November2010

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=Nov 29 - Dec 3, Zixu Zhang=

Title: Quench dynamics of the 2d XY model
Authors: [|Asja Jelic], [|Leticia F. Cugliandolo] Comments: 28 pages, 14 figures Subjects: Statistical Mechanics (cond-mat.stat-mech)

We investigate the out of equilibrium dynamics of the two-dimensional XY model when cooled across the Berezinskii-Kosterlitz-Thouless (BKT) phase transition using different protocols. We focus on the evolution of the growing correlation length and the density of topological defects (vortices). By using Monte Carlo simulations we first determine the time and temperature dependence of the growing correlation length after an infinitely rapid quench from above the transition temperature to the quasi-long range order region. The functional form is consistent with a logarithmic correction to the diffusive law and it serves to validate dynamic scaling in this problem. This analysis clarifies the different dynamic roles played by bound and free vortices. We then revisit the Kibble-Zurek mechanism in thermal phase transitions in which the disordered state is plagued with topological defects. We provide a theory of quenching rate dependence in systems with the BKT-type transition that goes beyond the equilibrium scaling arguments. Finally, we discuss the implications of our results to a host of physical systems with vortex excitations including planar ferromagnets and liquid crystals as well as the Ginzburg-Landau approach to bidimensional freely decaying turbulence.

Title: Fractionalization via $\mathbb{Z}_{2}$ Gauge Fields at a Cold Atom Quantum Hall Transition
Authors: [|Yafis Barlas], [|Kun Yang] Comments: 4+epsilon pages Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study a single species of fermionic atoms in an "effective" magnetic field at total filling factor $\nu_{f}=1$, interacting through a p-wave Feshbach resonance, and show that the system undergoes a quantum phase transition from a $\nu_{f} =1 $ fermionic integer quantum Hall state to $\nu_{b} =1/4 $ bosonic fractional quantum Hall state as a function of detuning. The transition is in the $(2+1)$-D Ising universality class. We formulate a dual theory in terms of quasiparticles interacting with a $\mathbb{Z}_{2}$ gauge field, and show that charge fractionalization follows from this topological quantum phase transition. Experimental consequences and possible tests of our theoretical predictions are discussed.

Title: Radiofrequency spectroscopy of a strongly interacting two-dimensional Fermi gas
Authors: [|Bernd Fröhlich], [|Michael Feld], [|Enrico Vogt], [|Marco Koschorreck], [|Wilhelm Zwerger], [|Michael Köhl] Comments: 4 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic and Molecular Clusters (physics.atm-clus); Quantum Physics (quant-ph)

We realize and study a strongly interacting two-component atomic Fermi gas confined to two dimensions in an optical lattice. Using radio-frequency spectroscopy we measure the interaction energy of the strongly interacting gas. We observe the confinement-induced Feshbach resonance and find the existence of confinement-induced molecules in very good agreement with theoretical predictions.

Title: Resistivity of a non-Galilean--invariant Fermi Liquid near Pomeranchuk Quantum Criticality
Authors: [|Dmitrii L. Maslov], [|Vladimir I. Yudson], [|Andrey V. Chubukov] Comments: 4 pages, 2 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We analyze the effect of the electron-electron interaction on the resistivity of a metal near a Pomeranchuk quantum phase transition (QPT). We show that Umklapp processes are not effective near a QPT, and one must consider both interactions and disorder to obtain finite and T dependent resistivity. By power counting, the correction to the residual resistivity at low T scales as AT^{(D+2)/3} near a Z=3 QPT. We show, however, that A=0 for a simply connected, convex Fermi surface in 2D, due to hidden integrability of the electron motion. We argue that A >0 in a two-band (s-d) model and propose this model as an explanation for the observed T^{(D+2)/3} behavior.

Title: Strongly-correlated superfluid near the Mott transition
Authors: [|A. Rancon], [|N. Dupuis] Comments: 4.1 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We study the Bose-Hubbard model using the non-perturbative renormalization group. We obtain a phase diagram in very good quantitative agreement with the quantum Monte Carlo result and recover the existence of two universality classes at the superfluid--Mott-insulator transition. Furthermore, we identify a sharp cross-over between a weakly- and a strongly-correlated superfluid phase. The latter is characterized by a strong reduction of the superfluid density and superfluid stiffness. The two fundamental length scales of the superfluid, the healing length $\xi_h$ and the Ginzburg length $\xi_G$ (the length scale beyond which the Bogoliubov theory breaks down), satisfy $\xi_G\gg\xi_h$ in the weakly-correlated superfluid phase but become of the same order in the strongly-correlated superfluid phase. We also briefly discuss the single-particle spectral function.

Title: The thermodynamic origin of the Contact and its relation to the gap in the BEC-BCS crossover
Authors: [|Victor Romero-Rochin] Comments: 19 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas)

As can be inferred from present experiments in ultracold gases, the scattering length is a quantity that determines the thermodynamic state of the gas. As such, there exists a conjugate thermodynamic to it. Here, we show that the recently introduced "contact" is the conjugate of the inverse of the scattering length. We find that this identification allows for a derivation of essentially all the known results regarding the contact. Using the mean-field theory for the Bose-Einstein (BEC) to Bardeen-Cooper-Schriefer (BCS) crossover, we also find that the contact is proportional to the square of the gap. We analyze in detail both a homogenous balanced mixture of fermions and its inhomogenous counterpart in a harmonic trap.

Title: The landscape of the Hubbard model
Authors: [|Subir Sachdev] Comments: 42 pages, 10 figures; Preliminary draft of TASI 2010 lectures; questions and feedback are welcome Subjects: High Energy Physics - Theory (hep-th) ; Strongly Correlated Electrons (cond-mat.str-el)

I present a pedagogical survey of a variety of quantum phases of the Hubbard model. The honeycomb lattice model has a conformal field theory connecting the semi-metal to the insulator with Neel order. States with fractionalized excitations are linked to the deconfined phases of gauge theories. I also consider the confining phases of such gauge theories, and show how Berry phases of monopoles induce valence bond solid order. Compressible metallic phases with Fermi surfaces are described, including the 'fractionalized Fermi liquid', using the Hubbard model on a bilayer triangular lattice. I make numerous connections to the AdS/CFT correspondence, reviewing insights gained and discussing open problems that the correspondence could address.

Title: Topological superconductivity and Majorana fermions in half-metal / superconductor heterostructure
Authors: [|Suk Bum Chung], [|Hai-Jun Zhang], [|Xiao-Liang Qi], [|Shou-Cheng Zhang] Comments: 7 pages, 4 figures Subjects: Superconductivity (cond-mat.supr-con)

A half-metal is by definition spin-polarized at its Fermi level and therefore was conventionally thought to have little proximity effect with an $s$-wave superconductor. Here we show that if there is spin-orbit coupling at the interface between a single-band half-metal and an $s$-wave superconductor, $p_x +ip_y$ superconductivity would be induced on the half-metal. This can give us a topological superconductor with a single chiral Majorana edge state. We show that two atomic layers of CrO$_2$ or CrTe give us the single-band half-metal and is thus a candidate material for realizing this physics.

Title: Emergence of a pair-coherent phase in many-body quenches of bosons
Authors: [|Uwe R. Fischer], [|Kang-Soo Lee], [|Bo Xiong] Comments: 4 pages of RevTex4-1, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph)

We investigate the dynamical level population statistics and associated first- and second-order coherence of an interacting bosonic two-mode model when the pair-exchange coupling is quenched from negative to positive values. In an adiabatic transition, this corresponds to going from coherent to fragmented phases. It is shown that for moderately rapid second-order transitions, a new pair-coherent phase emerges, which has neither conventional single-particle coherence nor fragmentation.

