Nov 28 - Dec 2, Saubhik Sarkar
Nov 21 - Nov 25, Xiaopeng Li
Dec 2
1. arXiv:1112.0013 [pdf, other]
Relaxation Dynamics and Pre-thermalization in an Isolated Quantum System
Michael Gring, Maximilian Kuhnert, Tim Langen, Takuya Kitagawa, Bernhard Rauer, Matthias Schreitl, Igor Mazets, David A. Smith, Eugene Demler, Jörg Schmiedmayer
Understanding relaxation processes is an important unsolved problem in many areas of physics. A key challenge in studying such non-equilibrium dynamics is the scarcity of experimental tools for characterizing their complex transient states. We employ measurements of full quantum mechanical probability distributions of matter-wave interference to study the relaxation dynamics of a coherently split one-dimensional Bose gas and obtain unprecedented information about the dynamical states of the system. Following an initial rapid evolution, the full distributions reveal the approach towards a thermal-like steady state characterized by an effective temperature eight times lower than the initial equilibrium temperature of the system before the splitting process. We conjecture that this state can be described through a generalized Gibbs ensemble and associate it with pre-thermalization.
Dec 1
1. arXiv:1111.7133 (cross-list from cond-mat.mes-hall) [pdf]
Sculpting oscillators with light within a nonlinear quantum fluid
G. Tosi, G. Christmann, N.G. Berloff, P. Tsotsis, T. Gao, Z. Hatzopoulos, P.G. Savvidis, J.J. Baumberg
Seeing macroscopic quantum states directly remains an elusive goal. Particles with boson symmetry can condense into such quantum fluids producing rich physical phenomena as well as proven potential for interferometric devices [1-10]. However direct imaging of such quantum states is only fleetingly possible in high-vacuum ultracold atomic condensates, and not in superconductors. Recent condensation of solid state polariton quasiparticles, built from mixing semiconductor excitons with microcavity photons, offers monolithic devices capable of supporting room temperature quantum states [11-14] that exhibit superfluid behaviour [15,16]. Here we use microcavities on a semiconductor chip supporting two-dimensional polariton condensates to directly visualise the formation of a spontaneously oscillating quantum fluid. This system is created on the fly by injecting polaritons at two or more spatially-separated pump spots. Although oscillating at tuneable THz-scale frequencies, a simple optical microscope can be used to directly image their stable archetypal quantum oscillator wavefunctions in real space. The self-repulsion of polaritons provides a solid state quasiparticle that is so nonlinear as to modify its own potential. Interference in time and space reveals the condensate wavepackets arise from non-equilibrium solitons. Control of such polariton condensate wavepackets demonstrates great potential for integrated semiconductor-based condensate devices.
2. arXiv:1111.7053 [pdf, other]
Driven-dissipative many-body pairing states for cold fermionic atoms in an optical lattice
W. Yi, S. Diehl, A. J. Daley, P. Zoller
We discuss the preparation of many-body states of cold fermionic atoms in an optical lattice via controlled dissipative processes induced by coupling the system to a reservoir. Based on a mechanism combining Pauli blocking and phase locking between adjacent sites, we construct complete sets of jump operators describing coupling to a reservoir that leads to dissipative preparation of pairing states for fermions with various symmetries in the absence of direct inter-particle interactions. We discuss the uniqueness of these states, and demonstrate it with small-scale numerical simulations. In the late time dissipative dynamics, we identify a "dissipative gap" that persists in the thermodynamic limit. This gap implies exponential convergence of all many-body observables to their steady state values. We then investigate how these pairing states can be used as a starting point for the preparation of the ground state of Fermi-Hubbard Hamiltonian via an adiabatic state preparation process also involving the parent Hamiltonian of the pairing state. We also provide a proof-of-principle example for implementing these dissipative processes and the parent Hamiltonians of the pairing states, based on Yb171 atoms in optical lattice potentials.
Nov 30
1.arXiv:1111.6778 [pdf, other]
Discrete Symmetry Breaking Transitions Between Paired Superfluids
M. J. Bhaseen, S. Ejima, F. H. L. Essler, H. Fehske, M. Hohenadler, B. D. Simons
We explore the zero-temperature phase diagram of bosons interacting via Feshbach resonant pairing interactions in one dimension. Using DMRG (Density Matrix Renormalization Group) and field theory techniques we characterize the phases and quantum phase transitions in this low-dimensional setting. We provide a broad range of evidence in support of an Ising quantum phase transition separating distinct paired superfluids, including results for the energy gaps, correlation functions and entanglement entropy. In particular, we show that the Ising correlation length, order parameter and critical properties are directly accessible from a ratio of the atomic and molecular two-point functions. We further demonstrate that both the zero-momentum occupation numbers and the visibility are in accordance with the absence of a purely atomic superfluid. We comment on the connection to recent studies of boson pairing in a generalized classical XY model.
