January2011

flat =Jan 31- Feb 4, Zixu Zhang=

Feb 04
1. [|arXiv:1102.0572] [[|pdf], [|ps], [|other]] Authors: [|S. Knoop], [|T. Schuster], [|R. Scelle], [|A. Trautmann], [|J. Appmeier], [|M. K. Oberthaler], [|E. Tiesinga], [|E. Tiemann] Comments: 11 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph) We have studied magnetic Feshbach resonances in an ultracold sample of Na prepared in the absolute hyperfine ground state. We report on the observation of three s-, eight d-, and three g-wave Feshbach resonances, including a more precise determination of two known s-wave resonances, and one s-wave resonance at a magnetic field exceeding 200mT. Using a coupled-channels calculation we have improved the sodium ground-state potentials by taking into account these new experimental data, and derived values for the scattering lengths. In addition, a description of the molecular states leading to the Feshbach resonances in terms of the asymptotic-bound-state model is presented.
 * Title: Feshbach spectroscopy and analysis of the interaction potentials of ultracold sodium**

2. [|arXiv:1102.0640] [[|pdf], [|ps], [|other]] Authors: [|F. Malet], [|T. Kristensen], [|S. M. Reimann], [|G. M. Kavoulakis] Comments: 6 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas) We study the rotational properties of a dipolar Bose-Einstein condensate confined in a quasi-two- dimensional anisotropic trap, for an arbitrary orientation of the dipoles with respect to their plane of motion. Within the mean-field approximation we find that the lowest-energy state of the system depends strongly on the relative strength between the dipolar and the contact interactions, as well as on the size and the orientation of the dipoles, and the size and the orientation of the deformation of the trapping potential.
 * Title: Rotational properties of dipolar Bose-Einstein condensates confined in anisotropic harmonic potentials**

3. [|arXiv:1102.0600] (cross-list from cond-mat.mes-hall) [[|pdf], [|ps], [|other]] Authors: [|Oleg L. Berman], [|Roman Ya. Kezerashvili], [|German V. Kolmakov] Comments: 34 pages, 8 figures Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Strongly Correlated Electrons (cond-mat.str-el) We review current understanding of the non-equilibrium dynamics of collective quantum systems. We describe an approach based on the Hamiltonian formulation of superfluid hydrodynamics. It is shown that, in the presence of constant energy pumping, the nonlinear coupling of fluctuations in the density and entropy strongly affects the nonequilibrium dynamics of the system. We use the results obtained to analyze the properties of out-of-equilibrium superfluid 4He and of exciton polariton Bose-Einstein condensates, both in semiconductor quantum wells and in graphene layers in presence of high magnetic field.
 * Title: Introduction to Nonlinear Phenomena in Superfluid Liquids and Bose-Einstein Condensates: Helium, Semiconductors and Graphene**

Feb 03
1. [|arXiv:1102.0340] [[|pdf], [|ps], [|other]] Authors: [|H. M. Price], [|N. R. Cooper] Subjects: Quantum Gases (cond-mat.quant-gas) We study theoretically the dynamics of two-component Bose-Einstein condensates in two dimensions, which admit topological excitations related to the Skyrmions of nuclear physics. We show that there exists a branch of uniformly propagating solitary waves characterised by a conserved momentum. These excitations exhibit a cross-over from spatially extended spin-wave states at low momentum to a localised "spin-wave droplet" at intermediate momentum; at still higher momentum, the configuration evolves continuously into a Skyrmion-antiSkyrmion pair. We discuss how these solitary waves can be generated and studied in experiment.
 * Title: Skyrmion-antiSkyrmion pairs in Ultracold Atomic Gases**

2. [|arXiv:1102.0350] [[|pdf], [|ps], [|other]] Authors: [|Yi Zhang], [|Tarun Grover], [|Ashvin Vishwanath] Comments: 6+ pages, 6 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) We use bipartite entanglement entropy, in particular the Renyi entropy $S_{2}$ to investigate the quantum structure of candidate ground states of critical spin-liquids. Using the Variational Monte-Carlo technique we calculate $S_2$ for the conjectured spinon Fermi sea state. This spin state is obtained by Gutzwiller projecting a spinon Fermi sea state on the triangular lattice. We find a violation of the boundary law, with $S_{2}$ enhanced by a logarithmic factor, an unusual result for a bosonic wave-function reflecting the presence of emergent fermions. Renyi entropy for algebraic spin liquids is found to obey the area law, consistent with the presence of emergent Dirac fermions in the system. Projection is found to significantly alter the entanglement properties of nested Fermi surface states. These results show that Renyi entropy calculations could serve as a diagnostic for gapless fractionalized phases.
 * Title: Entanglement entropy of critical spin liquids**

Feb 02
1. [|arXiv:1101.6025] (cross-list from cond-mat.quant-gas) [[|pdf], [|ps], [|other]] Authors: [|J. Kajala], [|F. Massel], [|P. Törmä] Comments: Supplementary material included Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con); Quantum Physics (quant-ph) Expansion dynamics of interacting fermions in a lattice are simulated within the one-dimensional (1D) Hubbard model, using the essentially exact time-evolving block decimation (TEBD) method. In particular, the expansion of an initial band-insulator state is considered. We analyze the simulation results based on the dynamics of a two-site two-particle system, the so-called Hubbard dimer. Our findings describe essential features of a recent experiment on the expansion of a Fermi gas in a two-dimensional lattice. We show that the Hubbard-dimer dynamics, combined with a two-fluid model for the paired and non-paired components of the gas, gives an efficient description of the full dynamics. This should be useful for describing dynamical phenomena of strongly interacting Fermions in a lattice in general.
 * Title: Expansion dynamics in the one-dimensional Fermi-Hubbard model**

2. [| arXiv:1102.0133] (cross-list from cond-mat.quant-gas) [[|pdf], [|ps], [|other]] Authors: [|J. Kajala], [|F. Massel], [|And P. Törmä] Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con) In this work we analyze the dynamical behavior of the collision between two clouds of fermionic atoms with opposite spin polarization. By means of the time-evolving block decimation (TEBD) numerical method, we simulate the collision of two one-dimensional clouds in a lattice. There is a symmetry in the collision behaviour between the attractive and repulsive interactions. We analyze the pair formation dynamics in the collision region, providing a quantitative analysis of the pair formation mechanism in terms of a simple two-site model.
 * Title: Collision of one dimensional (1D) spin polarized Fermi gases in an optical lattice**

Feb 01
1. [|arXiv:1101.5633] [[|pdf], [|ps], [|other]] Authors: [|Liang He], [|Walter Hofstetter] Comments: 8 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas) We study a system of ultra-cold fermionic polar molecules in a two-dimensional square lattice interacting via both the long-ranged dipole-dipole interaction and a short-ranged on-site attractive interaction. Singlet superfluid, charge density wave, and supersolid phases are found to exist in the system. We map out the zero temperature phase diagram and find that the supersolid phase is considerably stabilized by the dipole-dipole interaction and thus can exist over a large region of filling factors. We study the melting of the supersolid phase with increasing temperature, map out a finite temperature phase diagram of the system at fixed filling, and determine the parameter region where the supersolid phase can possibly be observed in experiments.
 * Title: Supersolid Phase of Cold Fermionic Polar Molecules in 2D Optical Lattices**

[|arXiv:1101.5639] [[|pdf], [|ps], [|other]] Authors: [|Stefan S. Natu], [|Erich J. Mueller] Comments: 4 pages, 3 figures, 1 table. Supplementary materials attached Subjects: Quantum Gases (cond-mat.quant-gas) We theoretically study the stability of a normal, spin disordered, homogenous spin-1 Bose gas against ferromagnetism, pairing, and condensation through a Random Phase Approximation which includes exchange (RPA-X). Repulsive spin-independent interactions stabilize the normal state against both ferromagnetism and pairing, and for typical interaction strengths leads to a direct transition from an unordered normal state to a fully ordered single particle condensate. Atoms with much larger spin-dependent interaction may experience a transition to a ferromagnetic normal state or a paired superfluid, but, within the RPA-X, there is no instability towards a normal state with spontaneous nematic order. We analyze the role of the quadratic Zeeman effect and finite system size.
 * Title: Pairing, Ferromagnetism, and Condensation of a normal spin-1 Bose gas**

