Mar 26 - Mar 30, Bin Wang
Mar 30
1. arXiv:1203.6367 [pdf, ps, other]
Stability of ultracold atomic Bose condensates with Rashba spin-orbit coupling against quantum and thermal fluctuations
Tomoki Ozawa, Gordon Baym
We study the stability of Bose condensates with Rashba-Dresselhaus spin-orbit coupling in three dimensions against quantum and thermal fluctuations. The ground state depletion of the plane-wave condensate due to quantum fluctuations is, as we show, finite, and therefore the condensate is stable. We also calculate the corresponding shift of the ground state energy. Although the system cannot condense in the absence of interparticle interactions, we show by estimating the number of excited particles that interactions stabilize the condensate even at non-zero temperature. Unlike in the usual Bose gas, the normal phase is not kinematically forbidden at any temperature; calculating the free energy of the normal phase at finite temperature, and comparing with the free energy of the condensed state, we infer that generally the system is condensed at zero temperature, and undergoes a transition to normal at non-zero temperature.
2. arXiv:1203.6363 [pdf, other]
Multigrid Algorithms for Tensor Network States
M. Dolfi, B. Bauer, M. Troyer, Z. Ristivojevic
The widely used density matrix renormalization group (DRMG) method often fails to converge in systems with multiple length scales, such as lattice discretizations of continuum models and dilute or weakly doped lattice models. The local optimization employed by DMRG to optimize the wave function is ineffective in updating large-scale features. Here we present a multigrid algorithm that solves these convergence problems by optimizing the wave function at different spatial resolutions. We demonstrate its effectiveness by simulating bosons in continuous space, and study non-adiabaticity when ramping up the amplitude of an optical lattice. The algorithm can be generalized to tensor network methods, and be combined with the contractor renormalization group (CORE) method to study dilute and weakly doped lattice models.
3. arXiv:1203.6359 [pdf, ps, other]
The Cooper problem and beyond in Bose-Fermi mixtures
Peter Anders, Philipp Werner, Matthias Troyer, Manfred Sigrist, Lode Pollet
We calculate the phase diagram of the Bose-Fermi Hubbard model on the 3d cubic lattice at fermionic half filling and bosonic unity filling by means of single-site dynamical mean-field theory (DMFT). For fast bosons, this is the Cooper problem in which the bosons can induce s-wave pairing, even between repulsively interacting fermions. Superfluid and supersolid phases are found, depending on the interspecies coupling strength. The modifications in the phase diagram compared to the purely fermionic case are well explained by renormalized hoppings and on-site interactions. Slow bosons favor fermionic charge density wave structures for attractive fermionic interactions.
4. arXiv:1203.6595 (cross-list from quant-ph) [pdf, other]
Engineered Open Systems and Quantum Simulations with Atoms and Ions
M. Müller, S. Diehl, G. Pupillo, P. Zoller
The enormous experimental progress in atomic, molecular and optical (AMO) physics during the last decades allows us nowadays to isolate single, a few or even many-body ensembles of microscopic particles, and to manipulate their quantum properties at a level of precision, which still seemed unthinkable some years ago. This versatile set of tools has enabled the development of the well-established concept of engineering of many-body Hamiltonians in various physical platforms. These available tools, however, can also be harnessed to extend the scenario of Hamiltonian engineering to a more general Liouvillian setting, which in addition to coherent dynamics also includes controlled dissipation in many-body quantum systems. Here, we review recent theoretical and experimental progress in different directions along these lines, with a particular focus on physical realizations with systems of atoms and ions. This comprises digital quantum simulations in a general open system setting, as well as engineering and understanding new classes of systems far away from thermodynamic equilibrium.
Mar 29
1. arXiv:1203.6348 [pdf, ps, other]
Induced superfluidity of imbalanced Fermi gases near unitarity
Kelly R. Patton, Daniel E. Sheehy
The induced intraspecies interactions among the majority species, mediated by the minority species, is computed for a population-imbalanced two-component Fermi gas. Although the Feshbach-resonance mediated interspecies interaction is dominant for equal populations, leading to singlet s-wave pairing, we find that in the strongly imbalanced regime the induced intraspecies interaction leads to p-wave pairing and superfluidity of the majority species. Thus, we predict that the observed spin-polaron Fermi liquid state in this regime is unstable to p-wave superfluidity, in accordance with the results of Kohn and Luttinger, below a temperature that, near unitarity, we find to be within current experimental capabilities. Possible experimental signatures of the p-wave state using radio-frequency spectroscopy as well as density-density correlations after free expansion are presented.
2. arXiv:1203.6292 [pdf, ps, other]
Probing superfluidity of a mesoscopic Tonks-Girardeau gas
C. Schenke, A. Minguzzi, F. W. J. Hekking
We study the dynamical response of a Tonks-Girardeau gas on a ring induced by a moving delta-barrier potential. An exact solution based on the time-dependent Bose-Fermi mapping allows to obtain the particle current, its fluctuations and the drag force acting on the barrier. The exact solution is analyzed numerically as well as analytically in the perturbative regime of weak barrier strength. In the weak barrier limit the stirring drives the system into a state with net zero current for velocities $v$ smaller than $v_c=\pi\hbar /mL$, with $m$ the atomic mass and $L$ the ring circumference. At $v$ approaching $v_c$ angular momentum can be transferred to the fluid and a nonzero drag force arises. The existence of a velocity threshold for current generation indicates superfluid-like behavior of the mesoscopic Tonks-Girardeau gas, different from the non-superfluid behavior predicted for Tonks-Girardeau gas in an infinite tube.
3. arXiv:1203.6065 [pdf, ps, other]
Global superfluid phase diagram of three component fermions with magnetic ordering
M. Kanasz-Nagy, G. Zarand
We investigate a three component fermion mixture in the presence of weak attractive interactions. We use a combination of the equation of motion and the Gaussian variational mean-field approaches, which both allow for simultaneous superfluid and magnetic ordering in an unbiased way, and capture the interplay between the two order parameters. This interplay significantly modifies the phase diagram, especially the superfluid-normal phase boundaries. In the close vicinity of the critical temperature and for small chemical potential imbalances, strong particle-hole symmetry breaking leads to a phase diagram similar to the one predicted by Cherng et al. [Phys. Rev. Lett. 99, 130406 (2007)], however, the overall phase diagram is markedly different: new chemical potential-driven first and second order transitions and triple points emerge as well as more exotic second order multicritical points, and bicritical lines with O(2,2) symmetry. We identify the terms which are necessary to capture this complex phase diagram in a Ginzburg-Landau approach, and determine the corresponding coefficients.
Mar 28
1. arXiv:1203.6054 [pdf, ps, other]
Pairing and radio-frequency spectroscopy in two-dimensional Fermi gases
Ville Pietilä
We theoretically study the normal phase properties of strongly interacting two-component Fermi gases in two spatial dimensions. In the limit of weak attraction, we find that even a balanced gas can be described in terms of effective polarons. As the attraction between fermions increases, we find a crossover from the gas of noninteracting polarons to a pseudogap phase. We investigate the signatures of this crossover in the radio-frequency (rf) spectroscopy. Our findings qualitatively explain the differences in the recent rf spectroscopy measurements of two-dimensional Fermi gases [Sommer et al., Phys. Rev. Lett. 108, 045302 (2012) and Zhang et al., arXiv:1201.3560].