Nov 29
1. arXiv:1111.6154 [pdf, ps, other]
Breathing oscillations of a trapped impurity in a Bose gas
T. H. Johnson, M. Bruderer, Y. Cai, S. R. Clark, W. Bao, D. Jaksch
Motivated by a recent experiment [J. Catani et al., arXiv:1106.0828v1 preprint, 2011], we study breathing oscillations in the width of a harmonically trapped impurity interacting with a separately trapped Bose gas. We provide an intuitive physical picture of such dynamics at zero temperature, using a time-dependent variational approach. In the Gross-Pitaevskii regime we obtain breathing oscillations whose amplitudes are suppressed by self trapping, due to interactions with the Bose gas. Introducing phonons in the Bose gas leads to the damping of breathing oscillations and non-Markovian dynamics of the width of the impurity, the degree of which can be engineered through controllable parameters. Our results reproduce the main features of the impurity dynamics observed by Catani et al. despite experimental thermal effects, and are supported by simulations of the system in the Gross-Pitaevskii regime. Moreover, we predict novel effects at lower temperatures due to self-trapping and the inhomogeneity of the trapped Bose gas.
Nov 28
1. arXiv:1111.6039 [pdf, other]
The effect of light assisted collisions on matter wave coherence in superradiant Bose-Einstein condensates
N. S. Kampel, A. Griesmaier, M.P. Hornbak Steenstrup, F. Kaminski, E. S. Polzik, J. H. Müller
We investigate experimentally the effects of light assisted collisions on the coherence between momentum states in Bose-Einstein condensates. The onset of superradiant Rayleigh scattering serves as a sensitive monitor for matter wave coherence. A subtle interplay of binary and collective effects leads to a profound asymmetry between the two sides of the atomic resonance and provides far bigger coherence loss rates for a condensate bathed in blue detuned light than previously estimated. We present a simplified quantitative model containing the essential physics to explain our experimental data and point at a new experimental route to study strongly coupled light matter systems.
Nov 24
1. arXiv:1111.5383 [pdf, ps, other]
Title: Probing the connections between superconductivity, stripe order, and structure in La1.905Ba0.095Cu1-yZnyO4
Author: Jinsheng Wen, Zhijun Xu, Guangyong Xu, Qing Jie, M. Huecker, A. Zheludev, Wei Tian, B. L. Winn, J. L. Zarestky, D. K. Singh, Tao Hong, Qiang Li, Genda Gu, J. M. Tranquada
The superconducting system La2-xBaxCuO4 is known to show a minimum in the transition temperature, Tc, at x = 1/8 where maximal stripe order is pinned by the anisotropy within the CuO2 planes that occurs in the low-temperature-tetragonal (LTT) crystal structure. For x = 0.095, where Tc reaches its maximum value of 32 K, there is a roughly coincident structural transition to a phase that is very close to LTT. Here we present a neutron scattering study of the structural transition, and demonstrate how features of it correlate with anomalies in the magnetic susceptibility, electrical resistivity, thermal conductivity, and thermoelectric power. We also present measurements on a crystal with 1% Zn substituted for Cu, which reduces Tc to 17 K, enhances the spin stripe order, but has much less effect on the structural transition. We make the case that the structural transition correlates with a reduction of the Josephson coupling between the CuO2 layers, which interrupts the growth of the superconducting order. We also discuss evidence for two-dimensional superconducting fluctuations in the normal state, analyze the effective magnetic moment per Zn impurity, and consider the significance of the anomalous thermopower often reported in the stripe-ordered phase.
2. arXiv:1111.5388 [pdf, ps, other]
Title: Superconducting gap structure of CeIrIn5 determined by field-angle-resolved specific heat measurements
Author: Shunichiro Kittaka, Yuya Aoki, Toshiro Sakakibara, Akito Sakai, Satoru Nakatsuji, Yasumasa Tsutsumi, Masanori Ichioka, Kazushige Machida
In order to identify the gap structure of CeIrIn5, we measured field-angle-resolved specific heat C(phi) by conically rotating the magnetic field H around the c axis at low temperatures down to 80 mK. We revealed that C(phi) exhibits a fourfold angular oscillation, and the oscillation amplitude decreases monotonically by tilting the field out of the ab plane. Detailed microscopic calculations based on the quasiclassical Eilenberger equation confirm that the observed features are uniquely explained by assuming the dx2-y2-wave gap. These results strongly indicate that CeIrIn5 is a dx2-y2-wave superconductor and suggest the universal pairing mechanism in CeMIn5 (M = Co,Rh, and Ir).
3. arXiv:1111.5446 [pdf, ps, other]
Title: Mechanical cat states in graphene resonators
Author: A. Voje, J. M. Kinaret, A. Isacsson
We study the quantum dynamics of a symmetric nanomechanical graphene resonator with degenerate flexural modes. Applying voltage pulses to two back gates, flexural vibrations of the membrane can be selectively actuated and manipulated. For graphene, nonlinear response becomes important already for amplitudes comparable to the magnitude of zero point fluctuations. We show, using analytical and numerical methods, that this allows for creation of cat-like superpositions of coherent states as well as superpositions of coherent cat-like non-product states.
