Robustness in Projected Entangled Pair States
J.I. Cirac, S. Michalakis, D. Perez-Garcia, N. Schuch
Comments: 11 pages, 4 figures
We analyze a criterion which guarantees that the ground states of certain many body systems are stable under perturbations. Specifically, we consider PEPS, which are believed to provide an efficient description, based on local tensors, for the low energy physics arising from local interactions. In order to assess stability in the framework of PEPS, one thus needs to understand how physically allowed perturbations of the local tensor affect the properties of the global state. In this paper, we show that a restricted version of the Local Topological Quantum Order (LTQO) condition provides a checkable criterion which allows to assess the stability of local properties of PEPS under physical perturbations. We moreover show that LTQO itself is stable under perturbations which preserve the spectral gap, leading to nontrivial examples of PEPS which possess LTQO and are thus stable under arbitrary perturbations.
June 17
1. arXiv:1306.3528 [pdf, other]
A Continuum Generalization of the Ising Model
Haley A. Yaple, Daniel M. Abrams
The Lenz-Ising model has served for almost a century as a basis for understanding ferromagnetism, and has become a paradigmatic model for phase transitions in statistical mechanics. While retaining the Ising energy arguments, we use techniques previously applied to sociophysics to propose a continuum model. Our formulation results in an integro-differential equation that has several advantages over the traditional version: it allows for asymptotic analysis of phase transitions, material properties, and the dynamics of the formation of magnetic domains.
2. arXiv:1306.3365 [pdf, other]
Efficient demagnetization cooling of atoms and its limits
Valentin V. Volchkov, Jahn Rührig, Tilman Pfau, Axel Griesmaier
Demagnetization cooling relies on spin-orbit coupling that brings motional and spin degrees of freedom into thermal equilibrium. In the case of a gas, one has the advantage that the spin degree of freedom can be cooled very efficiently using optical pumping. We investigate demagnetization cooling of a chromium gas in a deep optical dipole trap over a large temperature range and reach high densities up to 5*10^19m^-3. We study the loss mechanism under such extreme conditions and identify excited state collisions as the main limiting process. We discuss that demagnetization cooling has realistic potential of reaching degeneracy by optical cooling only.
3. arXiv:1305.6225 [pdf, other]
Distant multipartite entanglement in a first order phase transition
Julia Stasińska, Benjamin Rogers, Mauro Paternostro, Gabriele De Chiara, Anna Sanpera
We exploit rotationally invariant states as a reference test bed to detect beyond nearest-neighbor genuine tripartite entanglement in spin-1/2 models. We construct multipartite entanglement witnesses which act on a rotationally invariant subspace and use them to detect multipartite entanglement also in non rotationally invariant states. We exemplify our findings by analyzing in detail the anisotropic XXZ spin chain close to its phase transitions. We show how the emergence of multipartite entanglement between non-adjacent spins signals the breaking of the global SU(2) symmetry, a feature that is not captured by bipartite entanglement.
June 10 - June 14, Bo Liu
June 14
1. arXiv:1306.3189 [pdf, ps, other]
The Compressibility in Strongly Correlated Superconductors and Superfluids: From BCS to BEC
Hao Guo, Yan He, Chih-Chun Chien, K. Levin
We present a theoretical study of the compressibility, $\kappa$, in a Fermi gas with attractive contact interactions, providing predictions for the strongly-attractive regime and the superfluid phase. Our work emphasizes the compressibility sum rule and gauge invariance as constraints on $\kappa$ and we show how within a particular $t$-matrix approach, these can be satisfied in the normal phase when no approximations are made. For tractability, approximations must be introduced, and it is believed that thermodynamical approaches to $\kappa$ are more reliable, than correlation function based schemes. Contrasting with other studies in the literature, we present thermodynamic calculations of $\kappa$; these yield semi-quantitative agreement with experiment and provide physical insight into similar results obtained via quantum Monte Carlo simulations.