2. arXiv:1203.6006 [pdf, other]
Quantum theory of bright matter wave solitons in harmonic confinement
David I. H. Holdaway, Christoph Weiss, Simon A. Gardiner
This paper investigates bright quantum-matter-wave solitons beyond the Gross Pitaevskii equation (GPE). As proposals for interferometry and creating nonlocal quantum superpositions have been formed, it has become necessary to investigate effects not present in mean field models. We investigate the effect of harmonic confinement on the internal degrees of freedom, as the ratio of zero point harmonic oscillator length to classical soliton length, for different numbers of atoms. We derive a first-order energy correction for the addition of a harmonic potential to the many-body wavefunction and use this to create a variational technique based on energy minimization of this wavefunction for an arbitrary number of atoms, and include numerics based on diagonalization of the Hamiltonian in a basis of harmonic oscillator Fock states. Finally we compare agreement between a Hartree product ground state and the Bethe ansatz solution with a Gaussian envelope localizing the center of mass and show a region of good agreement.
3. arXiv:1203.5999 [pdf, ps, other]
Dimer, trimer and FFLO liquids in mass- and spin-imbalanced trapped binary mixtures in one dimension
M. Dalmonte, K. Dieckmann, T. Roscilde, C. Hartl, A. E. Feiguin, U. Schollwöck, F. Heidrich-Meisner
We present a systematic investigation of attractive binary mixtures in presence of both spin- and mass-imbalance in one dimensional setups described by the Hubbard model. After discussing typical cold atomic experimental realizations and the relation between microscopic and effective parameters, we study several many-body features of trapped Fermi-Fermi and Bose-Bose mixtures such as density profiles, momentum distributions and correlation functions by means of numerical density-matrix-renormalization-group and Quantum Monte Carlo simulations. In particular, we focus on the stability of Fulde-Ferrell-Larkin-Ovchinnikov, dimer and trimer fluids in inhomogeneous situations, as typically realized in cold gas experiments due to the harmonic confinement. We finally consider possible experimental signatures of these phases both in the presence of a finite polarization and of a finite temperature.
4. arXiv:1203.5949 [pdf, ps, other]
Relativistic quantum effects of Dirac particles simulated by ultracold atoms
Dan-Wei Zhang, Zi-Dan Wang, Shi-Liang Zhu
Quantum simulation is a powerful tool to study a variety of problems in physics, ranging from high-energy physics to condensed-matter physics. In this article, we review the recent theoretical and experimental progress in quantum simulation of Dirac equation with tunable parameters by using ultracold neutral atoms trapped in optical lattices or subject to light-induced synthetic gauge fields. The effective theories for the quasiparticles become relativistic under certain conditions in these systems, making them ideal platforms for studying the exotic relativistic effects. We focus on the realization of one, two, and three dimensional Dirac equations as well as the detection of some relativistic effects, including particularly the well-known Zitterbewegung effect and Klein tunneling. The realization of quantum anomalous Hall effects is also briefly discussed.
5. arXiv:1203.5801 (cross-list from quant-ph) [pdf, ps, other]
Criticality without frustration for quantum spin-1 chains
Sergey Bravyi, Libor Caha, Ramis Movassagh, Daniel Nagaj, Peter Shor
Frustration-free (FF) spin chains have a property that their ground state minimizes all individual terms in the chain Hamiltonian. We ask how entangled the ground state of a FF quantum spin-s chain with nearest-neighbor interactions can be for small values of s. While FF spin-1/2 chains are known to have unentangled ground states, the case s=1 remains less explored. We propose the first example of a FF translation-invariant spin-1 chain that has a unique highly entangled ground state and exhibits some signatures of a critical behavior. The ground state can be viewed as the uniform superposition of balanced strings of left and right parentheses separated by empty spaces. Entanglement entropy of one half of the chain scales as log(n)/2 + O(1), where n is the number of spins. We prove that the energy gap above the ground state is polynomial in 1/n. The proof relies on a new result concerning statistics of Dyck paths which might be of independent interest.
Mar 27
1. arXiv:1203.5629 [pdf, ps, other]
Quantum and Thermal Transitions Out of the Pair-Supersolid Phase of Two-Species Bosons in Lattice
Chia-Min Chung, Shiang Fang, Pochung Chen
We investigate two-species bosons in a two-dimensional square lattice by quantum Monte Carlo method. We show that the inter-species attraction and nearest-neighbor intra-species repulsion results in the pair-supersolid phase, where a diagonal solid order coexists with an off-diagonal pair-superfluid order. The quantum and thermal transitions out of the pair-supersolid phase are characterized. It is found that there is a direct first order transition from the pair-supersolid phase to the double-superfluid phase without an intermediate region. Furthermore, the melting of the pair-supersolid occurs in two steps. Upon heating, first the pair-superfluid is destroyed via a KT transition then the solid order melts via an Ising transition.
2. arXiv:1203.5357 [pdf, other]
Dynamics of parametric matter wave amplification
Robert Bücker, Ulrich Hohenester, Tarik Berrada, Sandrine van Frank, Aurélien Perrin, Stephanie Manz, Thomas Betz, Julian Grond, Thorsten Schumm, Jörg Schmiedmayer
We develop a model for parametric amplification, based on a density matrix approach, which naturally accounts for the peculiarities arising for matter waves: significant depletion and explicit time-dependence of the source state population, long interaction times, and spatial dynamics of the amplified modes. We apply our model to explain the details in an experimental study on twin-atom beam emission from a one-dimensional degenerate Bose gas.
3. arXiv:1203.5541 [pdf, other]
Unwinding of a one-dimensional topological insulators
Achim Rosch
We show that a topological insulator made of four chains of superconducting spinless fermions characterized by four Majorana edge states can adiabatically be deformed into a trivial band insulator. To unwind this time-reversal invariant topological insulator, interactions to {\em spinful} fermions are switched on along an adiabatic path. Thereby, we couple modes which belong to two different representations of the time-reversal symmetry operator T with T^2=1 and T^2=-1, respectively. This observation can easily be understood by investigating how the relevant symmetries act on the entanglement spectrum giving rise to a Z_4 instead of a Z_8 classification of the interacting system. We also show that a simple level crossing of doubly and singly degenerate states occurs in the entanglement spectrum upon deforming the quantum state.