T itle: Bose-Einstein condensation with a finite number of particles in a power-law trap
Authors: [|Amine Jaouadi] (ISMO, LSAMA), [|Mourad Telmini] (LSAMA, CNSTN), [|Eric Charron] (ISMO) S ubjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph)

Bose-Einstein condensation (BEC) of an ideal gas is investigated, beyond the thermodynamic limit, for a finite number $N$ of particles trapped in a generic three-dimensional power-law potential. We derive an analytical expression for the condensation temperature $T_c$ in terms of a power series in $x_0=\epsilon_0/k_BT_c$, where $\epsilon_0$ denotes the zero-point energy of the trapping potential. This expression, which applies in cartesian, cylindrical and spherical power-law traps, is given analytically at infinite order. It is also given numerically for specific potential shapes as an expansion in powers of $x_0$ up to the second order. We show that, for a harmonic trap, the well known first order shift of the critical temperature $\Delta T_c/T_c \propto N^{-1/3}$ is inaccurate when $N \leqslant 10^{5}$, the next order (proportional to $N^{-1/2}$) being significant. We also show that finite size effects on the condensation temperature cancel out in a cubic trapping potential, \textit{e.g.} $V(\mathbi{r}) \propto r^3$. Finally, we show that in a generic power-law potential of higher order, \textit{e.g.} $V(\mathbi{r}) \propto r^\alpha$ with $\alpha > 3$, the shift of the critical temperature becomes positive. This effect provides a large increase of $T_c$ for relatively small atom numbers. For instance, an increase of about +40% is expected with $10^4$ atoms in a $V(\mathbi{r}) \propto r^{12}$ trapping potential.

Title: Crossover from adiabatic to sudden interaction quench in a Luttinger liquid
Authors: [|Balázs Dóra], [|Masud Haque], [|Gergely Zaránd] Comments: 6 pages, 2 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)

Motivated by recent experiments on interacting cold atoms, we analyze interaction quenches in Luttinger liquids (LL), where the interaction is ramped from zero to a finite value within a finite time \tau. The adiabatic ground state energy is approached with increasing quenching rate as ~\ln(\tau)/\tau^2. The fermionic single particle density matrix reveals several regions of spatial and temporal coordinates relative to the quench time, termed as Fermi liquid, sudden quench, and adiabatic LL regimes, and a LL regime with time dependent exponent. The various regimes can also be observed in the momentum distribution of the fermions, directly accessible through time of flight experiments. Most of our results apply to arbitrary quench protocols.

Title: Generation and Dynamics of Quantized Vortices in a Unitary Fermi Superfluid
Authors: [|Aurel Bulgac], [|Yuan-Lung] (Alan)Luo, [|Piotr Magierski], [|Kenneth J. Roche], [|Yongle Yu] Comments: 5 pages main text + 20 pages supplemental material. 32 accompanying videos can be viewed and downloaded from [|this http URL] (Please follow instructions in supplemental material.) Subjects: Quantum Gases (cond-mat.quant-gas) ; Nuclear Theory (nucl-th)

Superfluidity and superconductivity are remarkable manifestations of quantum coherence at a macroscopic scale. The dynamics of superfluids has dominated the study of these systems for decades now, but a comprehensive theoretical framework is still lacking. We introduce a local extension of the time-dependent density functional theory to describe the dynamics of fermionic superfluids. Within this approach one can correctly represent vortex quantization, generation, and dynamics, the transition from a superfluid to a normal phase and a number of other large amplitude collective modes which are beyond the scope of two-fluid hydrodynamics, Ginzburg-Landau and/or Gross-Pitaevskii approaches. We illustrate the power of this approach by studying the generation of quantized vortices, vortex rings, vortex reconnection, and transition from a superfluid to a normal state in real time for a unitary Fermi gas. We predict the emergence of a new qualitative phenomenon in superfluid dynamics of gases, the existence of stable superfluidity when the systems are stirred with velocities significantly exceeding the nominal Landau critical velocity in these systems.

Title: Spin Liquid Ground State of the $S=1/2$ Kagome Heisenberg Model
Authors: [|Simeng Yan], [|David A. Huse], [|Steven R. White] Comments: 8 pages, 7 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Quantum Physics (quant-ph)

Condensed matter physicists have long sought a realistic two-dimensional (2D) magnetic system whose ground state is a {\it spin liquid}---a zero temperature state in which quantum fluctuations have melted away any form of magnetic order. The nearest-neighbor $S=1/2$ Heisenberg model on the kagome lattice has seemed an ideal candidate, but in recent years some approximate numerical approaches to it have yielded instead a valence bond crystal. We have used the density matrix renormalization group to perform very accurate simulations on numerous cylinders with circumferences up to 12 lattice spacings, finding instead of the valence bond crystal a singlet-gapped spin liquid with substantially lower energy that appears to have $Z_2$ topological order. Our results, through a combination of very low energy, short correlation lengths and corresponding small finite size effects, a new rigorous energy bound, and consistent behavior on many cylinders, provide strong evidence that the 2D ground state of this model is a gapped spin liquid.

Title: On the problem of van der Waals forces in dielectric media
Authors: [|Lev P. Pitaevskii] Comments: 10 pages, 2 figures. To be published in "Casimir Physics", Lecture Notes in Physics Series, Springer Subjects: Statistical Mechanics (cond-mat.stat-mech)

A short review of the problems which arise in the generalization of the Lifshitz theory of van der Waals force in the case of forces inside dielectric media is presented, together with some historical remarks. General properties of the stress tensor of equilibrium electromagnetic field in media are discussed, and the importance of the conditions of mechanical equilibrium is stressed. The physical meaning of the repulsive van der Waals interaction between bodies immersed in a liquid is discussed.

Title: Design of a technique to measure the density of ultracold atoms in a short-period optical lattice in three dimensions with single atom sensitivity
Authors: [|Martin Shotter] Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph)

A measurement technique is described which has the potential to map the atomic site occupancies of ultracold atoms in a short-period three-dimensional optical lattice. The method uses accordion and pinning lattices, together with polarization gradient cooling and fluorescence detection, to measure the positions of individual atoms within the sample in three dimensions at a resolution of around half the atomic resonant wavelength. The method measures the site occupancy, rather than the parity of the site occupancy, of atoms in the lattice. It is expected that such measurements hold significant potential for the study of ultracold quantum dynamics.

Title: Diagrammatic Pairing Fluctuations Approach to the BCS-BEC Crossover
Authors: [|G. C. Strinati] Comments: 32 pages, 13 fugures, chapter in "BCS-BEC Crossover and the Unitary Fermi Gas" (Lecture Notes in Physics), edited by Wilhelm Zwerger (Springer, 2011) Subjects: Quantum Gases (cond-mat.quant-gas)

This paper gives a survey of a diagrammatic approach for fermionic pairing fluctuations, which are relevant to the BCS-BEC crossover realized with ultracold Fermi gases. Emphasis will be given to the physical intuition about the relevant physical processes that can be associated with this approach. Specific results will be presented for thermodynamic and dynamical quantities, where a critical comparison with alternative diagrammatic approaches will also be attempted.