2. arXiv:1306.3124 [pdf, other]
From antiferromagnetic order to magnetic textures in the two dimensional Fermi Hubbard model with synthetic spin orbit interaction
Jiří Minář, Benoît Grémaud
We study the interacting Fermi-Hubbard model in two spatial dimensions with synthetic gauge coupling of the spin orbit Rashba type, at half-filling. Using real space mean field theory, we numerically determine the phase as a function of the interaction strength for different values of the gauge field parameters. For a fixed value of the gauge field, we observe that when the strength of the repulsive interaction is increased, the system enters into an antiferromagnetic phase, then undergoes a first order phase transition to an non collinear magnetic phase. Depending on the gauge field parameter, this phase further evolves to the one predicted from the effective Heisenberg model obtained in the limit of large interaction strength. We explain the presence of the antiferromagnetic phase at small interaction from the computation of the spin-spin susceptibility which displays a divergence at low temperatures for the antiferromagnetic ordering. We discuss, how the divergence is related to the nature of the underlying Fermi surfaces. Finally, the fact that the first order phase transitions for different gauge field parameters occur at unrelated critical interaction strengths arises from a Hofstadter-like situation, i.e. for different magnetic phases, the mean-field Hamiltonians have different translational symmetries.
June 13
1. arXiv:1306.2829 [pdf, ps, other]
Atomic spin-orbit coupling synthesized with magnetic-field-gradient pulses
Zhi-Fang Xu, Li You, Masahito Ueda
We discuss a general scheme for creating atomic spin-orbit coupling (SOC) such as the Rashba or Dresselhaus types using magnetic-field-gradient pulses. In contrast to conventional schemes based on adiabatic center-of-mass motion with atomic internal states restricted to a dressed-state subspace, our scheme works for the complete subspace of a hyperfine-spin manifold by utilizing the coupling between the atomic magnetic moment and external magnetic fields. A spatially dependent pulsed magnetic field acts as an internal-state-dependent impulse, thereby coupling the atomic internal spin with its orbital center-of-mass motion, as in the Einstein-de Haas effect. This effective coupling can be dynamically manipulated to synthesize SOC of any type (Rashba, Dresselhaus, or any linear combination thereof). Our scheme can be realized with most experimental setups of ultracold atoms and is especially suited for atoms with zero nuclear spins.
2.arXiv:1306.2650 [pdf, other]
Bose-Hubbard vs Mean-Field approaches in quantum fluids confined in triple well potentials: Dynamics and Decoherence
Arturo Camacho, Rosario Paredes
The transition from Josephson oscillations to self-trapping regime in a system of three weakly coupled Bose-Einstein condensates in one dimension is investigated. We address the stationary and dynamical properties within both mean field and exact N-particle Bose-Hubbard schemes. Regarding the stationary properties we summarize the predictions of both approaches in a phase diagram as a function of interparticle interaction strength. From this analysis we identify the value of the critical interaction for which the transition occurs, and establish the limitations of the mean field against the Bose-Hubbard description, namely, that most of the stationary states are not accounted for by the former. Then, we characterize the dynamics and find that both schemes predict a continuous transition. By tracking the time evolution of the one-particle reduced density matrix in the N-particle Bose-Hubbard model we find the existence of stationary or equilibrium states, that appear as a consequence of intrinsic decoherence.
June 12
1. arXiv:1306.2606 [pdf, other]
Magnetically generated spin-orbit coupling for ultracold atoms
Brandon M. Anderson, I. B. Spielman, Gediminas Juzeliūnas
We present a new technique for producing two and three dimensional Rashba-type spin-orbit coupling for ultra cold atoms without involving light. The method relies on a sequence of pulsed inhomogeneous magnetic fields imprinting suitable phase gradients on the atoms. For sufficiently short pulse durations, the time-averaged Hamiltonian well approximates the Rashba Hamiltonian. Higher order corrections to the energy spectrum are calculated exactly for spin-1/2 and pertur- batively for higher spins. The pulse sequence does not modify the form of rotationally symmetric atom-atom interactions. Finally, we present a straightforward implementation of this pulse sequence on an atom-chip.
2. arXiv:1306.2510 [pdf, ps, other]
Universal behavior of two-dimensional bosonic gases at Berezinskii-Kosterlitz-Thouless transitions
G. Ceccarelli, J. Nespolo, A. Pelissetto, E. Vicari
We study the universal critical behavior of two-dimensional (2D) lattice bosonic gases at the Berezinskii-Kosterlitz-Thouless (BKT) transition, which separates the low-temperature superfluid phase from the high-temperature normal phase. For this purpose, we perform quantum Monte Carlo simulations of the hard-core Bose-Hubbard (BH) model at zero chemical potential. We determine the critical temperature by using a matching method that relates finite-size data for the BH model with corresponding data computed in the classical XY model. In this approach, the neglected scaling corrections decay as inverse powers of the lattice size L, and not as powers of 1/lnL, as in more standard approaches, making the estimate of the critical temperature much more reliable. Then, we consider the BH model in the presence of a trapping harmonic potential, and verify the universality of the trap-size dependence at the BKT critical point. This issue is relevant for experiments with quasi-2D trapped cold atoms.