Mar 26
1. arXiv:1203.5120 [pdf, ps, other]
Pair density waves and vortices in an elongated two-component Fermi gas
Ran Wei, Erich J. Mueller
We study the vortex structures of a two-component Fermi gas experiencing a uniform effective magnetic field in an anisotropic trap that interpolates between quasi-one dimensional (1D) and quasi-two dimensional (2D). At a fixed chemical potential, reducing the anisotropy (or equivalently increasing the attractive interactions or increasing the magnetic field) leads to instabilities towards pair density waves, and vortex lattices. Reducing the chemical potential stabilizes the system. We calculate the phase diagram, and explore the density and pair density. The structures are similar to those predicted for superfluid Bose gases. We further calculate the paired fraction, showing how it depends on chemical potential and anisotropy.
Mar 19 - Mar 23, Johannes Schachenmayer
Mar 23
1. arXiv:1203.5094 [pdf, other]
Interaction-induced conducting-nonconducting transition of ultra-cold atoms in 1D optical lattices
Chih-Chun Chien, Massimiliano Di Ventra, Michael ZwolakThe study of time-dependent, many-body transport phenomena is increasingly within reach of ultra-cold atom experiments. We show that the introduction of spatially inhomogeneous interactions, e.g., generated by optically-controlled collisions, induce a conducting-to-nonconducting transition in the transport of atoms in 1D optical lattices. Specifically, we simulate the dynamics of interacting fermionic atoms via a micro-canonical transport formalism within both mean-field and other approximations. For weakly repulsive interactions, a quasi steady-state atomic current develops that is similar to the situation occurring for electronic systems subject to an external voltage bias. As the interactions exceed a threshold value, a mean-field conducting-to-nonconducting transition occurs due to energetic constraints. This transition is preceded by the atomic equivalent of negative differential conductivity observed in transport across solid-state structures, but the broad tunability of cold atoms allows for richer non-equilibrium physics.
2. arXiv:1203.4890 [pdf, ps, other]
Detecting the superfluid critical momentum of Bose gases in optical lattices through dipole oscillations
Takuya Saito, Ippei Danshita, Takeshi Ozaki, Tetsuro NikuniWe study stability of superflow of Bose gases in optical lattices by analyzing the Bose-Hubbard model within the Gutzwiller mean-field approximation. We calculate the excitation spectra of the homogeneous Bose-Hubbard model at unit filling to determine the critical momenta for the Landau and dynamical instabilities. These two critical momenta are shown to approach each other when the on-site interaction increases towards the Mott transition point. In order to make a direct connection with realistic experiments, we next take into account a parabolic trapping potential and compute the real-time dynamics of dipole oscillations induced by suddenly displacing the trap center. We consider the following two cases: standard softcore bosons, whose interparticle interactions include the on-site one only, and hardcore bosons with long-range dipole-dipole interactions. For both cases, we show that the dipole oscillation is significantly damped when the maximum local momentum exceeds a certain threshold, which quantitatively agrees with the critical momentum for the dynamical instability in the homogeneous system. In the case of dipolar hardcore bosons, the dynamical instability of dipole oscillations leads to the formation of checkerboard density waves in the superfluid phase near the boundary to the supersolid phase.
3. arXiv:1203.4819 [pdf, other]
Quantum flutter of supersonic particles in one-dimensional quantum liquids
Charles J. M. Mathy, Mikhail B. Zvonarev, Eugene DemlerThe non-equilibrium dynamics of strongly correlated many-body systems exhibits some of the most puzzling phenomena and challenging problems in condensed matter physics. Here we report on essentially exact results on the time evolution of an impurity injected at a finite velocity into a one-dimensional quantum liquid. We provide the first quantitative study of the formation of the correlation hole around a particle in a strongly coupled many-body quantum system, and find that the resulting correlated state does not come to a complete stop but reaches a steady state which propagates at a finite velocity. We also uncover a novel physical phenomenon when the impurity is injected at supersonic velocities: the correlation hole undergoes long-lived coherent oscillations around the impurity, an effect we call quantum flutter. We provide a detailed understanding and an intuitive physical picture of these intriguing discoveries, and propose an experimental setup where this physics can be realized and probed directly.
4. arXiv:1203.4973 [pdf, ps, other]
Boltzmann-type approach to transport in weakly interacting one-dimensional fermionic systems
Christian Bartsch, Jochen Gemmer
We investigate transport properties of one-dimensional fermionic tight binding models featuring nearest and next-nearest neighbor hopping, where the fermions are additionally subject to a weak short range mutual interaction. To this end we employ a pertinent approach which allows for a mapping of the underlying Schr\"odinger dynamics onto an adequate linear quantum Boltzmann equation. This approach is based on a suitable projection operator method. From this Boltzmann equation we are able to numerically obtain diffusion coefficients in the case of non-vanishing next-nearest neighbor hopping, i.e., the non-integrable case, whereas the diffusion coefficient diverges without next-nearest neighbor hopping. For the latter case we analytically investigate the decay behavior of the current with the result that arbitrarily small parts of the current relax arbitrarily slowly which suggests anomalous diffusive transport behavior within the scope of our approach.
Mar 22
1. arXiv:1203.4658 [pdf, ps, other]
Advantages of mass-imbalanced ultracold fermionic mixtures for approaching quantum magnetism in optical lattices
Andrii Sotnikov, Daniel Cocks, Walter HofstetterWe study magnetic phases of two-component mixtures of ultracold fermions with repulsive interactions in optical lattices in the presence of hopping imbalance. Our analysis is based on dynamical mean-field theory (DMFT) and its real-space generalization at finite temperature. We study the temperature dependence of the transition into the ordered state as a function of the interaction strength and the imbalance parameter in two and three spatial dimensions. We show that below the critical temperature for Neel order mass-imbalanced mixtures also exhibit a charge-density wave, which provides a directly observable signature of the ordered state. For the trapped system, we compare our results obtained by real-space DMFT to a local-density approximation. We calculate the entropy for a wide range of parameters and identify regions, in which mass-imbalanced mixtures have clear advantages over balanced mixtures for the purpose of obtaining and detecting quantum magnetism.
2. arXiv:1203.4657 [pdf, other]
Dynamic spin response of a strongly interacting Fermi gas
S. Hoinka, M. Lingham, M. Delehaye, C. J. ValeWe present an experimental investigation of the dynamic spin response of a strongly interacting Fermi gas using Bragg spectroscopy. By varying the detuning of the Bragg lasers, we show that it is possible to measure the response in the spin and density channels separately. At low Bragg energies, the spin response is suppressed due to pairing, whereas the density response is enhanced. These experiments provide the first independent measurements of the spin-parallel and spin-antiparallel dynamic and static structure factors and open the way to a complete study of the structure factors at any momentum. At high momentum the spin-antiparallel dynamic structure factor displays a universal high frequency tail, proportional to $\omega^{-5/2}$, where $\hbar \omega$ is the probe energy.
Mar 21
1. arXiv:1203.4341 [pdf, ps, other]
Rydberg crystallization detection by statistical means
David Breyel, Thomas L. Schmidt, Andreas KomnikWe investigate an ensemble of atoms which can be excited into a Rydberg state. Using a disordered quantum Ising model, we perform a numerical simulation of the experimental procedure and calculate the probability distribution function $P(M)$ to create a certain number of Rydberg atoms $M$, as well as their pair correlation function. Using the latter, we identify the critical interaction strength above which the system undergoes a phase transition to a Rydberg crystal. We then show that this phase transition can be detected using $P(M)$ alone.