[|arXiv:1101.5726] [[|pdf], [|ps], [|other]] Authors: [|K. W. Mahmud], [|E. N. Duchon], [|Y. Kato], [|N. Kawashima], [|R. T. Scalettar], [|N. Trivedi] Comments: 13 pages, 9 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Statistical Mechanics (cond-mat.stat-mech) We use quantum Monte Carlo (QMC) simulations to study the combined effects of harmonic confinement and temperature for bosons in a two dimensional optical lattice. The scale invariant, finite temperature, state diagram is presented for the Bose-Hubbard model in terms of experimental parameters -- the particle number, confining potential and interaction strength. To distinguish the nature of the spatially separated superfluid, Mott Insulator and normal Bose liquid phases, we examine the local density, compressibility, superfluid density and Green's function. In the annular superfluid rings, as the width of the ring decreases, the long range superfluid correlations start to deviate from an equivalent homogeneous 2D system. At zero temperature, the correlation decay is intermediate between 1D and 2D, while at finite temperature, the decay is similar to that in 1D at a much lower temperature. The calculations reveal shortcomings of the local density approximation (LDA) in describing superfluid properties of trapped bosons. We also present the finite temperature phase diagram for the homogeneous two dimensional Bose-Hubbard model. We compare our state diagram with the results of a recent experiment at NIST on a harmonically trapped 2D lattice [Phys. Rev. Lett. 105, 110401 (2010)], and identify a finite temperature effect in the experiment.
 * Title: Finite temperature study of bosons in a two dimensional optical lattice**

4. [|arXiv:1101.6067] [[|pdf], [|ps], [|other]] Authors: [|M. Iskin] Comments: 4 pages with 2 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el) We use the Gutzwiller ansatz and analyze the phase diagram of the extended Bose-Hubbard Hamiltonian with on-site (U) and nearest-neighbor (V) repulsions. For d-dimensional hypercubic lattices, when 2dV < U, it is well-known that the ground state alternates between the charge-density-wave (CDW) and Mott insulators, and the supersolid (SS) phase occupies small regions around the CDW insulators. However, when 2dV > U, in this paper, we show that the ground state has only CDW insulators, and more importantly, the SS phase occupies a much larger region in the phase diagram, existing up to very large hopping values which could be orders of magnitude higher than that of the well-known case. In particular, the SS-superfluid phase boundary increases linearly as a function of hopping when 2dV \gtrsim 1.5U, for which the prospects of observing the SS phase with dipolar Bose gases loaded into optical lattices is much higher.
 * Title: Route to supersolidity for dipolar Bose gases on optical lattices**

5. [|arXiv:1101.5900] (cross-list from quant-ph) [[|pdf], [|ps], [|other]] Authors: [|James R. Wootton], [|Jiannis K. Pachos] Comments: 5 pages, 2 figures. Presented as a contributed talk at QIP 2011 Subjects: Quantum Physics (quant-ph) ; Strongly Correlated Electrons (cond-mat.str-el) The ability to reliably store quantum states is an essential element for any task in quantum information. Topological systems promise to protect quantum information by topological and energetic considerations. If undesired anyonic excitations are propagated at large distances, either through coherent or probabilistic processes, they can cause logical errors in the topologically encoded information. Here we show that Anderson localisation induced by disorder in the system can successfully protect topological quantum memories from the coherent propagation of anyons. For concreteness we employ the toric code model at zero temperature. It is known that in the absence of a magnetic field it can tolerate a finite initial density of anyonic errors. In the presence of a spurious magnetic field anyonic quantum walks are induced and the tolerable density becomes zero. We demonstrate that disorder in the couplings of the model can successfully localise anyons. This allows the topological quantum memory to tolerate a finite initial density of anyonic errors for arbitrarily long times. We anticipate that disorder inherent in any physical realisation of topological systems will help to strengthen the fault-tolerance of quantum memories.
 * Title: Bringing order through disorder: Localisation of errors in topological quantum memories**

Jan 31
1. [|arXiv:1101.5467] [[|pdf], [|ps], [|other]] Authors: [|Yinyin Qian], [|Ming Gong], [|Chuanwei Zhang] Comments: 7 pages, 8 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Materials Science (cond-mat.mtrl-sci) We numerically investigate, using the time evolving block decimation algorithm, the quantum transport of ultra-cold bosonic atoms in a double well optical lattice through periodic modulation of the lattice parameters (intra- and inter-well tunneling, chemical potential, etc.). The transport of atoms depends on the atom filling in each double well and the interaction between atoms. In the strongly interacting region, the bosonic atoms share the same transport properties as non-interacting fermions with quantized transport at the half filling and no atom transport at the integer filling. In the weakly interacting region, the number of the transported atoms is proportional to the atom filling. The effects of harmonic trapping and disorder potentials are characterized. A Berry phase model is developed to explain the numerically observed transport of bosonic atoms in the non-interacting and strongly interacting limits.
 * Title: Quantum Transport of Bosonic Cold Atoms in Double Well Optical Lattices**

2. [|arXiv:1101.5593] [[|pdf], [|other]] Authors: [|Marco Roncaglia], [|Matteo Rizzi], [|Jean Dalibard] Comments: 9 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas) Considerable efforts are currently devoted to the preparation of ultracold neutral atoms in the emblematic strongly correlated quantum Hall regime. The routes followed so far essentially rely on thermodynamics, i.e. imposing the proper Hamiltonian and cooling the system towards its ground state. In rapidly rotating 2D harmonic traps the role of the transverse magnetic field is played by the angular velocity. For particle numbers significantly larger than unity, the required angular momentum is very large and it can be obtained only for spinning frequencies extremely near to the deconfinement limit; consequently, the required control on experimental parameters turns out to be far too stringent. Here we propose to follow instead a dynamic path starting from the gas confined in a rotating ring. The large moment of inertia of the fluid facilitates the access to states with a large angular momentum, corresponding to a giant vortex. The initial ring-shaped trapping potential is then adiabatically transformed into a harmonic confinement, which brings the interacting atomic gas in the desired quantum Hall regime. We provide clear numerical evidence that for a relatively broad range of initial angular frequencies, the giant vortex state is adiabatically connected to the bosonic $\nu=1/2$ Laughlin state, and we discuss the scaling to many particles.
 * Title: From Rotating Atomic Rings to Quantum Hall States**

3. [|arXiv:1101.5594] [[|pdf], [|ps], [|other]] Authors: [|Marcus Barth], [|Wilhelm Zwerger] Comments: 8 figures, 26 pages Subjects: Quantum Gases (cond-mat.quant-gas) We derive exact relations that connect the universal C/k^4-decay of the momentum distribution at large k with both thermodynamic properties and correlation functions of two-component Fermi gases in one dimension with contact interactions. The relations are analogous to those obtained by Tan in the three-dimensional case and are derived from an operator product expansion of the one- and two-particle density matrix. They extend earlier results by Olshanii and Dunjko [Phys. Rev. Lett. 91, 090401 (2003)] for the bosonic Lieb-Liniger gas. As an application, we calculate the pair distribution function at short distances and the dimensionless contact in the limit of infinite repulsion. The ground state energy approaches a universal constant in this limit, a behavior that also holds in the three-dimensional case. In both one and three dimensions, the Tan relations rule out a Stoner instability to a saturated ferromagnet for repulsive Femions with zero range interactions.
 * Title: Tan relations in one dimension**

=Jan 24-28, Chungwei Lin=

Jan 28
1. [|arXiv:1101.5272] [[|pdf], [|other]] Authors: [|Martin Esmann], [|Niklas Teichmann], [|Christoph Weiss] Comments: 9 pages, 10 figures Subjects: Quantum Gases (cond-mat.quant-gas) Fractional photon-assisted tunneling is investigated both analytically and numerically for few interacting ultra-cold atoms in the double-wells of an optical superlattice. This could be realized experimentally by adding periodic shaking to an existing experimental setup [Phys. Rev. Lett. 101, 090404 (2008)]. In order to understand the physics of the photon-assisted tunneling, an effective model based on the rotating wave approximation is introduced. The validity of this effective approach is tested for wide parameter ranges which are accessible to experiments in double-well lattices. The effective model goes well beyond previous perturbation theory approaches and is useful to investigate in particular the fractional photon-assisted tunneling resonances. Analytic results on the level of the experimentally realizable two-particle quantum dynamics show very good agreement with the numerical solution of the time-dependent Schrodinger equation. Far from being a small effect, both the one-half-photon and the one-third-photon resonance are shown to be leading order effects.
 * Title: Fractional photon-assisted tunneling in an optical superlattice: a leading order effect**