June 11
1. arXiv:1306.1965 [pdf, other]
A Raman-induced Feshbach resonance in an effectively single-component Fermi gas
R. A. Williams, M. C. Beeler, L. J. LeBlanc, K. Jimenez-Garcia, I. B. Spielman
Ultracold gases of interacting spin-orbit coupled fermions are predicted to display exotic phenomena such as topological superfluidity and its associated Majorana fermions. Here, we experimentally demonstrate a route to strongly-interacting single-component atomic Fermi gases by combining an s-wave Feshbach resonance (giving strong interactions) and spin-orbit coupling (creating an effective p-wave channel). We identify the Feshbach resonance by its associated atomic loss feature and show that, in agreement with our single-channel scattering model, this feature is preserved and shifted as a function of the spin-orbit coupling parameters.
2. arXiv:1306.2162 [pdf, other]
Quantum simulation of a lattice Schwinger model in a chain of trapped ions
Philipp Hauke, David Marcos, Marcello Dalmonte, Peter Zoller
We discuss how a lattice Schwinger model can be realized in a linear ion trap, allowing a detailed study of the physics of Abelian lattice gauge theories related to one-dimensional quantum electrodynamics. Relying on the rich quantum-simulation toolbox available in state-of-the-art trapped-ion experiments, we show how one can engineer an effectively gauge-invariant dynamics by imposing energetic constraints, provided by strong Ising-like interactions. Applying exact diagonalization to ground-state and time-dependent properties, we study the underlying microscopic model, and discuss undesired interaction terms and other imperfections. As our analysis shows, the proposed scheme allows for the observation in realistic setups of spontaneous parity- and charge-symmetry breaking, as well as false-vacuum decay. Besides an implementation aimed at larger ion chains, we also discuss a minimal setting, consisting of only four ions in a simpler experimental setup, which enables to probe basic physical phenomena related to the full many-body problem. The proposal opens a new route for analog quantum simulation of high-energy and condensed-matter models where gauge symmetries play a prominent role.
June 10
1.arXiv:1306.1796 [pdf, other]
Self-consistent tight-binding description of Dirac points moving and merging in two dimensional optical lattices
Julen Ibañez-Azpiroz, Asier Eiguren, Aitor Bergara, Giulio Pettini, Michele Modugno
We present an accurate ab initio tight-binding model, capable of describing the dynamics of Dirac points in tunable honeycomb optical lattices following a recent experimental realization [L. Tarruell et al., Nature 483, 302 (2012)]. Our scheme is based on first-principle maximally localized Wannier functions for composite bands. The tunneling coefficients are calculated for different lattice configurations, and the spectrum properties are well reproduced with high accuracy. In particular, we show which tight binding description is needed in order to accurately reproduce the position of Dirac points and the dispersion law close to their merging, for different laser intensities.
2.arXiv:1306.1711 [pdf, ps, other]Recombination rates from potential models close to the unitary limit
E. Garrido, M. Gattobigio, A. Kievsky
We investigate universal behavior in the recombination rate of three bosons close to threshold. Using the He-He system as a reference, we solve the three-body Schr\"odinger equation above the dimer threshold for different potentials having large values of the two-body scattering length $a$. To this aim we use the hyperspherical adiabatic expansion and we extract the $S$-matrix through the integral relations recently derived. The results are compared to the universal form, $\alpha\approx 67.1\sin^2[s_0\ln(\kappa_*a)+\gamma]$, for different values of $a$ and selected values of the three-body parameter $\kappa_*$. A good agreement with the universal formula is obtained after introducing a particular type of finite-range corrections, which have been recently proposed by two of the authors in Ref.[1]. Furthermore, we analyze the validity of the above formula in the description of a very different system: neutron-neutron-proton recombination. Our analysis confirms the universal character of the process in systems of very different scales having a large two-body scattering length.