Title: Excitation Spectra of Bosons in Optical Lattices from Schwinger-Keldysh Calculation
Authors: [|T. D. Grass], [|F. E. A. dos Santos], [|A. Pelster] Subjects: Quantum Gases (cond-mat.quant-gas)

Within the Schwinger-Keldysh formalism we derive a Ginzburg-Landau theory for the Bose-Hubbard model which describes the real-time dynamics of the complex order parameter field. Analyzing the excitations in the vicinity of the quantum phase transition it turns out that particle/hole dispersions in the Mott phase map continuously onto corresponding amplitude/phase excitations in the superfluid phase, which have been detected recently by Bragg spectroscopy measurements.

Title: Particle-hole symmetry and interaction effects in the Kane-Mele-Hubbard model
Authors: [|Dong Zheng], [|Congjun Wu], [|Guang-Ming Zhang] Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We find that the Kane-Mele-Hubbard model with purely imaginary next-nearest-neighbor hoppings has a particle-hole symmetry at half-filling. Such a symmetry has interesting consequences including the absence of charge and spin currents along open edges, and the absence of the sign problem in the determinant quantum Monte-Carlo simulations. Consequentially, the interplay between band topology and strong correlations can be studied at high numeric precisions. The process that the topological band insulator evolves into the antiferromagnetic Mott insulator as increasing interaction strength is studied by calculating both the bulk and edge electronic properties.

=Nov 22 - Nov 26, Chungwei Lin=

Nov 26
Thanksgiving, no new papers.

Nov 25
1. [|arXiv:1011.5336] [[|pdf], [|ps], [|other]] Authors: [|O.H.T. Nummi], [|J.J. Kinnunen], [|P. Törmä] Comments: 14 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con) We study a three-component superfluid Fermi gas in a spherically symmetric harmonic trap using the Bogoliubov-deGennes method. We predict a coexistence phase in which two pairing field order parameters are simultaneously nonzero, in stark contrast to studies performed for trapped gases using local density approximation. We also discuss the role of atom number conservation in the context of a homogeneous system.
 * Title: Coexistence of pairing gaps in three-component Fermi gases**

2. [|arXiv:1011.5337] [[|pdf], [|ps], [|other]] Authors: [|Renyuan Liao], [|Joachim Brand] Comments: 4 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) Families of dark solitons exist in superfluid Fermi gases. The energy-velocity dispersion and number of depleted particles completely determines the dynamics of dark solitons on a slowly-varying background density. For the unitary Fermi gas we determine these relations from general scaling arguments and conservation of local particle number. We find solitons to oscillate sinusoidally at the trap frequency reduced by a factor of $1/\sqrt{3}$. Numerical integration of the time-dependent Bogoliubov-de Gennes equation determines spatial profiles and soliton dispersion relations across the BEC-BCS crossover and proves consistent with the scaling relations at unitarity.
 * Title: Traveling Dark Solitons in Superfluid Fermi Gases**

3. [|arXiv:1011.5387] [[|pdf], [|ps], [|other]] Authors: [|Xiaoming Cai], [|Shu Chen], [|Yupeng Wang] Comments: 4 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) Using the exact Bose-Fermi mapping, we study universal properties of ground-state density distributions and finite-temperature quantum critical behavior of one-dimensional hard-core bosons in trapped incommensurate optical lattices. Through the analysis of universal scaling relations in the quantum critical regime, we demonstrate that the superfluid to Bose glass transition and the general phase diagram of disordered hard-core bosons can be uniquely determined from finite-temperature density distributions of the trapped disordered system.
 * Title: Quantum criticality in disordered bosonic optical lattices**

4. [|arXiv:1007.5177] (cross-list from cond-mat.other) [[|pdf], [|ps], [|other]] Authors: [|Mateusz Łącki], [|Simone Paganelli], [|Veronica Ahufinger], [|Anna Sanpera], [|Jakub Zakrzewski] Subjects: Other Condensed Matter (cond-mat.other) ; Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph) We study the zero temperature phase diagram of the disordered spin-1 Bose-Hubbard model in a 2-dimensional square lattice. To this aim, we use a mean field Gutzwiller ansatz and a probabilistic mean field perturbation theory. The spin interaction induces two different regimes corresponding to a ferromagnetic and antiferromagnetic order. In the ferromagnetic case, the introduction of disorder reproduces analogous features of the disordered scalar Bose-Hubbard model, consisting in the formation of a Bose glass phase between Mott insulator lobes. In the antiferromagnetic regime the phase diagram differs more from the scalar case. Disorder in the chemical potential can lead to the disappearance of Mott insulator lobes with odd integer filling factor and, for sufficiently strong spin coupling, to Bose glass of singlets between even filling Mott insulator lobes. Disorder in the spinor coupling parameter results in the appearance of a Bose glass phase only between the n and n+1 lobes for n odd. Disorder in the scalar Hubbard interaction inhibits Mott insulator regions for occupation larger than a critical value.
 * Title: Disordered spinor Bose-Hubbard model**

5. [|arXiv:1011.5243] [[|pdf], [|other]] Authors: [|T. Pereg-Barnea], [|G. Refael] Comments: 5 pages; 2 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) We explore the possibility of inducing a topological insulator phase in a honeycomb lattice using a metallic gate. We start with a lattice model which does not include spin-orbit interaction. A metallic layer is added above the honeycomb sheet and density-density interaction without particle tunneling between the two systems is allowed. After integrating out the metal an interacting graphene sheet with long range interaction remains. The interaction may induce various ordered states, among them a topological insulator with complex next-nearest-neighbor hopping, as in the Haldane model. We find the ground state of the interacting system in a variational mean-field method and show that the Fermi wave vector, k_F of the metal determines which phase occurs. In analogy to the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, the metal's k_F determines the interaction profile as a function of the distance. Tuning it correctly may lead to an anomalous Hall state or an anomalous spin Hall state.
 * Title: Inducing topological order in a honeycomb lattice**

Nov 24
1. [|arXiv:1011.4967] [[|pdf], [|ps], [|other]] Authors: [|Chungwei Lin], [|Xiaopeng Li], [|W. Vincent Liu] Comments: 4+ pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el) We study Kosterlitz-Thouless (KT) transitions of the Larkin-Ovchinnikov (LO) phase for a two-dimensional system composed of coupled one-dimensional tubes of fermions. The LO phase here is characterized by a stripe structure (periodic in only one direction) in the order parameter. The low energy excitations involve the oscillation of the stripe and the fluctuation of the phase, which can be described by an effective theory composed of two anisotropic XY models. We compute from a microscopic model the coefficients of the XY models from which the KT transition temperatures are determined. We found the $T^{KT} \propto t_{\perp}$ for small intertube tunneling $t_{\perp}$. As $t_{\perp}$ increases the system undergoes a first-order transition to the normal phase at zero temperature. Our method can be used to determine the Goldstone excitations of any stripe order involving charge or spin degrees of freedom.
 * Title: U(1) $\times$ U(1) / Z$_2$ Kosterlitz-Thouless transition of the Larkin-Ovchinnikov phase in an anisotropic two-dimensional system**