2. arXiv:1203.4262 [pdf, ps, other]
Anisotropic pair-superfluidity of trapped two-component Bose gases
Yongqiang Li, Liang He, Walter HofstetterWe theoretically investigate the pair-superfluid phase of two-component ultracold gases with negative inter-species interactions in an optical lattice. We establish the phase diagram for filling $n=1$ at zero and finite temperature, by applying Bosonic Dynamical Mean-Field Theory, and confirm the stability of pair-superfluidity for asymmetric hopping of the two species. While the pair superfluid is found to be robust in the presence of a harmonic trap, we observe that it is destroyed already by a small population imbalance of the two species.
3. arXiv:1203.4402 [pdf, ps, other]
Multi-orbital bosons in bipartite optical lattices
Jani-Petri Martikainen, Jonas LarsonWe study interacting bosons in a two dimensional bipartite optical lattice. By focusing on the regime where the first three excited bands are nearly degenerate we derive a three orbital tight-binding model which captures the most relevant features of the bandstructure. In addition, we also derive a corresponding generalized Bose-Hubbard model and solve it numerically under different situations, both with and without a confining trap. It is especially found that the hybridization between sublattices can strongly influence the phase diagrams and in a trap enable even appearances of condensed phases intersecting the same Mott insulating plateaus.
4. arXiv:1203.4540 [pdf, ps, other]
Interaction-induced impeding of decoherence and anomalous diffusion
Dario Poletti, Jean-Sebastien Bernier, Antoine Georges, Corinna KollathWe study how the interplay of dissipation and interactions affects the dynamics of a bosonic many-body quantum system. In the presence of both dissipation and strongly repulsive interactions, observables such as the coherence and the compressibility display three dynamical regimes: an initial exponential variation followed by a power-law regime and finally a slow exponential convergence to their asymptotic values corresponding to the infinite temperature state. These very long-time scales arise as dissipation forces the population of states disfavored by interactions. The long-time, strong coupling dynamics are understood by performing a mapping onto a classical diffusion process displaying non-Brownian behavior. While both dissipation and strong interactions tend to suppress coherence when acting separately, we find that strong interaction impedes the decoherence process generated by the dissipation.
Mar 20
1. arXiv:1203.4162 [pdf, ps, other]
Negative Temperature States in the Discrete Nonlinear Schroedinger Equation
S. Iubini, R. Franzosi, R. Livi, G.-L. Oppo, A. PolitiWe explore the statistical behavior of the discrete nonlinear Schroedinger equation. We find a parameter region where the system evolves towards a state characterized by a finite density of breathers and a negative temperature. Such a state is metastable but the convergence to equilibrium occurs on astronomical time scales and becomes increasingly slower as a result of a coarsening processes. Stationary negative-temperature states can be experimentally generated via boundary dissipation or from free expansions of wave packets initially at positive temperature equilibrium.
2. arXiv:1203.4050 [pdf, ps, other]
Crossover trimers connecting continuous and discrete scaling regimes
Shimpei Endo, Pascal Naidon, Masahito UedaFor a system of two identical fermions and one distinguishable particle interacting via a short-range potential with a large s-wave scattering length, the Efimov trimers and Kartavtsev-Malykh trimers exist in different regimes of the mass ratio. The Efimov trimers are known to exhibit a discrete scaling invariance, while the Kartavtsev-Malykh trimers feature a continuous scaling invariance. We point out that a third type of trimers, "crossover trimers", exist universally regardless of short-range details of the potential. These crossover trimers have neither the discrete nor continuous scaling invariance. We show that the crossover trimers continuously connect the discrete and continuous scaling regimes as the mass ratio and the scattering length are varied. We identify the regions for the Kartavtsev-Malykh trimers, Efimov trimers, crossover trimers, and non-universal trimers as a function of the mass ratio and the s-wave scattering length by investigating the scaling property and model-independence of the trimers.
3. arXiv:1203.3807 [pdf, ps, other]
Detecting D-Wave Pairing and Collective Modes in Fermionic Condensates with Bragg Scattering
G. R. Boyd, V. Galitski, V. M. YakovenkoWe show how the appearance of d-wave pairing in fermionic condensates manifests itself in inelastic light scattering. Specifically, we calculate the Bragg scattering intensity from the dynamic structure factor and the spin susceptibility, which can be inferred from spin flip Raman transitions. This information provides a precise tool with which we can identify nontrivial correlations in the state of the system beyond the information contained in the density profile imaging alone. Due to the lack of Coulomb effects in neutral superfluids, this is also an opportunity to observe the Anderson-Bogoliubov collective mode.
4. arXiv:1203.4136 [pdf, other]
Encoding relativistic potential dynamics into free evolution
C. Sabín, J. Casanova, J. J. García-Ripoll, L. Lamata, E. Solano, J. LeónWe propose a method to simulate a Dirac or Majorana equation evolving under a potential with the use of the corresponding free evolution, while the potential dynamics is encoded in a static transformation upon the initial state. We extend our results to interacting two-body systems.
Mar 19
1. arXiv:1203.3745 [pdf, other]
Spin-Seebeck effect in a strongly interacting Fermi gas
C. H. Wong, H.T.C. Stoof, R.A. Duine
We study the spin-Seebeck effect in a strongly interacting, two-component Fermi gas and propose an experiment to measure this effect by relatively displacing spin up and spin down atomic clouds in a trap using spin-dependent temperature gradients. We compute the spin-Seebeck coefficient and related spin-heat transport coefficients as functions of temperature and interaction strength. We find that when the inter-spin scattering length becomes larger than the Fermi wavelength, the spin-Seebeck coefficient changes sign as a function of temperature, and hence so does the direction of the spin-separation. We compute this zero-crossing temperature as a function of interaction strength and in particular in the unitary limit for the inter-spin scattering.
2. arXiv:1203.3728 [pdf, ps, other]
Wave function Monte Carlo method for polariton condensates
Michiel WoutersWe present a quantum jump approach to describe coupled quantum and classical systems in the context of Bose-Einstein condensation in the solid state. In our formalism, the excitonic gain medium is described by classical rate equations, while the polariton modes are described fully quantum mechanically. We show the equivalence of our method with a master equation approach. As an application, we compute the linewidth of a single mode polariton condensate. Both the line broadening due to the interactions between polaritons and the interactions with the reservoir excitons is taken into account.
3. arXiv:1203.3657 [pdf, ps, other]
Beyond mean-field dynamics in open Bose-Hubbard chains
D. Witthaut, F. Trimborn, H. Hennig, G. Kordas, T. Geisel, S. WimbergerWe investigate the effects of phase noise and particle loss on the dynamics of a Bose-Einstein condensate in an optical lattice. Starting from the many-body master equation, we discuss the applicability of generalized mean-field approximations in the presence of dissipation as well as methods to simulate quantum effects beyond mean-field by including higher-order correlation functions. It is shown that localized particle dissipation leads to surprising dynamics, as it can suppress decay and restore the coherence of a Bose-Einstein condensate. These effects can be applied to engineer coherent structures such as stable discrete breathers and dark solitons.