4. arXiv:1111.5594 [pdf, other]
Title: Bilayer Rydberg atoms as a quantum simulator for unconventional superconductors
Author: J. P. Hague, C. MacCormick
We show how systems of cold fermionic Rydberg atoms in a bilayer lattice can be used as a quantum simulator for electron-phonon interactions in the presence of strong electronic correlation: a scenario found in many unconventional superconductors. We discuss the experimental system for making such a simulator, show that the resulting Hamiltonian can be mapped to an extended Hubbard-Holstein model, and make comparison with numerics. Fermions can be simulated, and the proposed quantum simulator can cope with systems where numerics are difficult to apply.
Nov 23
1. arXiv:1111.5594 [pdf, other]
Title: Anisotropic sound and shock waves in dipolar Bose-Einstein condensate
Author: P. Muruganandama, S. K. Adhikari
We study the propagation of anisotropic sound and shock waves in dipolar Bose-Einstein condensate in three dimensions (3D) as well as in quasi-two (2D, disk shape) and quasi-one (1D, cigar shape) dimensions using the mean-field approach. In 3D, the propagation of sound and shock waves are distinct in directions parallel and perpendicular to dipole axis with the appearance of instability above a critical value corresponding to attraction. Similar instability appears in 1D and not in 2D. The numerical anisotropic Mach angle agrees with theoretical prediction. The numerical sound velocity in all cases agrees with that calculated from Bogoliubov theory. A movie of the anisotropic wave propagation in a dipolar condensate is made available as supplementary material.
2. arXiv:1111.5225 [pdf, other]
Title: Finite-temperature mutual information in a simple phase transition
Author: J. Wilms, J. Vidal, F. Verstraete, S. Dusuel
We study the finite-temperature behavior of the Lipkin-Meshkov-Glick model, with a focus on correlation properties as measured by the mutual information. The latter, which quantifies the amount of both classical and quantum correlations, is computed exactly in the two limiting cases of vanishing magnetic field and vanishing temperature. For all other situations, numerical results provide evidence of a finite mutual information at all temperatures except at criticality. There, it diverges as the logarithm of the system size, with a prefactor that can take only two values, depending on whether the critical temperature vanishes or not. Our work provides a simple example in which the mutual information appears as a powerful tool to detect finite-temperature phase transitions, contrary to entanglement measures such as the concurrence.
3. arXiv:1111.5085 [pdf, ps, other]
Title: A projection operator approach to the Bose-Hubbard model
Author: A. Dutta, C. Trefzger, K. Sengupta
We develop a projection operator formalism for studying both the zero temperature equilibrium phase diagram and the non-equilibrium dynamics of the Bose-Hubbard model. Our work, which constitutes an extension of Phys. Rev. Lett. {\bf 106}, 095702 (2011), shows that the method provides an accurate description of the equilibrium zero temperature phase diagram of the Bose-Hubbard model for several lattices in two- and three-dimensions (2D and 3D). We show that the accuracy of this method increases with the coordination number $z$ of the lattice and reaches to within 0.5% of quantum Monte Carlo data for lattices with $z=6$. We also show that the same method may be used to analyze the non-equilibrium dynamics of the model both in the Mott phase and near the superfluid-insulator quantum critical point where the hopping amplitude $J$ and the on-site interaction $U$ satisfies $zJ/U \ll 1$. In particular, we study the non-equilibrium dynamics of the model both subsequent to a sudden quench of the hopping amplitude $J$ and during a ramp from $J_i$ to $J_f$ characterized by a ramp time $\tau$ and exponent $\alpha$: $J(t)=J_i +(J_f-J_i) (t/\tau)^{\alpha}$. We compute the wavefunction overlap $F$, the residual energy $Q$, the superfluid order parameter $\Delta(t)$, the equal-time order parameter correlation function $C(t)$, and the defect formation probability $P$ for the above-mentioned protocols. We find that $Q$, $F$, and $P$ do not exhibit the expected universal scaling. We explain this absence of universality and show that our results for linear ramps compare well with the recent experimental observations.