2. [|arXiv:1011.5019] [[|pdf], [|ps], [|other]] Authors: [|Shu Chen], [|Junpeng Cao], [|Shi-Jian Gu] Comments: 5 pages, 3 figures Journal-ref: Phys. Rev. A 82, 053625 (2010) Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) We study the Bose-Fermi mixture with infinitely boson-boson repulsion and finite boson-Fermion repulsion. By using a generalized Jordan-Wigner transformation, we show that the system can be mapped to a repulsive Hubbard model and thus can be solved exactly for the case with equal boson and fermion masses. By using the Bethe-ansatz solutions, we investigate the ground state properties of the mixture system. Our results indicate that the system with commensurate filling $n=1$ is a charge insulator but still a superfluid with non-vanishing superfluid density. We also briefly discuss the case with unequal masses for bosons and fermions.
 * Title: Mixture of Tonks-Girardeau gas and Fermi gas in one-dimensional optical lattices**

3. [|arXiv:1011.5156] [[|pdf], [|ps], [|other]] Authors: [|Yurii Slyusarenko], [|Andrii Sotnikov] Comments: 6 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) We study theoretically some effects caused by a propagation of the charged particle in a dilute gas of alkali-metal atoms in the state with Bose-Einstein condensates. The energy change of the high-speed (relativistic) particle based on the Cherenkov effect in this system is investigated. We note that the particle in some cases not only emits the energy but also can be accelerated by the ultracold atomic gas. The necessary conditions for the acceleration of the particle by the condensate are found. We study also a possibility of defining the spectral characteristics of atoms by the detection of the Cherenkov radiation in the condensate.
 * Title: Propagation of the relativistic charged particle in an ultracold atomic gas with Bose-Einstein condensates**

4. [|arXiv:1011.5192] [[|pdf], [|ps], [|other]] Authors: [|A. Trenkwalder], [|C. Kohstall], [|M. Zaccanti], [|D. Naik], [|A. I. Sidorov], [|F. Schreck], [|R. Grimm] Subjects: Quantum Gases (cond-mat.quant-gas) We report on the expansion of a Fermi-Fermi mixture of Li-6 and K-40 atoms under conditions of strong interactions realized near the center of an interspecies Feshbach resonance. We observe two different phenomena of hydrodynamic behavior. The first one is the well-known inversion of the aspect ratio. The second one is a collective expansion, where both species stick together and despite of their different masses expand jointly. Our work constitutes a first step to explore the intriguing many-body physics of this novel system.
 * Title: Hydrodynamic Expansion of a Strongly Interacting Fermi-Fermi Mixture**

5. [|arXiv:1011.5207] [[|pdf], [|ps], [|other]] Authors: [|Xiong-Jun Liu], [|Yang Liu], [|Xin Liu] Comments: 4 pages, 4 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Quantum Physics (quant-ph) The strong correlation effects on topological insulator are studied in a two-sublattice system with an onsite single-particle energy difference $\Delta$ between two sublattices. At $\Delta=0$, increasing the onsite interaction strength $U$ drives the transition from the quantum spin Hall insulating state to the non-topological antiferromagnetic Mott-insulating (AFMI) state. When $\Delta$ is larger than a certain value, a topologically trivial band insulator or AFMI at small values of $U$ may change into a quantum anomalous Hall state with antiferromagnetic ordering at intermediate values of $U$. Further increasing $U$ drives the system back into the topologically trivial state of AFMI. The corresponding phenomena is observable in the solid state and cold atom systems. We also propose a scheme to realize and detect these effects in cold atom systems.
 * Title: Correlation effects on topological insulator**

Nov 23
1. [|arXiv:1011.4536] [[|pdf], [|ps], [|other]] Authors: [|Kelly R. Patton], [|Daniel E. Sheehy] (Louisiana State University) Comments: 5 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Superconductivity (cond-mat.supr-con) The induced interaction among the majority spin species, due to the presence of the minority species, is computed for the case of a population-imbalanced resonantly-interacting Fermi gas. It is shown that this interaction leads to an instability, at low temperatures, of the recently observed polaron Fermi liquid phase of strongly imbalanced Fermi gases to a p-wave superfluid state. We find that the associated transition temperature, while quite small in the weakly interacting BCS regime, is experimentally accessible in the strongly interacting unitary regime.
 * Title: Induced p-wave superfluidity in strongly interacting imbalanced Fermi gases**

2. [|arXiv:1011.4537] [[|pdf], [|ps], [|other]] Authors: [|E. Toth], [|P.B. Blakie] Comments: 5 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) Conventional wisdom is that increasing temperature causes quantum coherence to decrease. Using finite temperature perturbation theory and exact calculations for the strongly correlated bosonic Mott insulating state we show a practical counter-example that can be explored in optical lattice experiments: the short-range coherence of the Mott insulating phase can increase substantially with increasing temperature. We demonstrate that this phenomenon originates from thermally produced defects that can tunnel with ease. Since the near zero temperature coherence properties have been measured with high precision we expect these results to be verifiable in current experiments.
 * Title: Thermally induced coherence in a Mott insulator of bosonic atoms**

3. [|arXiv:1011.4893] [[|pdf], [|ps], [|other]] Authors: [|Luca Salasnich] Comments: 7 pages, 6 figures, accepted to publication in Phys. Rev. A Subjects: Quantum Gases (cond-mat.quant-gas) ; Nuclear Theory (nucl-th) We investigate the low-temperature thermodynamics of the unitary Fermi gas by introducing a model based on the zero-temperature spectra of both bosonic collective modes and fermonic single-particle excitations. We calculate the Helmholtz free energy and from it we obtain the entropy, the internal energy and the chemical potential as a function of the temperature. By using these quantities and the Landau's expression for the superfluid density we determine analytically the superfluid fraction, the critical temperature, the first sound velocity and the second sound velocity. We compare our analytical results with other theoretical predictions and experimental data of ultracold atoms and dilute neutron matter.
 * Title: Low-temperature thermodynamics of the unitary Fermi gas: superfluid fraction, first sound and second sound**

4. [|arXiv:1011.4563] (cross-list from cond-mat.stat-mech) [[|pdf], [|ps], [|other]] Authors: [|Federico Crecchi], [|Ettore Vicari] Comments: 4 pages Subjects: Statistical Mechanics (cond-mat.stat-mech) ; Quantum Gases (cond-mat.quant-gas) We investigate the effects of a trapping space-dependent potential on the low-temperature quasi-long-range order phase of two-dimensional particle systems with a relevant U(1) symmetry, such as quantum atomic gases. We characterize the universal features of the trap-size dependence using scaling arguments. The resulting scenario is supported by numerical Monte Carlo simulations of a classical two-dimensional XY model with a space-dependent hopping parameter whose inhomogeneity is analogous to that arising from the trapping potential in experiments of atomic gases.
 * Title: Quasi-long-range order in trapped systems**

5. [|arXiv:1011.4543] [[|pdf], [|ps], [|other]] Authors: [|Sumilan Banerjee], [|T V Ramakrishnan], [|C Dasgupta] Comments: 11 pages, 8 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) Over the years, Angle Resolved Photo Emission Spectroscopy (ARPES) has uncovered a number of unusual spectral properties of electrons with definite in-plane momenta near the Fermi energy in the hole doped cuprates. We describe here a minimal theory of tight binding electrons moving on the square planar Cu lattice of the cuprates, mixed quantum mechanically with pairs of them (Cooper pairs). Superconductivity occurring at the transition temperature $T_c$ is the long-range phase coherence of Cooper pairs with $d$-wave symmetry. Fluctuations necessarily associated with incipient long-range superconducting order have a generic large-distance behaviour near $T_c$. We calculate the spectral density of electrons coupled to such Cooper pair fluctuations and show that many features observed in ARPES experiments on different cuprates above $T_c$ as a function of doping and temperature emerge naturally in this description. These features include `Fermi arcs' with temperature-dependent length and an antinodal pseudogap which fills up linearly as the temperature increases towards the pseudogap temperature. Below $T_c$, the effects of nonzero superfluid density and thermal fluctuations are calculated and compared successfully with experiment.
 * Title: Pairing Fluctuations Determine Low Energy Electronic Spectra in Cuprate Superconductors**