June 3 - June 7, Xiaopeng Li
June 7
1. arXiv:1306.1528 [pdf, other]
Superconductivity with intrinsic topological order induced by pure Coulomb interaction and time-reversal symmetry breaking
Evelyn Tang, Xiao-Gang Wen
Recently, in certain flat band lattice systems at commensurate fillings, fractional quantum Hall states have been found -- which have anyonic excitations. We study such systems away from commensuration, i.e. the ground state of an anyon gas in such a system. The presence of the underlying lattice allows access to an entirely new regime where the anyon kinetic energy can be larger than their interaction energy. Within the flux-attachment approach, using mean-field then adding fluctuations, we find several possible superfluid states. Two have intrinsic topological order, i.e. fractionalized quasiparticles with a fusion structure of (Z_2)^4 and (Z_8)^2 respectively, and a third has no fractionalized excitations similar to a BCS-type state. This represents a mechanism for superconductivity driven purely by strong repulsion and complex hopping of electrons.
2.arXiv:1306.1253 [pdf, ps, other]
Transverse magnetic field and chiral-nonchiral transition in vortex states for nearly B//ab in chiral p-wave superconductors
Masahiro Ishihara, Yuujirou Amano, Masanori Ichioka, Kazushige Machida (Okayama Univ.)
On the basis of Eilenberger theory, we study the vortex state when a magnetic field is applied nearly parallel to the ab plane in a chiral p-wave superconductor with a large anisotropy ratio of ab and c, as in Sr2RuO4. We quantitatively estimate the field dependence of the pair potential, magnetization, and flux line lattice form factor, and study the transition from the chiral p_- state at low fields to the nonchiral p_y state at high fields. Even for exactly parallel fields to the ab plane, transverse fields exist in the chiral state. The chiral-nonchiral transition disappears when the magnetic field orientation is tilted within 1 degree from the ab plane. This may be a reason why the experimental detection of this transition is difficult.
3. arXiv:1306.1234 [pdf, other]
Probing the chiral anomaly with nonlocal transport in Weyl semimetals
S.A. Parameswaran, T. Grover, D. A. Abanin, D. A. Pesin, A. Vishwanath
Weyl semimetals are three-dimensional crystalline systems where pairs of bands touch at points in momentum space, termed Weyl nodes, that are characterized by a definite topological charge: the chirality. Consequently, they exhibit the Adler-Bell-Jackiw anomaly, which in this condensed matter realization implies that application of parallel electric (E) and magnetic (B) fields pumps electrons between nodes of opposite chirality at a rate proportional to E dot B. We argue that this pumping is measurable via nonlocal transport experiments, in the limit of weak internode scattering. Specifically, we show that as a consequence of the anomaly, applying a local magnetic field parallel to an injected current induces a valley imbalance that diffuses over long distances. A probe magnetic field can then convert this imbalance into a measurable voltage drop far from source and drain. Such nonlocal transport vanishes when the injected current and magnetic field are orthogonal, and therefore serves as a test of the chiral anomaly. Since the nodes are analogous to valley degrees of freedom in semiconductors, this suggests that valley currents in Weyl semimetals can be controlled using electric fields, which has potential practical 'valleytronic' applications.
June 6
1.arXiv:1306.1190 [pdf, other]
Chiral Ladders and the Edges of Chern Insulators
Dario Hügel, Belén Paredes
The realization and detection of topological phases with ultracold atomic gases is at the frontier of current theoretical and experimental research. Here, we identify cold atoms in optical ladders subjected to synthetic magnetic fields as readily realizable bridges between one-dimensional spin-orbit (time reversal) topological insulators and two-dimensional Chern insulators. We reveal three instances of their promising potential: i) they realize spin-orbit coupling, with the left-right leg degree of freedom playing the role of an effective spin, ii) their energy bands and eigenstates exactly reproduce the topological chiral edge modes of two-dimensional Chern insulators, and iii) they can be tailored to realize a topological phase transition from a trivial to a topological insulating phase. We propose realistic schemes to observe the chiral and topological properties of ladder systems with current optical lattice-based experiments. Our findings open a door to the exploration of the physics of the edges of Chern insulators and to the realization of spin-orbit coupling and topological superfluid phases with ultracold atomic gases.