4. arXiv:1111.4627 (cross-list from hep-ph) [pdf, ps, other]
Title: Topology of quantum vacuum
Author: G. E. Volovik
Topology in momentum space is the main characteristics of the ground states of a system at zero temperature, the quantum vacua. The gaplessness of fermions in bulk, on the surface or inside the vortex core is protected by topology. Irrespective of the deformation of the parameters of the microscopic theory, the energy spectrum of these fermions remains strictly gapless. This solves the main hierarchy problem in particle physics. The quantum vacuum of Standard Model is one of the representatives of topological matter alongside with topological superfluids and superconductors, topological insulators and semi-metals, etc. There is a number of of topological invariants in momentum space of different dimensions. They determine universality classes of the topological matter and the type of the effective theory which emerges at low energy, give rise to emergent symmetries, including the effective Lorentz invariance, and emergent gauge and gravitational fields. The topological invariants in extended momentum and coordinate space determine the bulk-surface and bulk-vortex correspondence, connecting the topology in bulk with the real space. The momentum space topology gives some lessons for quantum gravity. In effective gravity emerging at low energy, the collective variables are the tetrad field and spin connections, while the metric is the composite object of tetrad field. This suggests that the Einstein-Cartan-Sciama-Kibble theory with torsion field is more relevant. There are also several scenarios of Lorentz invariance violation governed by topology, including splitting of Fermi point and development of the Dirac points with quadratic and cubic spectrum. The latter leads to the natural emergence of the Horava-Lifshitz gravity.
Nov 22
1. arXiv:1111.4922 [pdf, ps, other]
Title: Casimir force induced by imperfect Bose gas
Author: Marek Napiorkowski, Jaroslaw Piasecki
We present a study of the Casimir effect in an imperfect (mean-field) Bose gas contained between two infinite parallel plane walls. The derivation of the Casimir force follows from the calculation of the excess grand canonical free energy density under periodic, Dirichlet, and Neumann boundary conditions with the use of the steepest descent method. In the one-phase region the force decays exponentially fast when distance $D$ between the walls tends to infinity. When Bose-Einstein condensation point is approached the decay length in the exponential law diverges with critical exponent $\nu_{IMP}=1$, which differs from the perfect gas case where $\nu_{P}=1/2$. In the two-phase region the Casimir force is long-range, and decays following the power law $D^{-3}$, with the same amplitude as in the perfect gas.
2. arXiv:1111.4778 [pdf, ps, other]
Title: Tunable Spin-orbit Coupling and Quantum Phase Transition in a Trapped Bose-Einstein Condensate
Author: Yongping Zhang, Gang Chen, Chuanwei Zhang
Spin-orbit coupling (SOC), the intrinsic interaction between a particle spin and its motion, is responsible for various important phenomena, ranging from atomic fine structure to topological condensed matter physics. The recent experimental breakthrough on the realization of SOC for ultra-cold atoms provides a completely new platform for exploring spin-orbit coupled superfluid physics. However, the SOC strength in the experiment, determined by the applied laser wavelengths, is not tunable. In this Letter, we propose a scheme for tuning the SOC strength through a fast and coherent modulation of the laser intensities. We show that the many-body interaction between atoms, together with the tunable SOC, can drive a \textit{quantum phase transition} (QPT) from spin-balanced to spin-polarized ground states in a harmonic trapped Bose-Einstein condensate (BEC). This transition realizes the long-sought QPT in the quantum Dicke model, and may have important applications in quantum optics and quantum information. We characterize the QPT using the periods of collective oscillations (center of mass motion and scissors mode) of the BEC, which show pronounced peaks and damping around the quantum critical point.
Nov 21
1. arXiv:1111.4471 [pdf, ps, other]
Title: Intrinsic Hall effect in a multiband chiral superconductor
Author: Edward Taylor, Catherine Kallin
We identify an intrinsic, dissipationless Hall effect in multiband chiral superconductors in the absence of a magnetic field (i.e., an \emph{anomalous} Hall effect). This effect arises from interband tunneling of chiral Cooper pairs. We discuss the implications of this effect for the putative chiral p-wave superconductor, Strontium Ruthenate, and show that it can contribute significantly to polar Kerr rotation experiments. Since the magnitude of the effect depends on the structure of the order parameter across the bands, this result may be used to distinguish between different models proposed for the superconducting state of Strontium Ruthenate.
2. arXiv:1111.3080 (cross-list from quant-ph) [pdf, other]
Title: When does a quantum mechanical system depend on the initial conditions of the system or the environment?
Author: Adrian Hutter, Stephanie Wehner
Deriving rigorous bounds for the time scales that are needed for thermalization forms one of the most vexing problems when it comes to understanding statistical mechanics from the principles of quantum mechanics. One central aspect in obtaining such bounds is to determine how long a system retains memory of its initial conditions. By viewing this problem from an quantum information theory perspective, we are able to simplify part of this task in a very natural and easy way. We first show that for any interaction between the system and the environment, and almost all initial states of the system, the question of how long such memory lasts can be answered by studying the temporal evolution of just one special initial state. This special state thereby depends only on our knowledge of macroscopic parameters of the system. We provide a simple entropic inequality for this state that can be used to determine whether mosts states of the system have, or have not become independent of their initial conditions after time t. Analyzing the rate of entropy change over time for a particular kind of interaction then allows us to place rigorous bounds on such time scales. We make a similar statement for almost all initial states of the environment, and finally provide a sufficient condition for which a system never thermalizes, but remains close to its initial state for all times.