Nov 22
1. [| arXiv:1011.4301] [[|pdf], [|other]] Authors: [|Kai Sun], [|W. Vincent Liu], [|S. Das Sarma] Comments: 6 pages 5 figures 9 pages of Supplementary Information Subjects: Quantum Gases (cond-mat.quant-gas) We demonstrate that a novel topological semimetal emerges as a parity-protected critical theory for fermionic atoms loaded in the $p$ and $d$ orbital bands of a two-dimensional optical lattice. The new quantum state is characterized by a parabolic band-degeneracy point with Berry flux $2\pi$, in sharp contrast to the $\pi$ flux of Dirac points as in graphene. We prove that this topological liquid is a universal property for all lattices of D$_4$ point group symmetry and the band degeneracy is protected by odd parity. Turning on inter-particle repulsive interaction, the system undergoes a phase transition to a topological insulator, whose experimental signature includes chiral gapless domain-wall modes, reminiscent of quantum Hall edge states.
 * Title: Topological semimetal: a probable new state of quantum optical lattice gases protected by D$_4$ symmetry**

2. [| arXiv:1011.4450] [[|pdf], [|ps], [|other]] Authors: [|R.G. Scott], [|F. Dalfovo], [|L.P. Pitaevskii], [|S. Stringari] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) We study soliton oscillations in a trapped superfluid Fermi gas across the Bose-Einstein condensate to Bardeen-Cooper-Schrieffer (BEC-BCS) crossover. We derive an exact equation relating the phase jump across the soliton to its energy, and hence obtain an expression for the soliton period. Our analytic approach is supported by simulations of the time-dependent Bogoliubov-de Gennes equations, which show that the period dramatically increases as the soliton becomes shallower on the BCS side of the resonance. Finally, we propose an experimental protocol to test our predictions.
 * Title: The dynamics of dark solitons in a trapped superfluid Fermi gas**

3. [| arXiv:1011.4503] [[|pdf], [|ps], [|other]] Authors: [|Seiji Sugawa], [|Kensuke Inaba], [|Shintaro Taie], [|Rekishu Yamazaki], [|Makoto Yamashita], [|Yoshiro Takahashi] Subjects: Quantum Gases (cond-mat.quant-gas) Many-body effects are at the very heart of diverse phenomena found in condensed-matter physics. One striking example is the Mott insulator phase where conductivity is suppressed as a result of a strong repulsive interaction. Advances in cold atom physics have led to the realization of the Mott insulating phases of atoms in an optical lattice, mimicking the corresponding condensed matter systems. Here, we explore an exotic strongly-correlated system of Interacting Dual Mott Insulators of bosons and fermions. We reveal that an inter-species interaction between bosons and fermions drastically modifies each Mott insulator, causing effects that include melting, generation of composite particles, an anti-correlated phase, and complete phase-separation. Comparisons between the experimental results and numerical simulations indicate intrinsic adiabatic heating and cooling for the attractively and repulsively interacting dual Mott Insulators, respectively.
 * Title: Interaction and filling induced quantum phases of dual Mott insulators of bosons and fermions**

=Nov 15 - Nov 19, Xiaopeng Li=

Nov 19
1. ** [|arXiv:1011.4164] [ [|pdf], [|ps] , [|other] ]** **Title: On a microcanonical relation between continuous and discrete spin models** Authors: Lapo Casetti, Cesare Nardini, Rachele Nerattini A relation between a class of stationary points of the energy landscape of continuous spin models on a lattice and the configurations of a Ising model defined on the same lattice suggests an approximate expression for the microcanonical density of states. Based on this approximation we conjecture that if a O(n) model with ferromagnetic interactions on a lattice has a phase transition, its critical energy density is equal to that of the n = 1 case, i.e., a system of Ising spins with the same interactions. The conjecture holds true in the case of long-range interactions. For nearest-neighbor interactions, numerical results are consistent with the conjecture for n=2 and n=3 in three dimensions. For n=2 in two dimensions (XY model) the conjecture yields a prediction for the critical energy of the Berezinskij-Kosterlitz-Thouless transition, which would be equal to that of the two-dimensional Ising model. We discuss available numerical data in this respect.

2. ** [|arXiv:1011.4173] [ [|pdf], [|ps] , [|other] ]** Authors: Oleg Kupervasser, Hrvoje Nikolic, Vinko Zlatić Statistical physics cannot explain why a thermodynamic arrow of time exists, unless one postulates very special and unnatural initial conditions. Yet, we argue that statistical physics can explain why the thermodynamic arrow of time is universal, i.e., why the arrow points in the same direction everywhere. Namely, if two subsystems have opposite arrow-directions initially, the interaction between them makes the configuration statistically unstable and causes a decay towards a system with a universal direction of the arrow of time. We present general qualitative arguments for that claim and support them by a detailed analysis of a toy model based on the baker's map.
 * Title: The Universal Arrow of Time**

**Nov 18 **
**Title: Ab initio modeling of Bose-Einstein condensation in Pb2V3O9** Authors: Alexander A. Tsirlin, Helge Rosner We apply density functional theory band structure calculations and quantum Monte Carlo simulations to investigate the Bose-Einstein condensation in the spin-1/2 quantum magnet Pb2V3O9. In contrast to previous conjectures on the one-dimensional nature of this compound, we present a quasi-two-dimensional model of spin dimers with ferromagnetic and antiferromagnetic interdimer couplings. Our model is well justified microscopically and provides a consistent description of the experimental data on the magnetic susceptibility, high-field magnetization, and field vs. temperature phase diagram. The Bose-Einstein condensation in the quasi-two-dimensional spin system of Pb2V3O9 is largely governed by intralayer interactions, whereas weak interlayer couplings have a moderate effect on the ordering temperature. The proposed computational approach is an efficient tool to analyze and predict high-field properties of quantum magnets.
 * 1. ** ** [|arXiv:1011.3981] [ [|pdf], [|ps] , [|other] ]**

**Nov 17 **
**Title: Nematic spin fluid in the tetragonal phase of BaFe2As2** Authors: Leland W. Harriger, Huiqian Luo, Mengshu Liu, T. G. Perring, C. Frost, Jiangping Hu, M. R. Norman, Pengcheng Dai Magnetic interactions are generally believed to play a key role in mediating electron pairing for superconductivity in iron arsenides; yet their character is only partially understood. Experimentally, the antiferromagnetic (AF) transition is always preceded by or coincident with a tetragonal to orthorhombic structural distortion. Although it has been suggested that this lattice distortion is driven by an electronic nematic phase, where a spontaneously generated electronic liquid crystal state breaks the C4 rotational symmetry of the paramagnetic state, experimental evidence for electronic anisotropy has been either in the low-temperature orthorhombic phase or the tetragonal phase under uniaxial pressure that breaks this symmetry. Here we use inelastic neutron scattering to demonstrate the presence of a large in-plane spin anisotropy above TN in the unstressed tetragonal phase of BaFe2As2. In the low-temperature orthorhombic phase, we find highly anisotropic spin waves with a large damping along the AF a-axis direction. On warming the system to the paramagnetic tetragonal phase, the low-energy spin waves evolve into quasi-elastic excitations, while the anisotropic spin excitations near the zone boundary persist. These results strongly suggest that the spin nematicity we find in the tetragonal phase of BaFe2As2 is the source of the electronic and orbital anisotropy observed above TN by other probes, and has profound consequences for the physics of these materials.
 * 1.**  ** [|arXiv:1011.3771] [ [|pdf] ]**