Mar 12 - Mar 16, Saubhik Sarkar
Mar 16
1. arXiv:1203.3325 [pdf, ps, other]
Atypical BCS-BEC crossover induced by quantum-size effects
A. A. Shanenko, M. D. Croitoru, A. V. Vagov, V. M. Axt, A. Perali, F. M. Peeters
Quantum-size oscillations of the basic physical characteristics of a confined fermionic condensate are a well-known phenomenon. Its conventional understanding is based on the single-particle physics, whereby the oscillations follow the size-dependent changes in the single-particle density of states. Here we present a study of a cigar-shaped ultracold superfluid Fermi gas, which demonstrates an important many-body aspect of the quantum-size effects, overlooked previously. The many-body physics is revealed in the atypical crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluid to the Bose-Einstein condensate (BEC) induced by the size quantization of the particle motion. Quantized perpendicular spectrum results in the formation of single-particle subbands (shells) so that the aggregate fermionic condensate becomes a coherent mixture of subband condensates. Each time when the lower edge of a subband crosses the chemical potential, the BCS-BEC crossover is approached in this subband, and the aggregate condensate contains both the BCS and BEC-like components.
2. arXiv:1203.3254 [pdf, other]
A unified description of pairing effects, BKT physics, and superfluidity of 2D interacting Bose gases
Chih-Chun Chien, Jianhuang She, Fred CooperWe develop a unified description for two-dimensional (2D) interacting Bose gases at arbitrary temperatures. The genuine Bose-Einstein condensation with long-range coherence only survives at zero temperature. At finite temperatures, many-body pairing effects introduce a finite amplitude of the pairing density, which results in a finite superfluid density. The superfluid phase is only stable below the Berenzinskii-Kosterlitz-Thouless (BKT) temperature due to phase fluctuations. We present a finite-temperature phase diagram of 2D Bose gases. One salient signature of the finite amplitude of the pairing density field is a two-peak structure in the single-particle spectral function, resembling that of the pseudogap phase in 2D attractive Fermi gases.
3. arXiv:1203.3206 [pdf, ps, other]
Vortex macroscopic superpositions in ultracold bosons in a double-well potential
M.A. Garcia-March, Lincoln D. CarrWe study macroscopic superpositions in the orbital rather than the spatial degrees of freedom, in a three-dimensional double-well system. We show that the ensuing dynamics of $N$ interacting excited ultracold bosons, which in general requires at least eight single-particle modes and $\mathcal{O}(N^7)$ Fock states for large $N$, is described by a surprisingly small set of many-body states. An initial state with half the atoms in each well, and purposely excited in one of them, gives rise to the tunneling of axisymmetric and transverse vortex structures. This process generates macroscopic superpositions only distinguishable by their orbital properties and within experimentally realistic times.
Mar 15
1. arXiv:1203.3177 [pdf, other]
Quantum dynamics of matter waves in a pulsed disordered lattice
Bryce Gadway, Jeremy Reeves, Ludwig Krinner, Dominik Schneble
We experimentally study the dynamical response of weakly-interacting atomic matter waves to a periodically pulsed, disordered optical lattice potential consisting of two overlapping standing-waves of incommensurate spatial periodicity. For periodic driving with a single standing wave only, we observe behavior consistent with the kicked-rotor model, namely ballistic spreading of momentum wavepackets at quantum resonances, and dynamical localization otherwise. However, adding the second standing wave can greatly modify these two effects. In particular, we find that the addition of disorder destroys dynamical localization when the driving is off-resonant and suppresses delocalization for a resonant drive. Our findings directly relate to quantum localization phenomena in low-dimensional systems, and illustrate the role of decoherence in the dynamics of driven quantum systems.
2. arXiv:1203.3169 (cross-list from hep-lat) [pdf, other]
Lattice Monte Carlo calculations for unitary fermions in a finite box
Michael G. Endres, David B. Kaplan, Jong-Wan Lee, Amy N. Nicholson
We perform lattice Monte Carlo simulations for up to 66 unitary fermions in a finite box using a highly improved lattice action for nonrelativistic spin 1/2 fermions. We obtain on our largest spatial lattice a value of $0.3968^{+0.0076}_{-0.0077}$ for the Bertsch parameter, defined as the energy of the unitary Fermi gas measured in units of the free gas energy in the thermodynamic limit. In addition, for up to four unitary fermions, we compute the spectrum of the lattice theory by exact diagonalization of the transfer matrix projected onto irreducible representations of the octahedral group for small to moderate size lattices, providing an independent check of our few-body simulation results. We compare our exact numerical and simulation results for the spectrum to benchmark studies of other research groups, as well as perform an extended analysis of our lattice action improvement scheme, including analysis of the errors associated with higher partial waves and finite temporal discretization.
Mar 14
1. arXiv:1203.2850 [pdf, ps, other]
Controlling integrability in a quasi-1D atom-dimer mixture
D.S. Petrov, V. Lebedev, J.T.M. Walraven
We analytically study the atom-dimer scattering problem in the near-integrable limit when the oscillator length l_0 of the transverse confinement is smaller than the dimer size, ~l_0^2/|a|, where a<0 is the interatomic scattering length. The leading contributions to the atom-diatom reflection and break-up probabilities are proportional to a^6 in the bosonic case and to a^8 for the up-(up-down) scattering in a two-component fermionic mixture. We show that by tuning a and l_0 one can control the "degree of integrability" in a quasi-1D atom-dimer mixture in an extremely wide range leaving thermodynamic quantities unchanged. We find that the relaxation to deeply bound states in the fermionic (bosonic) case is slower (faster) than transitions between different Bethe ansatz states. We propose a realistic experiment for detailed studies of the crossover from integrable to nonintegrable dynamics.
2. arXiv:1203.2792 [pdf, ps, other]
Floquet analysis of modulated two-mode Bose-Hubbard model
Gentaro Watanabe, Harri Mäkelä
We study the tunneling dynamics in a time-periodically modulated two-mode Bose-Hubbard model using Floquet theory. We consider situations where the system is in the self-trapping regime and either the tunneling amplitude, the interaction strength, or the energy difference between the modes is modulated. In the former two cases, the tunneling is enhanced in a wide region of the modulation frequency, while in the latter case the resonance is narrow. We explain this difference with the help of Floquet analysis. If the modulation amplitude is weak, the locations of the resonances can be found using the spectrum of the non-modulated Hamiltonian. Furthermore, we use Floquet analysis to explain the coherent destruction of tunneling (CDT) occurring in a large-amplitude modulated system. Finally, we present two ways to create a NOON state. One is based on coherent oscillation between the two states corresponding to all particles being in mode 1 or in mode 2. The oscillation is caused by detuning from a partial CDT between these states. The other is based on an adiabatic variation of the modulation frequency. This results in a Landau-Zener type of transition between the ground state and a NOON-like state.