Nov 14 - Nov 18, Zixu Zhang
Nov 18
1. arXiv:1111.4102 [pdf, ps, other]
A multi-site mean-field theory for cold bosonic atoms in optical lattices
T. McIntosh, P. Pisarski, R. J. Gooding, E. Zaremba
We present a detailed derivation of a multi-site mean-field theory (MSMFT) used to describe the Mott-insulator to superfluid transition of bosonic atoms in optical lattices. The approach is based on partitioning the lattice into small clusters which are decoupled by means of a mean field approximation. This approximation invokes local superfluid order parameters defined for each of the boundary sites of the cluster. The resulting MSMFT grand potential has a non-trivial topology as a function of the various order parameters. An understanding of this topology provides two different criteria for the determination of the Mott insulator superfluid phase boundaries. We apply this formalism to $d$-dimensional hypercubic lattices in one, two and three dimensions, and demonstrate the improvement in the estimation of the phase boundaries when MSMFT is utilized for increasingly larger clusters, with the best quantitative agreement found for $d=3$. The MSMFT is then used to examine a linear dimer chain in which the on-site energies within the dimer have an energy separation of $\Delta$. This system has a complicated phase diagram within the parameter space of the model, with many distinct Mott phases separated by superfluid regions.
Nov 17
1. arXiv:1111.3656 [pdf, ps, other]
Symmetries and currents of the ideal and unitary Fermi gases
Xavier Bekaert, Elisa Meunier, Sergej Moroz
The maximal algebra of symmetries of the free single-particle Schroedinger equation is determined and its relevance for the holographic duality in non-relativistic Fermi systems is investigated. This algebra of symmetries is an infinite dimensional extension of the Schroedinger algebra, it is isomorphic to the Weyl algebra of quantum observables, and it may be interpreted as a non-relativistic higher-spin algebra. The associated infinite collection of Noether currents bilinear in the fermions are derived from their relativistic counterparts via a light-like dimensional reduction. The minimal coupling of these currents to background sources is rewritten in a compact way by making use of Weyl quantisation. Pushing forward the similarities with the holographic correspondence between the minimal higher-spin gravity and the critical O(N) model, a putative bulk dual of the unitary and the ideal Fermi gases is discussed.
2. arXiv:1111.3822 [pdf, other]
The tunneling density-of-states of interacting massless Dirac fermions
A. Principi, Marco Polini, Reza Asgari, A. H. MacDonald
We calculate the tunneling density-of-states (DOS) of a disorder-free two-dimensional interacting electron system with a massless-Dirac band Hamiltonian. The DOS exhibits two main features: i) linear growth at large energies with a slope that is suppressed by quasiparticle velocity enhancement, and ii) a rich structure of plasmaron peaks which appear at negative bias voltages in an n-doped sample and at positive bias voltages in a p-doped sample. We predict that the DOS at the Dirac point is non-zero even in the absence of disorder because of electron-electron interactions, and that it is then accurately proportional to the Fermi energy. The finite background DOS observed at the Dirac point of graphene sheets and topological insulator surfaces can therefore be an interaction effect rather than a disorder effect.
3. arXiv:1111.3870 [pdf, other]
The Fulde-Ferrell-Larkin-Ovchinnikov state in the dimensional crossover between 1D and 3D lattices
D.-H. Kim, P. Törmä
The interplay between fermion pairing and magnetism is at the heart of understanding strongly correlated systems ranging from unconventional superconductors and ultracold gases to neutron stars and quarks. The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state would arise with this interplay, but it still remains elusive in spite of indirect experimental evidence observed. Dimensionality is expected to play a key role in realizing this exotic state of a spatially modulated order parameter. It was argued that combining strong one-dimensional (1D) FFLO features with long-range order in a quasi-1D system would stabilize the state. Here, within the dynamical mean-field theory (DMFT), we present a full phase diagram for the 1D-3D crossover of the FFLO state in the attractive Hubbard model of 3D coupled chains. We predict that the optimal regime for the FFLO state is considerably extended to intermediate interchain couplings and polarizations, directly realizable with ultracold atomic gas systems.
Nov 16
1. arXiv:1111.3442 [pdf, ps, other]
Properties of super-Tonks-Girardeau state for attractive Fermi gases in an optical lattice
Li Wang, Zhihao Xu, Shu Chen
We investigate properties of the super-Tonks-Girardeau state for spin-1/2 ultracold fermions trapped in a one-dimensional deep optical lattice which can be well described by the Hubbard model. In contrast to the ground state of the attractive Hubbard model, the super-Tonks-Girardeau state is the lowest scattering state with no pairing between attractive fermions which can be realized by a sudden switch of interaction from the strongly repulsive regime to the strongly attractive regime. With the aid of Bethe-ansatz method, we calculate energies of both the Fermi Tonks-Girardeau gas and the Fermi super-Tonks-Girardeau state of spin-1/2 ultracold fermions and show that both energies approach to the same limit as the strength of the interaction goes to infinity. By exactly solving the quench dynamics of the Hubbard model, we demonstrate that the Fermi super-Tonks-Girardeau state can be a very stable dynamic state. This allows the experimental study of properties of Fermi super-Tonks-Girardeau gas in optical lattices.