2. [|arXiv:1101.5337] (cross-list from cond-mat.str-el) [[|pdf], [|ps], [|other]] Authors: [|M. A. Cazalilla], [|R. Citro], [|T. Giamarchi], [|E. Orignac], [|M. Rigol] Comments: RevTeX 4, 68 pages, 22 EPS figures, submitted to Rev. Mod. Phys Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Quantum Gases (cond-mat.quant-gas) We review the physics of one-dimensional interacting bosonic systems. Beginning with results from exactly solvable models and computational approaches, we introduce the concept of bosonic Tomonaga-Luttinger Liquids relevant for one-dimension, and compare it with Bose-Einstein condensates existing in dimensions higher than one. We discuss the effects of various perturbations on the Tomonaga-Luttinger liquid state as well as extensions to multicomponent and out of equilibrium situations. Finally, we review the experimental systems that can be described in terms of models of interacting bosons in one dimension.
 * Title: One dimensional Bosons: From Condensed Matter Systems to Ultracold Gases**

3. [| arXiv:1101.5290] (cross-list from cond-mat.mes-hall) [[|pdf], [|other]] Authors: [|M. Storni], [|R. H. Morf] Comments: 9 pages, 8 figures Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Strongly Correlated Electrons (cond-mat.str-el) Using exact diagonalization in the spherical geometry, we investigate systems of localized quasiholes at nu=5/2 for interactions interpolating between the pure Coulomb and the 3-body interaction for which the Moore-Read state is the exact ground state. We show that the charge e/4 quasihole can be easily localized by means of a delta-function pinning potential. Using a tuned smooth pinning potential, the quasihole radius can be limited to about three magnetic length units. For systems of 2 quasiholes, adiabatic continuity between the Moore-Read and the Coulomb limit holds for the ground state, while for 4 quasiholes, the lowest two energy states exhibit adiabatic continuity. This implies the existence of a Majorana fermion for pure Coulomb interaction. We also present preliminary results in the Coulomb limit for braiding in systems containing 4 quasiholes, with up to 14 electrons, diagonalizing in the full spin polarized sector of the second Landau level Hilbert space.
 * Title: Numerical Study of Localized Quasiholes at nu=5/2 and the Majorana Fermion**

Jan 27
1. [|arXiv:1101.5006] [[|pdf], [|ps], [|other]] Authors: [|Michiel Snoek] Comments: 4 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Statistical Mechanics (cond-mat.stat-mech) We provide a rigorous derivation of Gutzwiller mean-field dynamics for lattice bosons, showing that it is exact on fully connected lattices. We apply this formalism to quenches in the interaction parameter from the Mott insulator to the superfluid state. Although within mean-field the Mott insulator is a steady state, we show that a dynamical critical interaction U_d exists, such that for final interaction parameter U_f>U_d the Mott insulator is exponentially unstable towards emerging long-range superfluid order, whereas for U_f<U_d the Mott insulating state is stable.
 * Title: Rigorous mean-field dynamics of lattice bosons: quenches from the Mott insulator**

Jan 26
1. [|arXiv:1101.4701] (cross-list from cond-mat.supr-con) [[|pdf], [|other]] Authors: [|Elihu Abrahams], [|Qimiao Si] Comments: 20 pages, 9 figures Subjects: Superconductivity (cond-mat.supr-con) ; Strongly Correlated Electrons (cond-mat.str-el) Superconductivity in the iron pnictides and chalcogenides arises at the border of antiferromagnetism, which raises the question of the role of quantum criticality. In this topical review, we describe the theoretical work that led to the prediction for a magnetic quantum critical point arising out of a competition between electronic localization and itinerancy, and the proposal for accessing it by using isoelectronic P substitution for As in the undoped iron pnictides. We go on to compile the emerging experimental evidence in support of the existence of such a quantum critical point in isoelectronically-tuned iron pnictides. We close by discussing the implications of these results for the physics of the iron pnictides and chalcogenides.
 * Title: Quantum criticality in the iron pnictides and chalcogenides**

2. [|arXiv:1101.4663] [[|pdf], [|other]] Authors: [|Iva Brezinova], [|Lee Collins], [|Katharina Ludwig], [|Barry Schneider], [|Joachim Burgdorfer] Comments: 13 pages, 11 figures Subjects: Quantum Gases (cond-mat.quant-gas) The Gross-Pitaevskii equation (GPE) plays an important role in the description of Bose-Einstein condensates (BECs) at the mean-field level. The GPE belongs to the class of non-linear Schr\"odinger equations which are known to feature dynamical instability and collapse for attractive non-linear interactions. We show that the GPE with repulsive non-linear interactions typical for BECs features chaotic wave dynamics. We find positive Lyapunov exponents for BECs expanding in periodic and aperiodic smooth external potentials as well as disorder potentials. Our analysis demonstrates that wave chaos characterized by the exponential divergence of nearby initial wavefunctions is to be distinguished from the notion of non-integrability of non-linear wave equations. We discuss the implications of these observations for the limits of applicability of the GPE, the problem of Anderson localization, and the properties of the underlying many-body dynamics.
 * Title: Wave chaos in the non-equilibrium dynamics of the Gross-Pitaevskii equation**

3. [|arXiv:1101.4785] [[|pdf], [|ps], [|other]] Authors: [|Jonas Larson], [|Jani-Petri Martikainen] Comments: 9 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) We present a method for transferring bosonic atoms residing on the lowest s-band of an optical lattice to the first excited p-bands. Our idea hinges on resonant tunneling between adjacent sites of accelerated lattices. The acceleration effectively shifts the quasi-bound energies on each site such that the system can be cast into a Wannier-Stark ladder problem. By adjusting the acceleration constant, a situation of resonant tunneling between the s- and p-bands is achievable. Within a mean-field model, considering Rb(87) atoms, we demonstrate population transfer from the s- to the p-bands with around 95 % efficiency. Nonlinear effects deriving from atom-atom interactions, as well as coupling of the quasi bound Wannier-Stark states to the continuum, are considered.
 * Title: Loading of p-band bosons in optical lattices**

4. [|arXiv:1101.4845] [[|pdf], [|ps], [|other]] Authors: [|Gentaro Watanabe], [|Harri Mäkelä] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) We present a method to create phase- and number-squeezed states in two-mode Bose systems using dissipation. The effectiveness of this method is demonstrated by considering cold Bose gases trapped in a double-well potential. The extension of our formalism to an optical lattice gives control of the phase boundaries of the steady-state phase diagram, and we discover a new phase characterized by a non-zero condensate fraction and thermal-like particle number statistics. We also show how to perform amplitude squeezing for single-mode photons using dissipation.
 * Title: Dissipation-induced squeezing**

Jan 25
1. [|arXiv:1101.4461] [[|pdf], [|ps], [|other]] Authors: [|Kwai-Kong Ng], [|Min-Fong Yang] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) By means of quantum Monte Carlo simulations implemented with a two-loop update scheme, the finite-temperature phase diagram of a three-body constrained attractive Bose lattice gas is investigated. The nature of the thermal phase transitions around the dimer superfluid and the atomic superfluid is unveiled. We find that the $Z_2$ symmetry-breaking transitions between these two superfluid phases are of first order even at nonzero temperatures. More interestingly, the thermal transition from the dimer superfluid to the normal fluid is found to be consistent with the Kosterlitz-Thouless type but giving an anomalous universal stiffness jump. It demonstrates that this transition is driven by unbinding of pairs of fractional vortices.
 * Title: Thermal Phase transitions in attractive extended Bose-Hubbard Model with three-body constraint**