Mar 13
1. arXiv:1203.2521 [pdf, ps, other]
Ferromagnetic transition of a two-component Fermi gas of Hard Spheres
F. Arias de Saavedra, F. Mazzanti, J. Boronat, A. Polls
We use microscopic many-body theory to analyze the problem of itinerant ferromagnetism in a repulsive atomic Fermi gas of Hard Spheres. Using simple arguments, we show that the available theoretical predictions for the onset of the ferromagnetic transition predict a transition point at a density ($k_F a \sim 1$) that is too large to be compatible with the universal low-density expansion of the energy. We present new variational calculations for the hard-sphere Fermi gas, in the framework of Fermi hypperneted chain theory, that shift the transition to higher densities ($k_F a \sim 1.8$). Backflow correlations, which are mainly active in the unpolarized system, are essential for this shift.
2. arXiv:1203.2389 [pdf, ps, other]
BEC in Spin-Orbit Coupled Optical Lattices: Flat Bands and Superfluidity
Yongping Zhang, Chuanwei Zhang
Recently spin-orbit (SO) coupled superfluids in free space or harmonic traps have been extensively studied, motivated by the recent experimental realization of SO coupling for Bose-Einstein condensates (BEC). However, the rich physics of SO coupled BEC in optical lattices has been largely unexplored. In this Letter, we show that in suitable parameter region the lowest Bloch state forms an isolated flat band in a one dimensional (1D) SO coupled optical lattice, which thus provides an experimentally feasible platform for exploring the recently celebrated topological flat band physics in lattice systems. We show that the flat band is preserved even with the mean field interaction in BEC. We investigate the superfluidity of the BEC in SO coupled lattices through dynamical and Landau stability analysis, and show that the BEC is stable on the whole flat band.
Mar 12
1. arXiv:1203.2030 [pdf, other]
Scaling behaviour of trapped bosonic particles in two dimensions at finite temperature
Giacomo Ceccarelli, Christian Torrero
In the framework of the trap-size scaling theory, we study the scaling properties of the Bose-Hubbard model in two dimensions in the presence of a trapping potential at finite temperature. In particular, we provide results for the particle density and the density-density correlator at the Mott transitions and within the superfluid phase.
2. arXiv:1203.2020 [pdf, ps, other]
Excitation spectrum of a two-component Bose-Einstein condensate in a ring potential
J. Smyrnakis, M. Magiropoulos, A. D. Jackson, G. M. Kavoulakis
A mixture of two distinguishable Bose-Einstein condensates confined in a ring potential has numerous interesting properties under rotational and solitary-wave excitation. The lowest-energy states for a fixed angular momentum coincide with a family of solitary-wave solutions. In the limit of weak interactions, exact diagonalization of the many-body Hamiltonian is possible and permits evaluation of the complete excitation spectrum of the system.
3. arXiv:1203.1994 [pdf, ps, other]
Displaced dynamics of binary mixtures in linear and nonlinear optical lattices
Golam Ali Sekh, Mario Salerno, Aparna Saha, Benoy Talukdar
The dynamical behavior of matter wave solitons of two-component Bose-Einstein condensates (BEC) in combined linear and nonlinear optical lattices (OLs) is investigated. In particular, the dependence of the frequency of the oscillating dynamics resulting from initially slightly displaced components is investigated both analytically, by means of a variational effective potential approach for the reduced collective coordinate dynamics of the soliton, and numerically, by direct integrations of the mean field equations of the BEC mixture. We show that for small initial displacements binary solitons can be viewed as point masses connected by elastic springs of strengths related to the amplitude of the OL and to the intra and inter-species interactions. Analytical expressions of symmetric and anti-symmetric mode frequencies, are derived and occurrence of beatings phenomena in the displaced dynamics is predicted. These expressions are shown to give a very good estimation of the oscillation frequencies for different values of the intra-species interatomic scattering length, as confirmed by direct numerical integrations of the mean field Gross-Pitaevskii equations (GPE) of the mixture. The possibility to use displaced dynamics for indirect measurements of BEC mixture characteristics such as number of atoms and interatomic interactions is also suggested.
4. arXiv:1203.1927 [pdf, other]
Conduction of Ultracold Fermions Through a Mesoscopic Channel
Jean-Philippe Brantut, Jakob Meineke, David Stadler, Sebastian Krinner, Tilman Esslinger
In a mesoscopic conductor electric resistance is detected even if the device is defect-free. We engineer and study a cold-atom analog of a mesoscopic conductor. It consists of a narrow channel connecting two macroscopic reservoirs of fermions that can be switched from ballistic to diffusive. We induce a current through the channel and find ohmic conduction, even for a ballistic channel. An analysis of in-situ density distributions shows that in the ballistic case the dissipation is localized at the entrance and exit of the channel, revealing the presence of contact resistance. In contrast, a diffusive channel with disorder displays dissipation over the whole channel. Our approach opens the way towards quantum simulation of mesoscopic devices with quantum gases.
Mar 5 - Mar 9, Xiaopeng Li
Mar 9
1. arXiv:1203.1874 [pdf, ps, other]
Title: A simple solvable energy landscape model that shows a thermodynamic phase transition and a glass transition
Author: Gerardo G. Naumis
When a liquid melt is cooled, a glass or phase transition can be obtained depending on the cooling rate. Yet, this behavior has not been clearly captured in energy landscape models. Here a model is provided in which two key ingredients are considered based in the landscape, metastable states and their multiplicity. Metastable states are considered as in two level system models. However, their multiplicity and topology allows a phase transition in the thermodynamic limit, while a transition to the glass is obtained for fast cooling. By solving the corresponding master equation, the minimal speed of cooling required to produce the glass is obtained as a function of the distribution of metastable and stable states. This allows to understand cooling trends due to rigidity considerations in chalcogenide glasses.
2. arXiv:1203.1834 [pdf]
Title: NMR in the 245 iron-selenides Rb0.74Fe1.6Se2: phase separation between an antiferromagnet and a superconducting Rb0.3Fe2Se2
Author: Y. Texier, J. Deisenhofer, V. Tsurkan, A. Loidl, D. S. Inosov, G. Friemel, J. Bobroff
77Se and 87Rb nuclear magnetic resonance (NMR) experiments on Rb0.74Fe1.6Se2 demonstrate phase separation between a majority antiferromagnetic (AF) and a minority metallic/superconducting (SC) phase. The AF spectrum is compatible with the AF Fe-vacancy ordered structure reported previously. The SC spectrum displays a very small linewidth, implying the absence of Fe vacancies and any trace of AF order. Its composition is deduced from intensity measurements to be Rb0.3(1)Fe2Se2, i.e. 0.15 electrons/Fe, which makes superconductivity in the 245 family similar to the other Fe-based superconductors.