2. arXiv:1111.3460 [pdf, other]
Classical realization of two-site Fermi-Hubbard systems
S. Longhi, G. Della Valle, V. Foglietti
A classical wave optics realization of the two-site Hubbard model, describing the dynamics of interacting fermions in a double-well potential, is proposed based on light transport in evanescently-coupled optical waveguides.
3. arXiv:1111.3494 [pdf, other]
Glass to superfluid transition in dirty bosons on a lattice
Julia Stasińska, Pietro Massignan, Michael Bishop, Jan Wehr, Anna Sanpera, Maciej Lewenstein
We investigate the interplay between disorder and interactions in a Bose gas on a lattice in presence of randomly localized impurities. We compare the performance of two theoretical methods, the simple version of multi-orbital Hartree-Fock and the common Gross-Pitaevskii approach, showing how the former gives a better approximation to the ground state in the limit of weak interactions, where the superfluid fraction is small. We further prove rigorously that for this class of disorder the fractal dimension of the ground state d* tends to the physical dimension in the thermodynamic limit. This allows us to introduce a quantity, the fractional occupation, which gives insightful information on the crossover from a Lifshits to a Bose glass. Finally, we compare temperature and interaction effects, highlighting similarities and intrinsic differences.
Nov 15
1. arXiv:1111.2873 [pdf, ps, other]
Bond order solid of two-dimensional dipolar fermions
S. G. Bhongale, L. Mathey, Shan-Wen Tsai, Charles W. Clark, Erhai Zhao
Cold atoms provide a promising platform to solve problems that, although computationally infeasible, are of immense importance to condensed matter physics and material science. Ultra-cold bosonic atoms have been quite successful in emulating the Bose-Hubbard model. Experiments are now underway towards mapping out the unknown phase diagram of the Fermi-Hubbard model. Recent experimental advances in cooling dipolar gases to quantum degeneracy provide an unprecedented opportunity to engineer Hubbard-like models with long range interactions. Here we show that two new and exotic types of order emerge generically in dipolar fermion systems: bond order solids of p- and d-wave symmetry. Similar, but manifestly different, phases of two-dimensional correlated electronic systems have previously only been hypothesized. Our results suggest that these phases can be constructed flexibly with dipolar fermions, using currently available experimental techniques, providing detectable experimental signatures.
Nov 14
1. arXiv:1111.2706 [pdf, other]
Mixtures of ultra-cold atoms in 1D disordered potentials
Francois Crepin, Gergely Zarand, Pascal Simon
We study interacting 1D two-component mixtures of cold atoms in a random potential, and extend the results reported earlier [{\it Phys. Rev. Lett.} {\bf 105}, 115301 (2010)]. We construct the phase diagram of a disordered Bose-Fermi mixture as a function of the strength of the Bose-Bose and Bose-Fermi interactions, and the ratio of the bosonic sound velocity and the Fermi velocity. Performing renormalization group and variational calculations, three phases are identified: (i) a fully delocalized two-component Luttinger liquid with superfluid bosons and fermions (ii) a fully localized phase with both components pinned by disorder, and (iii) an intermediate phase where fermions are localized but bosons are superfluid. Within the variational approach, each phase corresponds to a different level of replica symmetry breaking. In the fully localized phase we find that the bosonic and fermionic localization lengths can largely differ. We also compute the momentum distribution as well as the structure factor of the atoms (both experimentally accessible), and discuss how the three phases can be experimentally distinguished.
Nov 7 - Nov 11, Bin Wang
Nov 11
1. arXiv:1111.2375 [pdf, ps, other]
Title: Quantum criticality of spin-1 bosons in a 1D harmonic trap
Authors: C. C. N. Kuhn, X. W. Guan, A. Foerster, M. T. Batchelor
Using the thermodynamic Bethe ansatz equations, we derive the equation of state of spin-1 bosons with strongly repulsive density-density and antiferromagnetic spin-exchange interactions. In an harmonic trap we find that a partially-polarized core is surrounded by two wings composed of either spin-singlet bosonic pairs or a fully spin-aligned Tonks-Girardeau gas. The universal scaling functions yield the same dynamical exponent $z=2$ and correlation length exponent $\nu=1/2$ for the thermodynamic properties of these different spin states. This reveals a subtle resemblance to the physics of the spin-1/2 attractive Fermi gas. We demonstrate that the phase diagram, the Tomonaga-Luttinger liquids and critical properties of the bulk system can be mapped out from the density profiles of the trapped spinor gas at finite temperatures.