2. [|arXiv:1101.4500] [[|pdf], [|other]] Authors: [|N. Goldman], [|D. F. Urban], [|D. Bercioux] Comments: 12 pages, 9 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph) We investigate the properties of the Lieb lattice, i.e a face-centered square lattice, subjected to external gauge fields. We show that an Abelian gauge field leads to a peculiar quantum Hall effect, which is a consequence of the single Dirac cone and the flat band characterizing the energy spectrum. Then we explore the effects of an intrinsic spin-orbit term - a non-Abelian gauge field - and demonstrate the occurrence of the quantum spin Hall effect in this model. Besides, we obtain the relativistic Hamiltonian describing the Lieb lattice at low energy and derive the Landau levels in the presence of external Abelian and non-Abelian gauge fields. Finally, we describe concrete schemes for realizing these gauge fields with cold fermionic atoms trapped in an optical Lieb lattice. In particular, we provide a very efficient method to reproduce the intrinsic (Kane-Mele) spin-orbit term with assisted-tunneling schemes. Consequently, our model could be implemented in order to produce a variety of topological states with cold-atoms.
 * Title: Topological Phases for Fermionic Cold Atoms on the Lieb Lattice**

3. [|arXiv:1101.4539] [[|pdf], [|other]] Authors: [|Henning Prüser], [|Martin Wenderoth], [|Piet E. Dargel], [|Alexander Weismann], [|Robert Peters], [|Thomas Pruschke], [|Rainer G. Ulbrich] Comments: 7 pages, 3 figures + 8 pages supplementary material; Nature Physics (Jan 2011 - advanced online publication) Subjects: Strongly Correlated Electrons (cond-mat.str-el) The Kondo effect, one of the oldest correlation phenomena known in condensed matter physics, has regained attention due to scanning tunneling spectroscopy (STS) experiments performed on single magnetic impurities. Despite the sub-nanometer resolution capability of local probe techniques one of the fundamental aspects of Kondo physics, its spatial extension, is still subject to discussion. Up to now all STS studies on single adsorbed atoms have shown that observable Kondo features rapidly vanish with increasing distance from the impurity. Here we report on a hitherto unobserved long range Kondo signature for single magnetic atoms of Fe and Co buried under a Cu(100) surface. We present a theoretical interpretation of the measured signatures using a combined approach of band structure and many-body numerical renormalization group (NRG) calculations. These are in excellent agreement with the rich spatially and spectroscopically resolved experimental data.
 * Title: Long-range Kondo signature of a single magnetic impurity**

Jan 24
1. [|arXiv:1101.4206] [[|pdf], [|ps], [|other]] Authors: [|David A. Smith] (1), [|Simon Aigner] (2), [|Sebastian Hofferberth] (3), [|Michael Gring] (1), [|Mauritz Andersson] (4), [|Stefan Wildermuth] (2), [|Peter Krüger] (5), [|Stephan Schneider] (1), [|Thorsten Schumm] (1), [|Jörg Schmiedmayer] (1,2) ((1) Vienna Center for Quantum Science and Technology, Atominstitut, TU Wien, Vienna, Austria, (2) Physikalisches Institut, Universität Heidelberg, Heidelberg, Germany, (3) Harvard-MIT Center for Ultracold Atoms, Harvard University, USA, (4) School of Information and Communication Technology, KTH, Stockholm, Sweden, (5) Midlands Ultracold Atom Research Centre, University of Nottingham, UK) Comments: 12 pages, 8 figures Subjects: Atomic Physics (physics.atom-ph) ; Quantum Gases (cond-mat.quant-gas) Imaging ultracold atomic gases close to surfaces is an important tool for the detailed analysis of experiments carried out using atom chips. We describe the critical factors that need be considered, especially when the imaging beam is purposely reflected from the surface. In particular we present methods to measure the atom-surface distance, which is a prerequisite for magnetic field imaging and studies of atom surface-interactions.
 * Title: Absorption Imaging of Ultracold Atoms on Atom Chips**

=Jan 17-21, Xiaopeng Li=

Jan 21
Author: Hiroshi Uchigaito, Masafumi Udagawa, Yukitoshi Motome We present our theoretical results on the ground states in layered triangular-lattice compounds ANiO2 (A=Na, Li, Ag). To describe the interplay between charge, spin, orbital, and lattice degrees of freedom in these materials, we study a doubly-degenerate Hubbard model with electron-phonon couplings by the Hartree-Fock approximation combined with the adiabatic approximation. In a weakly-correlated region, we find a metallic state accompanied by \sqroot3x\sqroot3 charge ordering. On the other hand, we obtain an insulating phase with spin-ferro and orbital-ferro ordering in a wide range from intermediate to strong correlation. These phases share many characteristics with the low-temperature states of AgNiO2 and NaNiO2, respectively. The charge-ordered metallic phase is stabilized by a compromise between Coulomb repulsions and effective attractive interactions originating from the breathing-type electronphonon coupling as well as the Hund's-rule coupling. The spin-orbital-ordered insulating phase is stabilized by the cooperative effect of electron correlations and the Jahn-Teller coupling, while the Hund's-rule coupling also plays a role in the competition with other orbital-ordered phases. The results suggest a unified way of understanding a variety of low-temperature phases in ANiO2. We also discuss a keen competition among different spin-orbital-ordered phases in relation to a puzzling behavior observed in LiNiO2.
 * 1. [|arXiv:1101.3828] [[|pdf], [|ps], [|other]] **
 * Title: Mean-field Study of Charge, Spin, and Orbital Orderings in Triangular-lattice Compounds ANiO2 (A=Na, Li, Ag)**

Author: Martin Greiter It is well established that the Hilbert space for charged particles in a plane subject to a uniform magnetic field can be described by two mutually commuting ladder algebras. We propose a similar formalism for Landau level quantization on a sphere with a magnetic monopole in the center, which involves two mutually commuting SU(2) algebras.
 * 2. [|arXiv:1101.3943] [[|pdf], [|ps], [|other]] **
 * Title: Formalism for Landau Level Quantization on the Sphere**

Jan 20
Author: Saptarshi Mandal, Naveen Surendran We study the excitations in a three dimensional version of Kitaev's spin-1/2 model on the honeycomb lattice introduced by the present authors recently. The gapped phase of the system is analyzed using a low energy effective Hamiltonian which is defined on the diamond lattice and consists of plaquette operators. The excitations of the effective Hamiltonian form loops in an embedded lattice. The elementary excitations, which are the shortest loops, are fermions. Moreover, the excitations obey nontrivial braiding rules: when a fermion winds through a loop, the wave function acquires a phase $\pi$.
 * 1. [|arXiv:1101.3718] [[|pdf], [|ps], [|other]] **
 * Title: Topological excitations in three dimensional Kitaev model**

Jan 19
Author: Florian Loder, Siegfried Graser, Arno P. Kampf, Thilo Kopp Striped high-$T_{\rm c}$ superconductors such as La$_{2-y-x}$Nd$_{y}$Sr$_x$CuO$_4$ and La$_{2-x}$Ba$_{x}$CuO$_4$ near $x=1/8$ show a fascinating competition between spin and charge order and superconductivity. A theory for these systems therefore has to capture both the spin correlations of an antiferromagnet and the pair correlations of a superconductor. For this purpose we present here an effective Hartree-Fock theory incorporating both electron pairing with finite center-of-mass momentum and antiferromagnetism. We show that this theory reproduces the key experimental features such as the formation of the antiferromagnetic stripe patterns at 7/8 band filling or the quasi one-dimensional electronic structure observed by photoemission spectroscopy.
 * 1. [|arXiv:1101.3402] [[|pdf], [|other]] **
 * Title: Pairing Theory of Striped Superconductivity**

Author: D. M. Jezek, H. M. Cataldo We study the winding number dependence of the stationary states of a Bose-Einstein condensate in a ring-shaped lattice. The system is obtained by confining atoms in a toroidal trap with equally spaced radial barriers. We calculate the energy and angular momentum as functions of the winding number and the barrier height for two quite distinct particle numbers. In both cases we observe two clearly differentiated regimes. For low barriers, metastable vortex states are obtained up to a maximum winding number which depends on the particle number and barrier height. In this regime, the angular momentum and energy show, respectively, almost linear and quadratic dependences on the winding number. For large barrier heights, on the other hand, stationary states are obtained up to a maximum winding number which depends only on the number of lattice sites, whereas energy and angular momentum are shown to be sinusoidal functions of the winding number.
 * 2. [|arXiv:1101.3362] [[|pdf], [|ps], [|other]] **
 * Title: Winding number dependence of Bose-Einstein condensates in a ring-shaped lattice**


 * 3. [|arXiv:1101.3609v1]**
 * Title: Hidden Fermi Liquid: Self-Consistent Theory for the Normal State of High-Tc Superconductors**

Author: Philip A. Casey, Philip W. Anderson Hidden Fermi liquid theory explicitly accounts for the effects of Gutzwiller projection in the t-J Hamiltonian, widely believed to contain the essential physics of the high-Tc superconductors. We derive expressions for the entire "strange metal", normal state relating angle-resolved photoemission, resistivity, Hall angle, and by generalizing the formalism to include the Fermi surface topology - angle-dependent magnetoresistance. We show this theory to be the first self-consistent description for the normal state of the cuprates based on transparent, fundamental assumptions. Our well-defined formalism also serves as a guide for further experimental confirmation.