3. arXiv:1203.1815 [pdf, other]
Title: Universal energy distribution of quasi-particles emitted in a local time dependent quench
Author: Pietro Smacchia, Alessandro Silva
We study the emission of quasi-particles in the scaling limit of the 1d Quantum Ising chain at the critical point perturbed by a time dependent local transverse field. We compute \it exactly \rm and for a \it generic \rm time dependence the average value of the transverse magnetization, its correlation functions, as well as the statistic of both the inclusive and exclusive work. We show that, except for a cyclic perturbation, the probability distribution of the work at low energies is a power law whose exponent is universal, i.e. does not depend on the specific time dependent protocol, but only on the final value attained by the perturbation.
4. arXiv:1203.1788 [pdf, other]
Title: Universal thermodynamics of a two-dimensional Bose gas
Author: A. Rancon, N. Dupuis
Using renormalization-group arguments we show that the low-temperature thermodynamics of a three- or two-dimensional dilute Bose gas is fully determined by a universal scaling function $\calF_d(\mu/k_BT,\tilde g(T))$ once the mass $m$ and the s-wave scattering length $a_d$ of the bosons are known ($d$ is the space dimension). Here $\mu$ and $T$ denote the chemical potential and temperature of the gas, and the temperature-dependent dimensionless interaction constant $\tilde g(T)$ is a function of $ma_d^2k_BT/\hbar^2$. We compute the scaling function $\calF_2$ using a nonperturbative renormalization-group approach and find that both the $\mu/k_BT$ and $\tilde g(T)$ dependencies are in very good agreement with recent experimental data obtained for a quasi-two-dimensional Bose gas with or without optical lattice. We also show that the nonperturbative renormalization-group estimate of the Berezinskii-Kosterlitz-Thouless transition temperature compares well with the result obtained from a quantum Monte Carlo simulation of an effective classical field theory.
5. arXiv:1203.1624 [pdf, ps, other]
Title: Anomalous Behavior of Spin Systems with Dipolar Interactions
Author: David Peter, Steffen Müller, Stefan Wessel, Hans Peter Büchler
We study the properties of spin systems realized by cold polar molecules interacting via dipole-dipole interactions in two-dimensions. Using a spin wave theory, that allows for the full treatment of the characteristic long-distance tail of the dipolar interaction, we find several anomalous features in the ground state correlations and the spin wave excitation spectrum, which are absent in their counterparts with short range interaction. The most striking consequence is the existence of true long-range order at finite temperature for a two-dimensional phase with a broken U(1) symmetry.
Mar 8
1. arXiv:1203.1436 [pdf, other]
Title: Temperature Induced Spin Density Wave in Magnetic Doped Topological Insulators
Author: Martha Lasia, Luis Brey
We study the magnetic properties of topological insulators doped with isoelectronic magnetic impurities. We obtain that at zero temperature the impurities order ferromagnetically, but when raising the temperature the topological insulator undergoes a first order phase transition to a spin density wave phase before the system reaches the paramagnetic phase. The origin of this phase is the non-trivial dependence of the topological insulator spin susceptibility on the momentum. We analyze the coupling of the non-uniform magnetic phase with the Dirac electronic system that occurs at the surfaces of the topological insulators.
2. arXiv:1203.1412 [pdf, ps, other]
Title : Three body on-site interactions in ultracold bosonic atoms in optical lattices and superlattices
Author: Manpreet Singh, Arya Dhar, Tapan Mishra, R. V. Pai, B. P. Das
The Mott insulator-superfluid transition for ultracold bosonic atoms in an optical lattice has been extensively studied in the framework of the Bose-Hubbard model with two-body on-site interactions. In this paper, we analyze the additional effect of the three-body on-site interactions on this phase transition in optical lattice and the transitions between the various phases that arise in an optical superlattice. Using the mean-field theory and the density matrix renormalization group method, we find the phase diagrams depicting the relationships between various physical quantities in an optical lattice and superlattice. We also suggest possible experimental signatures to observe the three-body interactions.
3. arXiv:1203.1362 [pdf, ps, other]
Title: Evidence for +-s-wave pairing symmetry in LiFeAs: specific heat study
Author: Dong-Jin Jang, J. B. Hong, Y. S. Kwon, T. Park, K. Gofryk, F. Ronning, J. D. Thompson, Yunkyu Bang
We report specific heat capacity measurements on a LiFeAs single crystal at temperatures down to 400 mK and magnetic fields up to 9 Tesla. A small specific heat jump at Tc and finite residual density of states at T=0 K in the superconducting (SC) state indicate that there are strong unitary scatterers that lead to states within the SC gap. A sub-linear magnetic field dependence of the Sommerfeld coefficient \gamma(H) at T=0 K is equally well fitted by both a nodal d-wave gap as well as a sign changing multiband \pm s-wave gap. When impurity effects are taken into account, however, the linear temperature dependence of the electronic specific heat C_{el}/T at low temperatures argues against a nodal d-wave superconducting gap. We conclude that the SC state of LiFeAs is most compatible with the multiband \pm s-wave SC state with the gap values \Delta_{small}=0.46 \Delta_{large}.
4. arXiv:1203.1345 (cross-list from quant-ph) [pdf, other]
Title: PT-symmetry breaking and universal chirality in a PT-symmetric ring
Author: Derek D. Scott, Yogesh N. Joglekar
We investigate the properties of an $N$-site tight-binding lattice with periodic boundary condition (PBC) in the presence of a pair of gain and loss impurities $\pm i\gamma$, and two tunneling amplitudes $t_0,t_b$ that are constant along the two paths that connect them. We show that the parity and time-reversal ($\mP\mT$)-symmetric phase of the lattice with PBC is robust, insensitive to the distance between the impurities, and that the critical impurity strength for PT-symmetry breaking is given by $\gamma_{PT}=|t_0-t_b|$. We study the time-evolution of a typical wave packet, initially localized on a single site, across the PT-symmetric phase boundary. We find that it acquires chirality with increasing $\gamma$, and the chirality reaches a universal maximum value at the threshold, $\gamma=\gamma_{PT}$, irrespective of the initial location of the wave packet or the lattice parameters. Our results imply that PT-symmetry breaking on a lattice with PBC has consequences that have no counterpart in open chains.
Mar 7
1. arXiv:1203.1246 [pdf, other]
Title: Detecting Chiral Edge States in the Hofstadter Optical Lattice
Author: Nathan Goldman, Jerome Beugnon, Fabrice Gerbier
We propose and describe a realistic scheme to detect topological edge states in an optical lattice subjected to a synthetic magnetic field, based on a generalization of Bragg spectroscopy sensitive to angular momentum. We demonstrate that the excitation fraction produced by a well-designed laser probe provides an unambiguous signature, which highlights the presence of topological edge states and establishes their chiral nature. This signature is present for a variety of boundaries, from a hard wall to a smooth harmonic potential added on top of the optical lattice. We investigate the effects of angular momentum transfer, with and without changing the internal atomic state. The latter method allows to independently detect the weak signal from the selected edge states on a dark background. Our method therefore offers the unique possibility to visualize topological edge states using in situ imaging in optical lattices.