===1.  [|arXiv:1011.3207] [__[|pdf]__, [|other] ] === **Title: Nonequilibrium Phase Diagram of a Driven-Dissipative Many-Body System** Authors: Andrea Tomadin, Sebastian Diehl, Peter Zoller <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We study the nonequilibrium dynamics of a many-body bosonic system on a lattice, subject to driving and dissipation. The time-evolution is described by a master equation, which we treat within a generalized Gutzwiller mean field approximation for density matrices. The dissipative processes are engineered such that the system, in the absence of interaction between the bosons, is driven into a homogeneous steady state with off-diagonal long range order. We investigate how the coherent interaction affects qualitatively the properties of the steady state of the system and derive a nonequilibrium phase diagram featuring a phase transition into a steady state without long range order. The phase diagram exhibits also an extended domain where an instability of the homogeneous steady state gives rise to a persistent density pattern with spontaneously broken translational symmetry. In the limit of small particle density, we provide a precise analytical description of the time-evolution during the instability. Moreover, we investigate the transient following a quantum quench of the dissipative processes and we elucidate the prominent role played by collective topological variables in this regime.
 * <span style="font-size: 1.3em; margin: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 5px;">Nov 16 **

===<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: 21px;">2. <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;"> ** [|arXiv:1011.3259] [ [|pdf], [|ps] , [|other] ]** ===

<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;"> **Title: Exactly solvable models and ultracold Fermi gases**
<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;">Authors: Murray T. Batchelor, Angela Foerster, Xiwen Guan, Carlos C. N. Kuhn <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: 18px;">Exactly solvable models of ultracold Fermi gases are reviewed via their thermodynamic Bethe Ansatz solution. Analytical and numerical results are obtained for the thermodynamics and ground state properties of two- and three-component one-dimensional attractive fermions with population imbalance. New results for the universal finite temperature corrections are given for the two-component model. For the three-component model, numerical solution of the dressed energy equations confirm that the analytical expressions for the critical fields and the resulting phase diagrams at zero temperature are highly accurate in the strong coupling regime. The results provide a precise description of the quantum phases and universal thermodynamics which are applicable to experiments with cold fermionic atoms confined to one-dimensional tubes.

<span style="font-size: 1.3em; margin: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 5px;">**Nov 15** <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;">**1. [|arXiv:1011.3013] [ [|pdf], [|ps] , [|other] ]** <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;">**Title: Anisotropic superfluidity in a dipolar Bose gas** Authors: Christopher Ticknor, Ryan M. Wilson, John L. Bohn <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: 19px;">We study the superfluid character of a dipolar Bose-Einstein condensate (DBEC) in a quasi-two dimensional (q2D) geometry. In particular, we allow for the dipole polarization to have some non-zero projection into the plane of the condensate so that the effective interaction is anisotropic in this plane, yielding an anisotropic dispersion for propagation of quasiparticles. By performing direct numerical simulations of a probe moving through the DBEC, we observe the sudden onset of drag or creation of vortex-antivortex pairs at critical velocities that depend strongly on the direction of the probe's motion. This anisotropy emerges because of the anisotropic manifestation of a roton-like mode in the system.

=Nov 8 - Nov 12, Zixu Zhang=

Title: The Prediction of a Gapless Topological "Haldane Liquid" Phase in a One-Dimensional Cold Polar Molecular Lattice
Authors: [|J. P. Kestner], [|Bin Wang], [|Jay D. Sau], [|S. Das Sarma] Comments: 4 pages + 5 pages auxiliary material Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph)

We show that ultracold two-component fermionic dipolar gases in an optical lattice with strong two-body on-site loss can be used to realize a tunable effective spin-one model. Fermion number conservation provides an unusual constraint that $\sum_i \left(S^z_i\right)^2$ is conserved, leading to a novel topological liquid phase in one dimension which can be thought of as the gapless analog of the Haldane gapped phase of a spin-one Heisenberg chain. The properties of this phase are calculated numerically via the infinite time-evolving block decimation method and analytically via a mapping to a one-mode Luttinger liquid with hidden spin information.

Title: Three-body exclusion principle, duality mapping, and exact ground state of a harmonically trapped, ultracold Bose gas with three-body hard-core interactions in one dimension
Authors: [|M.D. Girardeau] Comments: 4 pages, no figures, revtex 4. Submitted to Phys. Rev. Letters Subjects: Quantum Gases (cond-mat.quant-gas)

Motivated by previous suggestions that three-body hard-core interactions in lower-dimensional ultracold Bose gases might provide a way for creation of non-Abelian anyons, the exact ground state of a harmonically trapped 1D Bose gas with three-body hard-core interactions is constructed by duality mapping, starting from an $N$-particle ideal gas of mixed symmetry with three-body nodes, which has double occupation of the lowest harmonic oscillator orbital and single occupation of the next $N-2$ orbitals. It has some similarity to the ground state of a Tonks-Girardeau gas, but is more complicated. It is proved that in 1D any system of $N\ge 3$ bosons with three-body hard-core interactions also has two-body soft-core interactions of generalized Lieb-Liniger delta function form, as a consequence of the topology of the configuration space of $N$ particles in 1D, i.e., wave functions with \emph{only} three-body hard core zeroes are topologically impossible. This is in contrast with the case of 2D, where pure three-body hard-core interactions do exist, and are closely related to the fractional quantized Hall effect. The exact ground state is compared with a previously-proposed Pfaffian-like approximate ground state, which satisfies the three-body hard-core constraint but is not an exact energy eigenstate. Both the exact ground state and the Pfaffian-like approximation imply two-body soft-core interactions as well as three-body hard-core interactions, in accord with the general topological proof.

Title: Supersolid in a one-dimensional optical lattice in the presence of a harmonic trap
Authors: [|Tapan Mishra], [|Sunethra Ramanan], [|Ramesh V. Pai], [|Meetu Sethi Luthra], [|B. P. Das] Comments: 9 pages, 16 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We study a system of ultra-cold atoms possessing long range interaction (e.g. dipole-dipole interaction) in a one dimensional optical lattice in the presence of a confining harmonic trap. We have shown that for large enough on-site and nearest neighbor interaction a supersolid phase can be stabilized, consistent with the previous Quantum Monte Carlo and DMRG results for the homogeneous system. Due to the external harmonic trap potential the supersolid phase coexists with other phases. We emphasize on the experimental signatures of the various ground state phases in the presence of a trap.

Title: A Spectroscopic Method to Measure the Superfluid Fraction of an Ultracold Atomic Gas
Authors: [|S. T. John], [|Z. Hadzibabic], [|N. R. Cooper] Subjects: Quantum Gases (cond-mat.quant-gas)

We perform detailed analytical and numerical studies of a recently proposed method for a spectroscopic measurement of the superfluid fraction of an ultracold atomic gas [N. R. Cooper and Z. Hadzibabic, Phys. Rev. Lett. 104, 030401 (2010)]. Previous theoretical work is extended by explicitly including the effects of non-zero temperature and interactions, and assessing the quantitative accuracy of the proposed measurement for a one-component Bose gas. We show that for suitably chosen experimental parameters the method yields an experimentally detectable signal and a sufficiently accurate measurement. This is illustrated by explicitly considering two key examples: First, for a weakly interacting three-dimensional Bose gas it reproduces the expected result that below the critical temperature the superfluid fraction closely follows the condensate fraction. Second, it allows a clear quantitative differentiation of the superfluid and the condensate density in a strongly interacting Bose gas.