Jan 17
Author: Francisco Castilho Alcaraz, Miguel Ibanez Berganza, German Sierra In a quantum critical chain, the scaling regime of the energy and momentum of the ground state and low lying excitations are described by conformal field theory (CFT). The same holds true for the von Neumann and Renyi entropies of the ground state, which display a universal logarithmic behaviour depending on the central charge. In this letter we generalize this result to those excited states of the chain that correspond to primary fields in CFT. It is shown that the n-th Renyi entropy is related to a 2n-point correlator of primary fields. We verify this statement for the critical XX and XXZ chains. This result uncovers a new link between quantum information theory and CFT.
 * 1. [|arXiv:1101.2881] [[|pdf], [|ps], [|other]] **
 * Title: Entanglement of low-energy excitations in Conformal Field Theory**

=Jan 10-14, Zixu Zhang=

Title: Spectroscopy for cold atom gases in periodically phase-modulating optical lattices
Authors: [|Akiyuki Tokuno], [|Thierry Giamarchi] Comments: 4 pages, 2 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el)

The response of cold atom gases to small periodic phase modulation of an optical lattice is discussed. For bosonic gases, the energy absorption rate is given, within linear response theory, by imaginary part of the current correlation function. For fermionic gases in a strong lattice potential, the same correlation function can be probed via the production rate double occupancy. The phase modulation gives thus direct access to the conductivity of the system, as function of the modulation frequency. We give an example of application in the case of one dimensional bosons at zero temperature and discuss the link between the phase- and amplitude-modulation.

Title: Stability Condition of a Strongly Interacting Boson-Fermion Mixture across a Feshbach Resonance
Authors: [|Zeng-Qiang Yu], [|Shizhong Zhang], [|Hui Zhai] Comments: 4 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

In this letter we study the properties of dilute bosons immersed in a single component Fermi sea across a broad boson-fermion Feshbach resonance. The stability of the mixture requires that the bare interaction between bosons exceeds a critical value, which is a universal function of the boson-fermion scattering length, and exhibits a maximum in the unitary regime. We calculate the quantum depletion, momentum distribution and contact of bosons across the resonance. The transition from condensate to molecular Fermi gas is also discussed.

Title: Thermodynamics and quantum criticality of strongly attractive spin-imbalanced Fermi gases in a 1D harmonic trap
Authors: [|Xiangguo Yin], [|Xi-Wen Guan], [|Shu Chen], [|Murray T Batchelor] Comments: f pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We investigate thermodynamics and quantum criticality of strongly attractive Fermi gases confined in a one-dimensional (1D) harmonic trap. Finite temperature density profiles, entropy, compressibility and susceptibility of the trapped gas are studied using analytic results for the thermodynamics within the local density approximation. We demonstrate that current experiments are capable of measuring universal Tomonaga-Luttinger liquid physics and quantum criticality of 1D strongly interacting Fermi gases. The results provide insights on recent measurements of key features of the phase diagram of a spin-imbalanced atomic Fermi gas [Liao et al., Nature 467, 567 (2010)] and point to further study of quantum critical phenomena in ultracold atomic Fermi gases.

Title: Phase diagram of soft-core bosons in two dimensions
Authors: [|S. Saccani], [|S. Moroni], [|M. Boninsegni] Subjects: Quantum Gases (cond-mat.quant-gas)

The low temperature phase diagram of Bose soft disks in two dimensions is studied by numerical simulations. It is shown that a supersolid cluster phase exists, within a range of the model parameters, analogous to that recently observed for a system of aligned dipoles interacting via a softened potential at short distance. These findings indicate that a long-range tail of the interaction is unneeded to obtain such a phase, and that the soft-core repulsive interaction is the minimal model for supersolidity.

Title: Quantum criticality between topological and band insulators in $(3+1)$-dimensions
Authors: [|Pallab Goswami], [|Sudip Chakravarty] Comments: 16 pages, 14 figures Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn) ; Strongly Correlated Electrons (cond-mat.str-el) Four-component massive and massless Dirac fermions in the presence of long range Coulomb interaction and chemical potential disorder exhibit striking fermionic quantum criticality. For an odd number of flavors of Dirac fermions, the sign of the Dirac mass distinguishes the topological and the trivial band insulator phases, and the gapless semi-metallic phase corresponds to the quantum critical point that separates the two. Up to a critical strength of disorder, the semi-metallic phase remains stable, and the universality class of the direct phase transition between two insulating phases is unchanged. Beyond the critical strength of disorder the semi-metallic phase undergoes a phase transition into a disorder controlled diffusive metallic phase, and there is no longer a direct phase transition between the two types of insulating phases. Our results are also applicable to even number of flavors of Dirac fermions, and the band inversion transition in various non-topological narrow gap semiconductors.

Title: Spatially varying interactions induced in atomic Fermi gases by optical Feshbach resonance
Authors: [|Chih-Chun Chien] Comments: 4 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Superconductivity (cond-mat.supr-con)

Optical Feshbach resonance is capable of inducing spatially varying interactions in ultra-cold atoms. Its applications to atomic Fermi gases in a cigar-shaped trap enable one to study several fresh phenomena. With the longitudinal direction bisected into a weakly interacting Fermi gas on one side and weakly interacting dimers on the other with its condensate removed by Bragg scattering, a non-equilibrium electrochemical current of fermions is expected to flow across the junction. The ballistic transport of fermions is limited by the universal conductance, which can be inferred from the current. By applying spatially varying interactions in the transverse direction, one can produce a normal-gas core enclosed by a superfluid shell, which is very different from common structures observed in experiments so far. These phenomena are naturally connected to other fields including atomtronics, quantum transport, and heavily underdoped high-temperature superconductors.

Title: Universality of Entanglement Near a Quantum Phase Transition
Authors: [|R. Jafari] Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Statistical Mechanics (cond-mat.stat-mech)

We show that near a quantum phase transitions point, some of the well-known quantities in classical statistical mechanics, show divergence and scaling properties which capture the universal behavior of quantum phase transitions. This is in contrast to the emerging paradigm that only entanglement can capture the universal and scaling properties of quantum phase transitions. We show that the transverse susceptibility and derivative of correlation functions with respect to the parameter that drives the quantum phase transitions, exhibit divergence and finite size scaling around the critical point, as does the entanglement. In other words the non-analytic and finite size scaling behaviors of entanglement is not its intrinsic properties and inherit from the non-analytic and scaling behaviors of correlation functions. However we exhibit that the correlation functions (even nearest neighbor correlation functions) which are the local quantities show the global properties of the systems and could capture the quantum critical point without pre-assumed order parameters even for the cases where the two-body entanglement is absent.

Title: The Spin-Exchange Dynamical Structure Factor of the S=1/2 Heisenberg Chain
Authors: [|Antoine Klauser], [|Jorn Mossel], [|Jean-Sebastien Caux], [|Jeroen van den Brink]

Comments: 4 pages, 4 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Statistical Mechanics (cond-mat.stat-mech)

We determine the spin-exchange dynamical structure factor of the Heisenberg spin chain, as is measured by indirect Resonant Inelastic X-ray Scattering (RIXS). We find that two-spin RIXS excitations nearly entirely fractionalize into *two spinon* states. These share the same continuum lower bound as single-spin neutron scattering excitations, even if the relevant final states belong to orthogonal symmetry sectors. The RIXS spectral weight is mainly carried by higher-energy excitations, and is beyond the reach of the low-energy effective theories of Luttinger liquid type.