2. arXiv:1203.1236 [pdf, ps, other]
Title: Strong-coupling solution of the bosonic dynamical mean-field theory
Author: Anna Kauch, Krzysztof Byczuk, Dieter Vollhardt
We derive an approximate analytical solution of the self-consistency equations of the bosonic dynamical mean-field theory (B-DMFT) in the strong-coupling limit. The approach is based on a linked-cluster expansion in the hybridization function of normal bosons around the atomic limit. The solution is used to compute the phase diagram of the bosonic Hubbard model for different lattices. We compare our results with numerical solutions of the B-DMFT equations and numerically exact methods, respectively. The very good agreement with those numerical results demonstrates that our approach captures the essential physics of correlated bosons both in the Mott insulator and in the superfluid phase. Close to the transition into the superfluid phase the momentum distribution function at zero momentum is found to be strongly enhanced already in the normal phase. The linked-cluster expansion also allows us to compute dynamical properties such as the spectral function of bosons. The evolution of the spectral function across the transition from the normal to the superfluid phase is seen to be characteristically different for the interaction driven and density driven transition, respectively.
3. arXiv:1203.1186 [pdf, ps, other]
Title: The scaling of the decoherence factor of a qubit coupled to a spin chain driven across quantum critical points
Author: Tanay Nag, Uma Divakaran, Amit Dutta
In this paper, we study the scaling of the decoherence factor of a qubit (spin-1/2) using the central spin model in which the central spin (qubit) is globally coupled to a transverse XY spin chain. The aim here is to study the non-equilibrium generation of decoherence when the spin chain is driven across quantum critical points (or lines) and derive the scaling of the decoherence factor in terms of the driving rate and some of the exponents associated with the quantum critical points. Our studies show that the scaling of logarithm of decoherence is identical to that of the defect density in the final state of the spin chain following a quench across isolated quantum critical points for both linear and non-linear variations of a parameter even if the defect density may not satisfy the standard Kibble-Zurek scaling. However, one finds an interesting deviation when the spin chain is driven along a critical line which is not given by the scaling of the defect density. Our analytical results, obtained via the exact solution of the Schr\"odinger equations as well as using an alternative method of calculating the Loschmidt echo are in complete agreement with numerical results.
4. arXiv:1203.1181 [pdf, other]
Title: Signatures of orbital loop currents in the spatially resolved local density of states
Author: W. H. P. Nielsen, W. A. Atkinson, B. M. Andersen
Polarized neutron scattering measurements have suggested that intra-unit cell antiferromagnetism may be associated with the pseudogap phase. Assuming that loop current order is responsible for the observed magnetism, we calculate some signatures of such circulating currents in the local density of states around a single non-magnetic impurity in a coexistence phase with superconductivity. We find a distinct C4 symmetry breaking near the disorder which is also detectable in the resulting quasi-particle interference patterns.
Mar 6
1. arXiv:1203.1028 [pdf, ps, other]
Title: Simplified topological invariants for interacting insulators
Author: Zhong Wang, Shou-Cheng Zhang
We propose general topological order parameters for interacting insulators in terms of the Green's function at zero frequency. They provide an unified description of various interacting topological insulators including the quantum anomalous Hall insulators and the time reversal invariant insulators in four, three and two dimensions. Since only Green's function at zero frequency is used, our topological order parameters can be evaluated efficiently by most numerical and analytical algorithms for strongly interacting systems.
2. arXiv:1203.0948 [pdf]
Title: Coherent multi-flavour spin dynamics in a fermionic quantum gas
Author: Jasper Simon Krauser, Jannes Heinze, Nick Fläschner, Sören Götze, Christoph Becker, Klaus Sengstock
Microscopic spin interaction processes are fundamental for global static and dynamical magnetic properties of many-body systems. Quantum gases as pure and well isolated systems offer intriguing possibilities to study basic magnetic processes including non-equilibrium dynamics. Here, we report on the realization of a well-controlled fermionic spinor gas in an optical lattice with tunable effective spin ranging from 1/2 to 9/2. We observe long-lived intrinsic spin oscillations and investigate the transition from two-body to many-body dynamics. The latter results in a spin-interaction driven melting of a band insulator. Via an external magnetic field we control the system's dimensionality and tune the spin oscillations in and out of resonance. Our results open new routes to study quantum magnetism of fermionic particles beyond conventional spin 1/2 systems.
3. arXiv:1203.0925 [pdf, other]
Title: Inducing spin-dependent tunneling to probe magnetic correlations in optical lattices
Author: K. G. L. Pedersen, B. M. Andersen, O. F. Syljuasen, G. M. Bruun, A. S. SorensenWe suggest a simple experimental method for probing antiferromagnetic spin correlations of two-component Fermi gases in optical lattices. The method relies on a spin selective Raman transition to excite atoms of one spin species to their first excited vibrational mode where the tunneling is large. The resulting difference in the tunneling dynamics of the two spin species can then be exploited, to reveal the spin correlations by measuring the number of doubly occupied lattice sites at a later time. We perform quantum Monte Carlo simulations of the spin system and solve the optical lattice dynamics numerically to show how the timed probe can be used to identify antiferromagnetic spin correlations in optical lattices.
Mar 5
1.arXiv:1203.0520 [pdf]
Title: Three-Dimensional Spin Rotations at the Fermi Surface of a Strongly Spin-Orbit Coupled Surface System
Author: Philipp Hoepfner (1), Joerg Schaefer (1), Andrzej Fleszar (2), Jan Hugo Dil (3,4), Bartosz Slomski (3,4), Fabian Meier (3,4), Christoph Loho (1),Christian Blumenstein (1), Luc Patthey (3), Werner Hanke (2), Ralph Claessen (1)
The spin texture of the metallic two-dimensional electron system (root3 x root3)-Au/Ge(111) is revealed by fully three-dimensional spin-resolved photoemission, as well as by density functional calculations. The large hexagonal Fermi surface, generated by the Au atoms, shows a significant splitting due to spin-orbit interactions. The planar components of the spin exhibit helical character, accompanied by a strong out-of-plane spin component with alternating signs along the six Fermi surface sections. Moreover, in-plane spin rotations towards a radial direction are observed close to the hexagon corners. Such a threefold-symmetric spin pattern is not described by the conventional Rashba model. Instead, it reveals an interplay with Dresselhaus-like spin-orbit effects as a result of the crystalline anisotropies.
2. arXiv:1203.0343 [pdf, ps, other]
Title: Enhancement of antiferromagnetic correlations below superconducting transition temperature in bilayer superconductors
Author: Hiroyuki Yoshizumi, Takao Morinari, Takami Tohyama
Motivated by the recent experiment in multilayered cuprate superconductors reporting the enhancement of antiferromagnetic order below the superconducting transition temperature, we study the proximity effect of the antiferromagnetic correlation in a bilayer system and also examine the possibility of a coexistence of antiferromagnetic order and superconductivity. We present the result of mean field theory that is consistent with the experiment and supports the proximity effect picture.