2. arXiv:1111.2382 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: Electronic Pumping of Quasiequilibrium Bose-Einstein Condensed Magnons
Authors: Scott A. Bender, Rembert A. Duine, Yaroslav Tserkovnyak
We theoretically investigate spin transfer between a system of quasiequilibrated Bose-Einstein condensed magnons in an insulator in direct contact with a conductor. While charge transfer is prohibited across the interface, spin transport arises from the exchange coupling between insulator and conductor spins. In normal insulator phase, spin transport is governed solely by the presence of thermal and spin-diffusive gradients; the presence of Bose-Einstein condensation (BEC), meanwhile, gives rise to a temperature-independent condensate spin current. Depending on the thermodynamic bias of the system, spin may flow in either direction across the interface, engendering the possibility of a dynamical phase transition of magnons. We discuss experimental feasibility of observing a BEC steady state (fomented by a spin Seebeck effect), which is contrasted to the more familiar spin-transfer induced classical instabilities.
Nov 10
1. arXiv:1111.2210 [pdf, other]
Title: Merging and alignment of Dirac points in a shaken honeycomb optical lattice
Authors: Selma Koghee, Lih-King Lim, M. O. Goerbig, C. Morais Smith
Inspired by the recent creation of the honeycomb optical lattice and the realization of the Mott insulating state in a square lattice by shaking, we study here the shaken honeycomb optical lattice. For a periodic shaking of the lattice, a Floquet theory may be applied to derive a time-independent Hamiltonian. In this effective description, the nearest-neighbor hopping parameters are renormalized by a Bessel function, which depends on the shaking direction, amplitude and frequency. Consequently, the system becomes anisotropic and the hopping parameters can vanish and even change sign, thus yielding different band structures. Here, we study the merging and the alignment of Dirac points and dimensional crossovers from the two dimensional system to one dimensional chains and zero dimensional dimers. We also consider next-nearest-neighbor hopping, which breaks the particle-hole symmetry and leads to a metallic phase. Unlike the nearest-neighbor hopping, the next-nearest-neighbor hopping is not renormalized. Furthermore, we include weak repulsive on-site interactions and find the density profiles for different values of the hopping parameters and interactions, both in a homogeneous system and in the presence of a trapping potential. Our results may be experimentally observed by using momentum-resolved Raman spectroscopy.
2. arXiv:1111.2052 (cross-list from cond-mat.str-el) [pdf, other]
Title: Time-dependent spin-wave theory
Authors: Andreas Rückriegel, Andreas Kreisel, Peter Kopietz
we generalize the spin-wave expansion in powers of the inverse spin to time-dependent quantum spin models describing rotating magnets or magnets in time-dependent external fields. We show that in these cases the spin operators should be projected onto properly defined rotating reference frames before the spin components are bosonized using the Holstein-Primakoff transformation. As a first application of our approach, we calculate the re-organization of the magnetic state due to Bose-Einstein condensation of magnons in the magnetic insulator yttrium-iron garnet; we predict a characteristic dip in the magnetization which should be measurable in experiments.
Nov 9
1. arXiv:1111.2031 [pdf, other]
Title: Macroscopic superposition states of ultracold bosons in a double-well potential
Authors: M. A. Garcia-March. D. R. Dounas-Frazer, Lincoln D. Carr
We present a thorough description of the physical regimes for ultracold bosons in double wells, with special attention paid to macroscopic superpositions (MSs). We use a generalization of the Lipkin-Meshkov-Glick Hamiltonian of up to eight single particle modes to study these MSs, solving the Hamiltonian with a combination of numerical exact diagonalization and high-order perturbation theory. The MS is between left and right potential wells; the extreme case with all atoms simultaneously located in both wells and in only two modes is the famous NOON state, but our approach encompasses much more general MSs. Use of more single particle modes brings dimensionality into the problem, allows us to set hard limits on the use of the original two-mode LMG model commonly treated in the literature, and also introduces a new mixed Josephson-Fock regime. Higher modes introduce angular degrees of freedom and MS states with different angular properties.
2. arXiv:1111.1952 [pdf, ps, other]
Title: Topological Superfluid in one-dimensional Ultracold Atomic System with Spin-Orbit Coupling
Authors: Zhongbo Yan, Xiaosen Yang, Shaolong Wan
We propose a one-dimensional Hamiltonian which supports Majorana fermions when $d_{x^{2}-y^{2}}$-wave superfluid appears in the ultracold atomic system and obtain the phase-separation diagrams both for the time-reversal-invariant case and time-reversal-symmetry-broken case. From the phase-separation diagrams, we find that the single Majorana fermion exists in the topological superfluid (TSF) region, and we can reach this region by tuning the chemical potential $\mu$ and spin-orbit coupling $\alpha_{R}$. Importantly, we find that spin-orbit coupling can effectively broaden the range of $\mu$ to tune for TSF, therefore makes the experiment of finding single Majorana fermion more realizable. Limited to the Hamiltonian, we find in some parameter limit, the Hamiltonian will reduced to a Heisenberg Hamiltoian, and more phases will emerge.