Title: Charge-spin-orbital dynamics of one-dimensional two-orbital Hubbard model
Authors: [|Hiroaki Onishi] Comments: 4 pages, 3 figures, Proceedings of ICM2009 (July 26-31, 2009, Karlsruhe, Germany) Journal-ref: J. Phys.: Conf. Ser. 200, 012152 (2010) Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study the real-time evolution of a charge-excited state in a one-dimensional $e_{\rm g}$-orbital degenerate Hubbard model, by a time-dependent density-matrix renormalization group method. Considering a chain along the $z$ direction, electrons hop between adjacent $3z^2-r^2$ orbitals, while $x^2-y^2$ orbitals are localized. For the charge-excited state, a holon-doublon pair is introduced into the ground state at quarter filling. At initial time, there is no electron in a holon site, while a pair of electrons occupies $3z^2-r^2$ orbital in a doublon site. As the time evolves, the holon motion is governed by the nearest-neighbor hopping, but the electron pair can transfer between $3z^2-r^2$ orbital and $x^2-y^2$ orbital through the pair hopping in addition to the nearest-neighbor hopping. Thus holon and doublon propagate at different speed due to the pair hopping that is characteristic of multi-orbital systems.

Title: Multipole correlations of $t_{\rm 2g}$-orbital Hubbard model with spin-orbit coupling
Authors: [|Hiroaki Onishi] Comments: 3 pages, 3 figures, To appear in J. Phys. Soc. Jpn. Suppl., Proceedings of ICHE2010 (September 17-20, 2010, Hachioji, Japan) Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We investigate the ground-state properties of a one-dimensional $t_{\rm 2g}$-orbital Hubbard model including an atomic spin-orbit coupling by using numerical methods, such as Lanczos diagonalization and density-matrix renormalization group. As the spin-orbit coupling increases, we find a ground-state transition from a paramegnetic state to a ferromagnetic state. In the ferromagnetic state, since the spin-orbit coupling mixes spin and orbital states with complex number coefficients, an antiferro-orbital state with complex orbitals appears. According to the appearance of the complex orbital state, we observe an enhancement of $\Gamma_{4u}$ octupole correlations.

Title: Variational calculations for anisotropic solitons in dipolar Bose-Einstein condensates
Authors: [|Rüdiger Eichler], [|Jörg Main], [|Günter Wunner] Comments: 12 pages, 11 figures, submitted to Phys. Rev. A Subjects: Quantum Gases (cond-mat.quant-gas) ; Chaotic Dynamics (nlin.CD); Quantum Physics (quant-ph)

We present variational calculations using a Gaussian trial function to calculate the ground state of the Gross-Pitaevskii equation and to describe the dynamics of the quasi-two-dimensional solitons in dipolar Bose-Einstein condensates. Furthermore we extend the ansatz to a linear superposition of Gaussians improving the results for the ground state to exact agreement with numerical grid calculations using imaginary time and split-operator method. We are able to give boundaries for the scattering length at which stable solitons may be observed in an experiment. By dynamical calculations with coupled Gaussians we are able to describe the rather complex behavior of the thermally excited solitons. The discovery of dynamically stabilized solitons indicates the existence of such BECs at experimentally accessible temperatures.

Title: Evidence for a bicritical point in the XXZ Heisenberg antiferromagnet on a simple cubic lattice
Authors: [|Walter Selke] Comments: 4 pages, 6 figures Subjects: Statistical Mechanics (cond-mat.stat-mech)

The classical Heisenberg antiferromagnet with uniaxial exchange anisotropy, the XXZ model, in a magnetic field on a simple cubic lattice is studied with the help of extensive Monte Carlo simulations. Analyzing, especially, various staggered susceptibilities and Binder cumulants, we present clear evidence for the meeting point of the antiferromagnetic, spin--flop, and paramagnetic phases being a bicritical point with Heisenberg symmetry. Results are compared to previous predictions based on various theoretical approaches.

Title: Superconductivity in an extended Hubbard model with attractive interaction
Authors: [|E. J. Calegari], [|S. G. Magalhaes], [|C. M. Chaves], [|A. Troper] Comments: 13 pages, 5 figures Journal-ref: Supercond. Sci. Technol. 24 (2011) 035004 S ubjects: Strongly Correlated Electrons (cond-mat.str-el)

In this work, a two-dimensional one-band Hubbard model is investigated within a two-pole approximation. The model presents a non-local attractive potential $U (U<0)$ that allows the study of d-wave superconductivity and also includes hopping up to second-nearest-neighbors. The two-pole scheme has been proposed to improve the Hubbard-I approximation. The analytical results show a more complex form for the gap $\Delta(T)$, when compared to the one obtained in the latter approximation. Indeed, new anomalous correlation functions associated with the superconductivity are involved in the calculation of $\Delta(T)$. Numerical results in a range of temperatures are presented. Moreover, the structure of the quasiparticle bands and the topology of the Fermi surface are studied in detail in the normal state. Connections with some experimental results are also included.

Title: Spin imbalance effect on Larkin-Ovchinnikov-Fulde-Ferrel state
Authors: [|Ryosuke Yoshii], [|Shunji Tsuchiya], [|Giacomo Marmorini], [|Muneto Nitta] Comments: 5 pages, 2 figures Subjects: Superconductivity (cond-mat.supr-con) ; Quantum Gases (cond-mat.quant-gas)

We study spin imbalance effects on the Larkin-Ovchinikov-Fulde-Ferrel (LOFF) state relevant for superconductors under a strong magnetic field and spin polarized ultracold Fermi gas. We obtain the exact solution for the condensates with arbitrary spin imbalance and the fermion spectrum perturbatively in the presence of small spin imbalance. We also obtain fermion zero mode exactly without perturbation.

Title: Gauge theory of the Hubbard model on honeycomb lattice and its instanton effect
Authors: [|Abolhassan Vaezi], [|Xiao-Gang Wen] Comments: 9 pages, 4 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

In this paper we investigate possible spin disordered phase in the Hubbard model on the honeycomb lattice. Using a slave-particle theory that include the charge fluctuations, we find a meanfield spin disordered phase in a range of on-site repulsion $U$. The spin disordered state is described by gapped fermions coupled to compact U(1) gauge field. We study the confinement/deconfinement problem of the U(1) gauge theory due to the instantons proliferation. We calculate all allowed instanton terms and compute their quantum numbers. It is shown that the meanfield spin disordered phase is unstable. The instantons proliferation induce a translation symmetry breaking.

Title: Rough chains superfluidity in a polarized gas of dipolar molecules
Authors: [|Barbara Capogrosso-Sansone], [|Anatoly Kuklov] Subjects: Other Condensed Matter (cond-mat.other)

We study quantum chains in a gas of polar bosonic molecules confined in a stack of $N$ identical 1d (cigar type) and 2d (plane type) optical lattice layers and polarized perpendicularly to the layers. A single chain quantum roughening transition is observed in {\it ab initio} simulations. It is proven that no superfluid of chains with length shorter than $N$ is possible. Rough chain superfluid (RCSF) is found and analyzed within the J-current model approximation. A detection scheme is discussed. A possibility of RCSF in superstructures of indirect excitons is proposed.

===4. [|arXiv:1007.1172] (cross-list from quant-ph) [[|pdf], [|ps], [|other]] ===

Title: Entanglement from density measurements: analytical density-functional for the entanglement of strongly correlated fermions
Authors: [|Vivian V. França], [|Irene D'Amico] Comments: 6 pages, 3 figures Subjects: Quantum Physics (quant-ph) ; Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)

We derive an analytical density functional for the single-site entanglement of the one-dimensional homogeneous Hubbard model, by means of an approximation to the linear entropy. We show that this very simple density functional reproduces quantitatively the exact results. We then use this functional as input for a local density approximation to the single-site entanglement of inhomogeneous systems. We illustrate the power of this approach in a harmonically confined system, which could simulate recent experiments with ultracold atoms in optical lattices as well as in a superlattice and in an impurity system. The impressive quantitative agreement with numerical calculations -- which includes reproducing subtle signatures of the particle density stages -- shows that our density-functional can provide entanglement calculations for actual experiments via density measurements. Next we use our functional to calculate the entanglement in disordered systems. We find that, at contrast with the expectation that disorder destroys the entanglement, there exist regimes for which the entanglement remains almost unaffected by the presence of disordered impurities.