Title: Anisotropic spin Hall effect from first principles
Authors: [|Frank Freimuth], [|Stefan Blügel], [|Yuriy Mokrousov] Comments: Accepted for publication in Physical Review Letters Subjects: Materials Science (cond-mat.mtrl-sci)

We report on first principles calculations of the anisotropy of the intrinsic spin Hall conductivity (SHC) in nonmagnetic hcp metals and in antiferromagnetic Cr. For most of the metals of this study we find large anisotropies. We derive the general relation between the SHC vector and the direction of spin polarization and discuss its consequences for hcp metals. Especially, it is predicted that for systems where the SHC changes sign due to the anisotropy the spin Hall effect may be tuned such that the spin polarization is parallel either to the electric field or to the spin current.

Title: Personal history of my engagement with cuprate superconductivity, 1986-2010
Authors: [|Philip W Anderson] Subjects: Superconductivity (cond-mat.supr-con)

Six months ago I was asked to write a personal history of my engagement with the high-Tc problem of the cuprate superconductors, in rather informal and autobiographical style. As the work proceeded I realized that it was impossible and would have been dishonest to separate out my rather amusing but seminal early fumblings from the complete restructuring of the problem which I have achieved during the past decade. But the result became considerably too long, by over half, for its intended recipient. The assignment had left me with no obligation to deal with all the fascinating but irrelevant phenomenology which I had more or less instinctively ignored on my way, but that feature also fails to endear the article to any conceivable editorial board containing knowledgeable experts on the subject. Also, their purpose was for it to serve as a (quote) introduction to the more technical debates, but its message is that almost all of these are not relevant. They are not, on the whole, focused on achieving understanding of the crucial experimental anomalies, many, if not most, of which are now understood. The key to the problem is a new method of dealing with the constrained Hilbert space which follows from the necessity of Gutzwiller projection.

Title: Dynamic Stimulation of Quantum Coherence in Lattice Bosons
Authors: [|Andrew Robertson], [|Victor M. Galitski], [|Gil Refael] Comments: 4 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas)

Thermal fluctuations tend to destroy long-range phase correlations. Consequently, bosons in a lattice will undergo a transition from a phase-coherent superfluid to an insulator as the temperature rises. Contrary to common intuition, however, we show that non-equilibrium driving can be used to reverse this thermal decoherence. This is possible because the energy distribution at equilibrium is rarely optimal for the manifestation of a given quantum property. We demonstrate this in the Bose-Hubbard model by calculating the non-equilibrium spatial correlation function with periodic driving. We show that the non-equilibrium phase diagram at finite bath temperatures can be made qualitatively identical to the familiar zero-temperature phase diagram, and we discuss the experimental manifestation of this phenomenon in cold atoms.

Title: Orbital Order, Metal Insulator Transition, and Magnetoresistance-Effect in the two-orbital Hubbard model
Authors: [|Robert Peters], [|Norio Kawakami], [|Thomas Pruschke] Comments: 7 pages, 7 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study the effects of temperature and magnetic field on a two-orbital Hubbard model within dynamical mean field theory. We focus on the quarter filled system, which is a special point in the phase diagram due to orbital degeneracy. At this particular filling the model exhibits two different long-range order mechanisms, namely orbital order and ferromagnetism. Both can cooperate but do not rely on each other's presence, creating a rich phase diagram. Particularly, in the vicinity of the phase transition to an orbitally ordered ferromagnetic state, we observe a strong magnetoresistance effect. Besides the low temperature phase transitions, we also observe a crossover between a paramagnetic insulating and a paramagnetic metallic state for increasing Hund's coupling at high temperatures.

Title: Single particle Green's functions and interacting topological insulators
Authors: [|V. Gurarie] Comments: 16 pages, 2 figures Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)

We study topological insulators characterized by the integer topological invariant Z, in even and odd spacial dimensions. These are well understood in case when there are no interactions. We extend the earlier work on this subject to construct their topological invariants in terms of their Green's functions. In this form, they can be used even if there are interactions. Specializing to one and two spacial dimensions, we further show that if two topologically distinct topological insulators border each other, the difference of their topological invariants is equal to the difference between the number of zero energy boundary excitations and the number of zeroes of the Green's function at the boundary. In the absence of interactions Green's functions have no zeroes thus there are always edge states at the boundary, as is well known. In the presence of interactions, in principle Green's functions could have zeroes. In that case, there could be no edge states at the boundary of two topological insulators with different topological invariants. This may provide an alternative explanation to the recent results on one dimensional interacting topological insulators.

===4. [|arXiv:1007.1920] (cross-list from hep-ph) [[|pdf], [|ps], [|other]] ===

Title: Nambu-Jona-Lasinio model description of weakly interacting Bose condensate and BEC-BCS crossover in dense QCD-like theories
Authors: [|Lianyi He] C omments: 29 pages + 9 figures. Published version in PRD Journal-ref: Phys.Rev.D82, 096003(2010) Subjects: High Energy Physics - Phenomenology (hep-ph) ; Superconductivity (cond-mat.supr-con); Nuclear Theory (nucl-th)

QCD-like theories possess a positively definite fermion determinant at finite baryon chemical potential $\mu_{\text B}$ and the lattice simulation can be successfully performed. While the chiral perturbation theories are sufficient to describe the Bose condensate at low density, to describe the crossover from Bose-Einstein condensation (BEC) to BCS superfluidity at moderate density we should use some fermionic effective model of QCD, such as the Nambu-Jona-Lasinio model. In this paper, using two-color two-flavor QCD as an example, we examine how the Nambu-Jona-Lasinio model describes the weakly interacting Bose condensate at low density and the BEC-BCS crossover at moderate density. Near the quantum phase transition point $\mu_{\text B}=m_\pi$ ($m_\pi$ is the mass of pion/diquark multiplet), the Ginzburg-Landau free energy at the mean-field level can be reduced to the Gross-Pitaevskii free energy describing a weakly repulsive Bose condensate with a diquark-diquark scattering length identical to that predicted by the chiral perturbation theories. The Goldstone mode recovers the Bogoliubov excitation in weakly interacting Bose condensates. The results of in-medium chiral and diquark condensates predicted by chiral perturbation theories are analytically recovered. The BEC-BCS crossover and meson Mott transition at moderate baryon chemical potential as well as the beyond-mean-field corrections are studied. Part of our results can also be applied to real QCD at finite baryon or isospin chemical potential.

Title: Structural change of vortex patterns in anisotropic Bose-Einstein condensates
Authors: [|N. Lo Gullo], [|Th. Busch], [|M. Paternostro] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph); Quantum Physics (quant-ph)

We study the changes in the spatial distribution of vortices in a rotating Bose-Einstein condensate due to an increasing anisotropy of the trapping potential. Once the rotational symmetry is broken, we find that the vortex system undergoes a rich variety of structural changes, including the formation of zig-zag and linear configurations. These spatial re-arrangements are well signaled by the change in the behavior of the vortex-pattern eigenmodes against the anisotropy parameter. The existence of such structural changes opens up possibilities for the coherent exploitation of effective many-body systems based on vortex patterns.