2. arXiv:1306.1125 [pdf, ps, other]
Nematicity as a probe of superconducting pairing in iron-based superconductors
Rafael M. Fernandes, Andrew J. Millis
In several families of iron-based superconducting materials, a d-wave pairing instability may compete with the leading s-wave instability. Here we show that when both states have comparable free energies, superconducting and nematic degrees of freedom are strongly coupled. While nematic order causes a sharp non-analytic increase in $T_{c}$, nematic fluctuations can change the character of the s-wave to d-wave transition, favoring an intermediate state that does not break time-reversal symmetry but does break tetragonal symmetry. The coupling between superconductivity and nematicity is also manifested in the strong softening of the shear modulus across the superconducting transition. Our results show that nematicity can be used as a diagnostic tool to search for unconventional pairing states in iron pnictides and chalcogenides.
3.arXiv:1306.1104 [pdf, other]
Crossover from weak to unitarity-limited interactions in Bose gases
J.J.R.M. van Heugten, H.T.C. Stoof
We develop an analytical approach for the description of the crossover of an atomic Bose gas from small to infinitely large scattering length where the two-body interactions satisfy the unitarity bound. We obtain several properties of the Bose gas as a function of interaction strength, namely the chemical potential, the contact, the speed of sound, the condensate density, the effective interatomic interaction and the three-body recombination rate. Also, the energy dependence of the effective interaction at unitarity and the evolution of the Feshbach bound state are discussed. It is shown how the approach can be systematically improved with renormalization-group methods and how it reduces to the Bogoliubov theory in the weak-coupling limit.
4. arXiv:1306.0998 [pdf, ps, other]
Defects in Topological Three-bands
Gyungchoon Go, Kyeong Tae Kang, Jung Hoon Han
How a defect manifests itself in topological bands is examined for the three-band case in both one and two space dimensions. Extension of Jackiw-Rebbi theory of soliton formation based on two-component Dirac model is given for three-band solitons of irrational charge. Insertion of \pi-flux in one-dimensional topological diamond-chain lattice fails to produce soliton defects, while in two-dimensional Kagome topological three-band it leads to a pair of soliton defects, each one dictated by the standard Jackiw-Rebbi scenario.
Jun 5
1. arXiv:1306.0638 [pdf, ps, other]
Newton-Cartan Geometry and the Quantum Hall Effect
Dam Thanh Son
We construct an effective field theory for quantum Hall states, guided by the requirements of nonrelativistic general coordinate invariance and regularity of the zero mass limit. We propose Newton-Cartan geometry as the most natural formalism to construct such a theory. Universal predictions of the theory are discussed.
Jun 4
1. arXiv:1306.0245 [pdf, other]
Interband spin-orbit coupling between anti-parallel spin states in Pb quantum well states
Bartosz Slomski, Gabriel Landolt, Stefan Muff, Fabian Meier, Jürg Osterwalder, J. Hugo Dil
Using spin and angle-resolved photoemission spectroscopy we investigate a momentum region in Pb quantum well states on Si(111) where hybridization between Rashba-split bands alters the band structure significantly. Starting from the Rashba regime where the dispersion of the quasi-free two-dimensional electron gas is well described by two spin-polarized parabolas, we find a breakdown of the Rashba behavior which manifests itself (i) in a spin splitting that is no longer proportional to the in-plane momentum and (ii) in a reversal of the sign of the momentum splitting. Our experimental findings are well explained by including interband spin-orbit coupling that mixes Rashba-split states with anti-parallel rather than parallel spins. Similar results for Pb/Cu(111) reveal that the proposed hybridization scenario is independent on the supporting substrate.
2. arXiv:1306.0172 [pdf, other]
Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence
Michael Foss-Feig, Kaden R A Hazzard, John J Bollinger, Ana Maria Rey, Charles W Clark
Ising models, and the physical systems described by them, play a central role in generating entangled states for use in quantum metrology and quantum information. In particular, ultracold atomic gases, trapped ion systems, and Rydberg atoms realize long-ranged Ising models, which even in the absence of a transverse field can give rise to highly non-classical dynamics and long-range quantum correlations. In the first part of this paper, we present a detailed theoretical framework for studying the dynamics of such systems driven (at time t=0) into arbitrary unentangled non-equilibrium states, thus greatly extending and unifying the work of Ref. [1]. Specifically, we derive exact expressions for closed-time-path ordered correlation functions, and use these to study experimentally relevant observables, e.g. Bloch vector and spin-squeezing dynamics. In the second part, these correlation functions are then used to derive closed-form expressions for the dynamics of arbitrary spin-spin correlation functions in the presence of both T_1 (spontaneous spin relaxation/excitation) and T_2 (dephasing) type decoherence processes. Even though the decoherence is local, our solution reveals that the competition between Ising dynamics and T_1 decoherence gives rise to an emergent non-local dephasing effect, thereby drastically amplifying the degradation of quantum correlations. In addition to identifying the mechanism of this deleterious effect, our solution points toward a scheme to eliminate it via measurement-based coherent feedback.