Title: Two-dimensional imaging of gauge fields in optical lattices
Authors: [|Jaeyoon Cho], [|M. S. Kim] Comments: 4 pages, 3 figures Subjects: Quantum Physics (quant-ph) ; Quantum Gases (cond-mat.quant-gas)

We propose a scheme to generate an arbitrary Abelian vector potential for atoms trapped in a two-dimensional optical lattice. By making the optical lattice potential dependent on the atomic state, we transform the problem into that of a two-dimensional imaging. It is shown that an arbitrarily fine pattern of the gauge field in the lattice can be realized without need of diffraction-limited imaging.

Title: Dissipative dynamics of a harmonically confined Bose-Einstein condensate
Authors: [|Z. Wu], [|E. Zaremba] Comments: 5 pages, 2 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We study the dissipation of the centre of mass oscillation of a harmonically confined condensate in the presence of a disorder potential. An extension of the Generalized Harmonic Theorem allows one to formulate the dynamics from the point of view of an oscillating disorder potential. This formulation leads to a rigorous result for the damping rate in the limit of weak disorder.

Title: Spin correlations and phase diagram of the perturbed Kitaev model
Authors: [|S. Mandal], [|Subhro Bhattacharjee], [|K.Sengupta], [|R. Shankar], [|G. Baskaran] Comments: 4 pgs., 3 figs, v1 Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We present a general classification of the perturbations to the Kitaev model on the basis of their effect on it's spin correlation functions. We derive a necessary and sufficient condition for the spin correlators to exhibit a long ranged power-law behavior in the presence of such perturbations. We substantiate our result by a study of the phase diagram of the Kitaev model augmented by a loop term and perturbed by an Ising term, within a RVB mean-field theory. We estimate the stability of the spin-liquid phase against such perturbations and show that this model exhibits both confinement-deconfinement transitions from spin liquid to antiferromagnetic/spin-chain/ferromagnetic phases as well as topological quantum phase transitions between gapped and gapless spin liquid phases.

=Jan 3-7, Chungwei Lin=

Jan 7
1. [|arXiv:1101.1134] [[|pdf], [|ps], [|other]] Authors: [|Hui Hu], [|Xia-Ji Liu] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) Universality of strongly interacting fermions is a topic of great interest in diverse fields. Here we investigate theoretically the universal dynamic density response of resonantly interacting ultracold Fermi atoms in the limit of either high temperature or large frequency: (1) At high temperature, we use quantum virial expansion to derive universal, non-perturbative expansion functions of dynamic structure factor; (2) At large momentum, we identify that the second-order expansion function gives the Wilson coefficient used in the operator expansion product method. The dynamic structure factor is therefore determined by its second-order expansion function with an overall normalization factor given by Tan's contact parameter. We show that the spin parallel and antiparallel dynamic structure factors have respectively a tail of the form $\sim \pm \omega^{-5/2}$ for $\omega\rightarrow\infty$, decaying slower than the total dynamic structure factor found previously ($\sim \omega^{-7/2}$). Our predictions for dynamic structure factor at high temperature or large frequency are testable using Bragg spectroscopy for ultracold atomic Fermi gases.
 * Title: Universal dynamic structure factor of a strongly correlated Fermi gas**

2. [|arXiv:1101.1278] [[|pdf], [|ps], [|other]] Authors: [|M. Asad-uz-Zaman], [|D. Blume] Comments: 9 pages, 10 figures Subjects: Quantum Gases (cond-mat.quant-gas) We study the behavior of two coupled purely dipolar Bose-Einstein condensates, each located in a cylindrically symmetric pancake-shaped external confining potential, as the separation b between the traps along the tight confining direction is varied. The solutions of the coupled Gross-Pitaevskii and Bogoliubov-de Gennes equations, which account for the full dynamics, show that the system behavior is modified by the presence of the second dipolar BEC. For sufficiently small b, the presence of the second dipolar BEC destabilizes the system dramatically. In this regime, the coupled system collapses through a mode that is notably different from the radial roton mode that induces the collapse of the uncoupled system. Finally, we comment on the shortcomings of an approach that neglects the dynamics in the z-direction, which is assumed to be a good approximation for highly pancake-shaped dipolar BECs in the literature.
 * Title: Modification of roton instability due to the presence of a second dipolar Bose-Einstein condensate**

3. [|arXiv:1101.1236] [[|pdf], [|other]] Authors: [|Andreas Ruegg], [|Gregory A. Fiete] Comments: 12 pages, 12 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) We study a two-dimensional system of spin-polarized fermions on the kagome lattice at filling fraction f=1/3 interacting through a nearest-neighbor interaction V. Above a critical interaction strength V_c a charge-density wave with a broken Z_3 symmetry is stabilized. Using the unrestricted mean-field approximation, we present several arguments showing that elementary topological point defects in the order parameter bind a fractional charge. Our analysis makes use of two appealing properties of the model: (i) For weak interaction, the low-energy degrees of freedom are described by Dirac fermions coupled to a complex-valued mass field (order parameter). (ii) The nearest-neighbor interaction is geometrically frustrated at filling f=1/3. Both properties offer a route to fractionalization and yield a consistent value 1/2 for the fractional charge as long as the symmetry between the up and the down triangles of the kagome lattice is preserved. If this symmetry is violated, the value of the bound charge varies continuously with the strength of the symmetry-breaking term in the model. In addition, we have numerically computed the confining potential between two fractionally charged defects. We find that it grows linearly at large distances but can show a minimum at a finite separation for intermediate interactions. This indicates that the polaron state, formed upon doping the charge-density wave, can be viewed as a bound state of two defects.
 * Title: Fractionally charged topological point defects on the kagome lattice**

Jan 6
1. [|arXiv:1101.0913] [[|pdf], [|ps], [|other]] Authors: [|Lars Bonnes], [|Stefan Wessel] Comments: 8 pages, 11 figures Subjects: Quantum Gases (cond-mat.quant-gas) We explore the phase diagram of ultracold bosonic polar molecules confined to a planar optical lattice of triangular geometry. External static electric and microwave fields can be employed to tune the effective interactions between the polar molecules into a regime of extended two- and three-body repulsions of comparable strength, leading to a rich quantum phase diagram. An extended supersolid phase is found to persist deep into the three-body dominated regime. Addressing the critical properties of the supersolid-superfluid transition where the three-fold lattice symmetry is enhanced, we verify that the transition belongs to the three-dimensional O(2) universality class. While three-body interactions break particle-hole symmetry explicitly, a characteristic supersolid-supersolid quantum phase transition is observed, which indicates the restoration of particle-hole symmetry at half-filling.
 * Title: Supersolid Polar Molecules beyond Pairwise Interactions**

2. [|arXiv:1101.1006] [[|pdf], [|other]] Authors: [|Austen Lamacraft] Comments: 16 pages and as many figures Subjects: Quantum Gases (cond-mat.quant-gas) We study a spin 1 Bose condensate small enough to be treated as a single magnetic `domain': a system that we term a microcondensate. Because all particles occupy a single spatial mode, this quantum many body system has a well defined classical limit consisting of three degrees of freedom, corresponding to the three macroscopically occupied spin states. We study both the classical limit and its quantization, finding an integrable system in both cases. Depending on the sign of the ratio of the spin interaction energy and the quadratic Zeeman energy, the classical limit displays either a separartrix in phase space, or Hamiltonian monodromy corresponding to non-trivial phase space topology. We discuss the quantum signatures of these classical phenomena using semiclassical quantization as well as an exact solution using the Bethe ansatz.
 * Title: Spin 1 microcondensate in a magnetic field: semiclassics and exact solution**