3. arXiv:1111.1798 [pdf, ps, other]
Title: Probing Majorana fermions in spin-orbit coupled atomic Fermi gases
Authors: Xia-Ji Liu, Lei Jiang, Han Pu, Hui Hu
We examine theoretically the visualization of Majorana fermions in a two-dimensional trapped ultracold atomic Fermi gas with spin-orbit coupling. By increasing an external Zeeman field, the trapped gas transits from non-topological to topological superfluid, via a mixed phase in which both types of superfluids coexist. We show that the zero-energy Majorana fermion, supported by the topological superfluid and localized at the vortex core, is clearly visible through (i) the core density and (ii) the local density of states, which are readily measurable in experiment. We present a realistic estimate on experimental parameters for ultracold $^{40}$K atoms.
Nov 8
1. arXiv:1111.1694 [pdf, other]
Title: Spin-Nematic Squeezing in a Quantum Gas
Authors: C. D. Hamley, C. S. Gerving, T. M. Hoang, E. M. Bookjans, M. S. Chapman
Exotic types of magnetic order and phases resulting from collective behaviour of quantum spins are an important focus of many-body physics. Nematic or quadrupolar ordering of spins is one such example, which breaks O(3) rotational symmetry, has no magnetic moment and is analogous to the well-known ordering of molecules in nematic phases of liquid crystals. Spin nematic phases have been posited for a variety of condensed matter systems including frustrated quantum magnets, and heavy-fermion and iron-based superconductors, although they are challenging to detect directly. Spin-1 atomic Bose-Einstein condensates provide a natural system to investigate spin-nematic quantum phases with a key advantage that the nematic tensor is directly measurable. Here, we measure spin-nematic fluctuations in a spin-1 condensate following a quench through a nematic to ferromagnetic quantum phase transition and observe quadrature squeezing in the variance of the fluctuations up to -8.3 dB (-10.3 dB corrected for detection noise) below the standard quantum limit. Previous work in atomic squeezing has employed two-level systems exhibiting SU(2) symmetry on the Bloch sphere, while the squeezing observed here is an example of squeezing in an SU(3) system. These results demonstrate spin-nematic dynamics in the quantum regime and could form the basis of a quantum-enhanced magnetometer.
2. arXiv:1111.1267 [pdf, ps, other]
Title: Actinide Topological Insulator Materials with Strong Interaction
Authors: Xiao Zhang, Haijun Zhang, Claudia Felser, Shou-Cheng Zhang
Topological band insulators have recently been discovered in spin-orbit coupled two and three dimensional systems. In this work, we theoretically predict a class of topological Mott insulators where interaction effects play a dominant role. In actinide elements, simple rocksalt compounds formed by Pu and Am lie on the boundary of metal to insulator transition. We show that interaction drives a quantum phase transition to a topological Mott insulator phase with a single Dirac cone on the surface.
Nov 7
1. arXiv:1111.0999 [pdf, other]
Title: Clock shift in a strongly interacting two-dimensional Fermi gas
Authors: Christian Langmack, Marcus Barth, Wilhelm Zwerger, Eric Braaten
We derive universal relations for the radio-frequency (rf) spectroscopy of a two-dimensional Fermi gas consisting of two spin states with a resonant S-wave interaction. The rf transition rate has a high-frequency tail that is proportional to the contact and displays logarithmic scaling violations, decreasing asymptotically like $1/(\omega^2 \ln^2 \omega)$. Its coefficient is proportional to $\ln^2(a_{2D}'/a_{2D})$, where $a_{2D}$ and $a_{2D}'$ are the 2-dimensional scattering lengths associated with initial-state and final-state interactions. The clock shift is proportional to the contact and to $\ln(a_{2D}'/a_{2D})$. If $|\ln(a_{2D}'/a_{2D})| \gg 1$, the clock shift arises as a cancellation between much larger contributions proportional to $\ln^2(a_{2D}'/a_{2D})$ from bound-bound and bound-free rf transitions.
2. arXiv:1111.1108 (cross-list from quant-ph) [pdf, other]
Title: Dynamics and evaporation of defects in Mott-insulating clusters of boson pairs
Authors: Dominik Muth, David Petrosyan, Michael Fleischhauer
Repulsively bound pairs of particles in a lattice governed by the Bose-Hubbard model can form stable incompressible clusters of dimers corresponding to finite-size $n=2$ Mott insulators. Here we study the dynamics of hole defects in such clusters corresponding to unpaired particles which can resonantly tunnel out of the cluster into the lattice vacuum. Due to bosonic statistics, the unpaired particles have different effective mass inside and outside the cluster, and "evaporation" of hole defects from the cluster boundaries is possible only when their quasi-momenta are within a certain transmission range. We show that quasi-thermalization of hole defects occurs in the presence of catalyzing particle defects which thereby purify the Mott insulating clusters. We study the dynamics of one-dimensional system using analytical techniques and numerically exact t-DMRG simulations. We derive an effective strong-interaction model that enables simulations of the system dynamics for much longer times. We also discuss a more general case of two bosonic species which reduces to the fermionic Hubbard model in the strong interaction limit.