Jun 3
1. arXiv:1305.7500 [pdf, other]
Quantum Anomalous Hall Effect with Higher Plateaus
Jing Wang, Biao Lian, Haijun Zhang, Yong Xu, Shou-Cheng Zhang
Quantum anomalous Hall (QAH) effect in magnetic topological insulators is driven by the combination of spontaneous magnetic moments and spin-orbit coupling. Its recent experimental discovery raises the question if higher plateaus can also be realized. Here we present a general theory for QAH effect with higher Chern numbers, and show by first-principles calculations that thin film magnetic topological insulator of Cr-doped Bi$_2$(Se,Te)$_3$ is a candidate for the C=2 QAH insulator. Remarkably, whereas higher magnetic field leads to lower Hall conductance plateaus in the integer quantum Hall effect, higher magnetic moment leads to higher Hall conductance plateaus in the QAH effect.
2. arXiv:1305.7320 [pdf, ps, other]
Coupling, merging, and splitting Dirac points by electron-electron interaction
Balázs Dóra, Igor F. Herbut, Roderich Moessner
The manipulation and movement of Dirac points in the Brillouin zone by the electron-electron interaction is considered within leading order perturbation theory. At the merging point, an infinitesimal interaction is shown to cause opening of the gap or splitting of the Dirac points, depending on the inter- or intrasublattice nature of the merging and the sign of the interaction. The topology of the spectrum can therefore be efficiently changed by simply tuning the interaction between particles, as opposed to the usual careful band structure engineering. This is illustrated around the merging transition of one, two, and three dimensional Dirac-Weyl fermions. A simple Weyl-like Hamiltonian that describes the quadratic band-crossing in three dimensions is also proposed, and its stability under interactions is addressed.
3. arXiv:1305.7275 [pdf, other]
Phase Diagram of a Three Orbital Model for the high-T$_c$ cuprates
Cedric Weber, Thierry Giamarchi, Chandra M. Varma
We study the phase diagram of an effective three orbital model of the cuprates using Variational Monte-Carlo calculations (VMC) on asymptotically large lattices and exact diagonalization on a 24-site cluster. States with ordered orbital current loops (LC), itinerant Anti-ferromagnetism (AFM), d-wave superconductivity (SC), and the Fermi-liquid (FL) are investigated using appropriate Slater determinants refined by Jastrow functions for on-site and inter-site correlations. We find an LC state stable in the thermodynamic limit for a range of parameters compatible with the Fermi surface of a typical hole doped superconductors provided the transfer integrals between the oxygen atoms have signs determined by the effects of indirect transfer through the Cu-4s orbitals as suggested by O.K. Andersen. The results of the calculations are that this phase gives way at lower dopings to an AFM phase and at larger copings to a SC phase followed by a FL phase.
4. arXiv:1305.7233 [pdf, ps, other]
Odd parity superconductivity in Weyl semimetals
Huazhou Wei, Sung-Po Chao, Vivek Aji
Unconventional superconducting states of matter are realized in the presence of strong spin orbit coupling. In particular, non degenerate bands can support odd parity superconductivity with rich topological content. Here we study whether this is the case for Weyl semimetals. These are systems whose low energy sector, in the absence of interactions, is described by linearly dispersing chiral fermions in three dimensions. The energy spectrum has nodes at an even number of points in the Brillouin zone. Consequently both intranodal finite momentum pairing and internodal BCS superconductivity are allowed. For local attractive interaction the finite momentum pairing state with chiral p-wave symmetry is found to be most favorable at finite chemical potential. The state is an analog of the superfluid $^{3}$He A phase, with cooper pairs having finite center of mass momentum. For chemical potential at the node the state is preempted by a fully gapped charge density wave. For long range attraction the BCS state wins out for all values of the chemical potential.