3. [|arXiv:1101.0911] [[|pdf], [|other]] Authors: [|Shun-Li Yu], [|X.C. Xie], [|Jian-Xin Li] Comments: 5 pages, 4 figures. Due to the limit of file size, a clear Fig.4 is available on request Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall) We investigate the interplay between the strong correlation and the spin-orbital coupling in the Kane-Mele-Hubbard model and obtain the qualitative phase diagram via the variational cluster approach. We identify, through an increase of the Hubbard $U$, the transition from the topological band insulator to either the spin liquid phase or the easy-plane antiferromagnetic insulating phase, depending on the strength of the spin-orbit coupling. A nontrivial evolution of the bulk bands in the topological quantum phase transition is also demonstrated.
 * Title: Mott Physics and Topological Phase Transition in Correlated Dirac Fermions**

Jan 5
1. [|arXiv:1101.0728] [[|pdf], [|ps], [|other]] Title: Solitons as the early stage of quasicondensate formation during evaporative cooling Authors: [|E. Witkowska], [|P. Deuar], [|M. Gajda], [|K. Rz\każewski] Subjects: Quantum Gases (cond-mat.quant-gas) We calculate the evaporative cooling dynamics of trapped one-dimensional Bose-Einstein condensates for parameters leading to a range of condensates and quasicondensates in the final equilibrium state. We confirm that solitons are created during the evaporation process, but always eventually dissipate during thermalisation. The distance between solitons at the end of the evaporation ramp matches the coherence length in the final thermal state. Calculations were made using the classical fields method. They bridge the gap between the phase defect picture of the Kibble-Zurek mechanism and the long-wavelength phase fluctuations in the thermal state.

2. [|arXiv:1101.0780] [[|pdf], [|other]] Title: Universal Spin Transport in a Strongly Interacting Fermi Gas Authors: [|Ariel Sommer], [|Mark Ku], [|Giacomo Roati], [|Martin W. Zwierlein] Comments: 10 pages, 6 figures Subjects: Quantum Gases (cond-mat.quant-gas) Transport of fermions is central in many fields of physics. Electron transport runs modern technology, defining states of matter such as superconductors and insulators, and electron spin, rather than charge, is being explored as a new carrier of information [1]. Neutrino transport energizes supernova explosions following the collapse of a dying star [2], and hydrodynamic transport of the quark-gluon plasma governed the expansion of the early Universe [3]. However, our understanding of non-equilibrium dynamics in such strongly interacting fermionic matter is still limited. Ultracold gases of fermionic atoms realize a pristine model for such systems and can be studied in real time with the precision of atomic physics [4, 5]. It has been established that even above the superfluid transition such gases flow as an almost perfect fluid with very low viscosity [3, 6] when interactions are tuned to a scattering resonance. However, here we show that spin currents, as opposed to mass currents, are maximally damped, and that interactions can be strong enough to reverse spin currents, with opposite spin components reflecting off each other. We determine the spin drag coeffcient, the spin diffusivity, and the spin susceptibility, as a function of temperature on resonance and show that they obey universal laws at high temperatures. At low temperatures, the spin diffusivity approaches a minimum value set by the ratio of the reduced Planck's constant to the atomic mass. For repulsive interactions, our measurements appear to exclude a metastable ferromagnetic state [7-9].

Jan 4
1. [|arXiv:1101.0103] [[|pdf], [|ps], [|other]] Authors: [|G.E. Astrakharchik], [|M. D. Girardeau] Comments: 4 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el) The ground state properties of a single-component one-dimensional Coulomb gas are investigated. We use Bose-Fermi mapping for the ground state wave function which permits to solve the Fermi sign problem in the following respects (i) the nodal surface is known, permitting exact calculations (ii) evaluation of determinants is avoided, reducing the numerical complexity to that of a bosonic system, thus allowing simulation of a large number of fermions. Due to the mapping the energy and local properties in one-dimensional Coulomb systems are exactly the same for Bose-Einstein and Fermi-Dirac statistics. The exact ground state energy has been calculated in homogeneous and trapped geometries by using the diffusion Monte Carlo method. We show that in the low-density Wigner crystal limit an elementary low-lying excitation is a plasmon, which is to be contrasted with the large-density ideal Fermi gas/Tonks-Girardeau limit, where low lying excitations are phonons. Exact density profiles are confronted to the ones calculated within the local density approximation which predicts a change from a semicircular to inverted parabolic shape of the density profile as the value of the charge is increased.
 * Title: Exact ground state properties of the one-dimensional Coulomb gas**

2. [|arXiv:1101.0284] [[|pdf], [|ps], [|other]] Authors: [|Xibo Zhang], [|Chen-Lung Hung], [|Shih-Kuang Tung], [|Nathan Gemelke], [|Cheng Chin] Comments: 15 pages, 6 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Other Condensed Matter (cond-mat.other); Statistical Mechanics (cond-mat.stat-mech) Critical behavior developed near a quantum phase transition, interesting in its own right, offers exciting opportunities to explore the universality of strongly-correlated systems near the ground state. Cold atoms in optical lattices, in particular, represent a paradigmatic system, for which the quantum phase transition between the superfluid and Mott insulator states can be externally induced by tuning the microscopic parameters. In this paper, we describe our approach to study quantum criticality of cesium atoms in a two-dimensional lattice based on in situ density measurements. Our research agenda involves testing critical scaling of thermodynamic observables and extracting transport properties in the quantum critical regime. We present and discuss experimental progress on both fronts. In particular, the thermodynamic measurement suggests that the equation of state near the critical point follows the predicted scaling law at low temperatures.
 * Title: Exploring quantum criticality based on ultracold atoms in optical lattices**

3. [|arXiv:1101.0094] [[|pdf], [|ps], [|other]] Authors: [|Suhas Gangadharaiah], [|Bernd Braunecker], [|Pascal Simon], [|Daniel Loss] Comments: 4.2 page, 1 figure Subjects: Strongly Correlated Electrons (cond-mat.str-el) We show that one-dimensional electron systems in proximity of a superconductor that support Majorana edge states are extremely susceptible to electron-electron interactions. Strong interactions generically destroy the induced superconducting gap that stabilizes the Majorana edge states. For weak interactions, the renormalization of the gap is nonuniversal and allows for a regime, in which the Majorana edge states persist. We present strategies how this regime can be reached.
 * Title: Majorana edge states in interacting one-dimensional systems**

4. [|arXiv:1101.0314] [[|pdf], [|ps], [|other]] Authors: [|Yuanpei Lan], [|Shaolong Wan], [|Shou-Cheng Zhang] Comments: 3 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) The topological magnetoelectric effect (TME) is the fundamental quantization effect for topological insulators in units of the fine structure constant $\alpha$. In [Phys. Rev. Lett. 105, 166803(2010)], a topological quantization condition of the TME is given under orthogonal incidence of the optical beam, in which the wave length of the light or the thickness of the TI film must be tuned to some commensurate values. This fine tuning is difficult to realize experimentally. In this article, we give manifestly $SL(2,\mathbb{Z})$ covariant expressions for Kerr and Faraday angles at oblique incidence at a topological insulator thick film. We obtain a generalized quantization condition independent of material details, and propose a more easily realizable optical experiment, in which only the incidence angle is tuned, to directly measure the topological quantization associated with the TME.
 * Title: Generalized quantization condition in topological insulator**

5. [|arXiv:1101.0320] [[|pdf], [|ps], [|other]] Authors: [|Lukasz Fidkowski], [|T. S. Jackson], [|Israel Klich] Comments: 4 pages, 2 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Quantum Physics (quant-ph) We characterize gapless edge modes in arbitrary topological insulators. We show that the edge mode spectrum is a continuous deformation of the spectrum of a certain gluing function defining the occupied state bundle over the Brillouin zone (BZ). Topologically non-trivial gluing functions, corresponding to non-trivial bundles, then yield edge modes exhibiting spectral flow. We illustrate our results for the case of chiral edge states in two dimensional Chern insulators, as well as helical edges in quantum spin Hall states. Our central technical tool is the entanglement spectrum (ES), which has a dual nature as both an edge spectrum and a bulk quantity.
 * Title: From bulk invariants to edge modes via the entanglement spectrum**

Jan 3
N/A