Title: Bipartite Entanglement in an Out-of-equilibrium Correlated Many-body System
Authors: [|Didier Poilblanc] Comments: 4 pages, 5 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

Relaxation and thermalization after e.g a quantum quench play a central role in out-of-equilibrium physics (e.g. of ultracold atoms) and electronic transport phenomena. On the other hand, entanglement measures are surprisingly useful tools to investigate quantum phases of matter. Here, I show that **bipartite** entanglement entropy and entanglement spectra can provide key information on out-of-of equilibrium states. This is illustrated by considering a global quench in a XXZ spin-1/2 chain across its (zero-temperature) quantum critical point. I argue that the thermalized system can be described by a stationary statistical ensemble defined by the time-averaged **reduced** density matrix of an **extensive subsystem**. Generically, such an effective statistical distribution deviates strongly from a genuine Boltztman-Gibbs equilibrium distribution.

Title: Size and Dynamics of Vortex Dipoles in Dilute Bose-Einstein Condensates
Authors: [|Pekko Kuopanportti], [|Jukka A. M. Huhtamäki], [|Mikko Möttönen] Comments: 5 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas)

Recently, Freilich et al. [Science 329, 1182 (2010)] experimentally discovered stationary states of vortex dipoles, pairs of vortices of opposite circulation, in dilute Bose-Einstein condensates. To explain their observations, we perform simulations based on the Gross-Pitaevskii equation and obtain excellent quantitative agreement on the size of the stationary dipole. We also investigate how their imaging method, in which atoms are repeatedly extracted from a single condensate, affects the vortex dynamics. We find that it mainly induces isotropic size oscillations of the condensate without otherwise altering the vortex trajectories. Thus, the imaging technique appears a promising tool for studying real-time superfluid dynamics.

Title: Spin liquids in graphene
Authors: [|Minh-Tien Tran], [|Ki-Seok Kim] Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We reveal that local interactions in graphene allow novel spin liquids between the semi-metal and antiferromagnetic Mott insulating phases, identified with algebraic spin liquid and Z$_{2}$ spin liquid, respectively. We argue that the algebraic spin liquid can be regarded as the two dimensional realization of one dimensional spin dynamics, where antiferromagnetic correlations show exactly the same power-law dependence as valence bond correlations. Nature of the Z$_{2}$ spin liquid turns out to be $d + i d'$ singlet pairing, but time reversal symmetry is preserved, taking $d + i d'$ in one valley and $d - i d'$ in the other valley. We propose the Josephson effect between two valleys for the mechanism of time reversal symmetry and an experimental signature of the quantized thermal valley Hall effect. Quantum phase transitions among the semi-metal, algebraic spin liquid, and Z$_{2}$ spin liquid are shown to be continuous while the transition from the Z$_{2}$ spin liquid to the antiferromagnetic Mott insulator turns out to be the first order. We emphasize that both algebraic spin liquid and $d \pm id'$ Z$_{2}$ spin liquid can be verified by the quantum Monte Carlo simulation, showing the enhanced symmetry in the algebraic spin liquid and the quantized thermal valley Hall effect in the Z$_{2}$ spin liquid.

Title: Perturbative correction to the ground state properties of one-dimensional strongly interacting bosons in a harmonic trap
Authors: [|Francis N. C. Paraan], [|Vladimir E. Korepin] Comments: 4 pages, 3 PDF figures Subjects: Quantum Gases (cond-mat.quant-gas)

We calculate the first order perturbation correction to the ground state energy and chemical potential of a harmonically trapped boson gas with contact interactions about the infinite repulsion Tonks-Girardeau limit. With $c$ denoting the interaction strength, we find that for a large number of particles $N$ the $1/c$ correction to the ground state energy increases as $N^{5/2}$, in contrast to the unperturbed Tonks-Girardeau value that is proportional to $N^2$. We describe a thermodynamic scaling limit for the trapping frequency that yields an extensive ground state energy and reproduces the zero temperature thermodynamics obtained by a local density approximation.

Title: Non-perturbative predictions for cold atom Bose gases with tunable interactions
Authors: [|Fred Cooper], [|Chih-Chun Chien], [|Bogdan Mihaila], [|John F. Dawson], [|Eddy Timmermans] Comments: 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; High Energy Physics - Phenomenology (hep-ph); Nuclear Theory (nucl-th)

We derive a theoretical description for dilute Bose gases as a loop expansion in terms of composite-field propagators by rewriting the Lagrangian in terms of auxiliary fields related to the normal and anomalous densities. We demonstrate that already in leading order this non-perturbative approach describes a large interval of coupling-constant values, satisfies Goldstone's theorem, yields a Bose-Einstein transition that is second-order, and is consistent with the critical temperature predicted in the weak-coupling limit by the next-to-leading order large-N expansion.

Title: Crossover from 2D to 3D in a weakly interacting Fermi gas
Authors: [|P. Dyke], [|E. D. Kuhnle], [|S. Whitlock], [|H. Hu], [|M. Mark], [|S. Hoinka], [|M. Lingham], [|P. Hannaford], [|C. J. Vale] Comments: 5 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We have studied the transition from two to three dimensions in a low temperature weakly interacting $^6$Li Fermi gas. Below a critical atom number, $N_{2D}$, only the lowest transverse vibrational state of a highly anisotropic oblate trapping potential is occupied and the gas is two-dimensional. Above $N_{2D}$ the Fermi gas enters the quasi-2D regime where shell structure associated with the filling of individual transverse oscillator states is apparent. This dimensional crossover is demonstrated through measurements of the cloud size and aspect ratio versus atom number.

Title: Conservation laws, integrability and transport in one-dimensional quantum systems
Authors: [|J. Sirker], [|R.G. Pereira], [|I. Affleck] Comments: 22 pages, 9 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Statistical Mechanics (cond-mat.stat-mech)

In integrable one-dimensional quantum systems an infinite set of local conserved quantities exists which can prevent a current from decaying completely. For cases like the spin current in the XXZ model at zero magnetic field or the charge current in the attractive Hubbard model at half filling, however, the current operator does not have overlap with any of the local conserved quantities. We show that in these situations transport at finite temperatures is dominated by a diffusive contribution with the Drude weight being either small or even zero. For the XXZ model we discuss in detail the relation between our results, the phenomenological theory of spin diffusion, and measurements of the spin-lattice relaxation rate in spin chain compounds. Furthermore, we study the Haldane-Shastry model where the current operator is also orthogonal to the set of conserved quantities associated with integrability but becomes itself conserved in the thermodynamic limit.

Title: Topological phases and flat surface bands in superconductors without inversion symmetry
Authors: [|Andreas P. Schnyder], [|Shinsei Ryu] Comments: 8 pages, 5 figures Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Superconductivity (cond-mat.supr-con)

We examine different topological phases in three-dimensional non-centrosymmetric superconductors with time-reversal symmetry by using three different types of topological invariants. Due to the bulk boundary correspondence, a non-zero value of any of these topological numbers indicates the appearance of zero-energy Andreev surface states. In fully gapped phases the presence of these surface states is independent of the surface orientation, whereas in nodal superconducting phases the Andreev states appear only for certain orientations of the surface. We find that some of these boundary modes in nodal superconducting phases are dispersionless, i.e., they form a flat surface band. These dispersionless Andreev surface bound states have many observable consequences. In particular, they lead to a zero-bias conductance peak in the scanning tunneling spectra.

=﻿Nov 1 - Nov 5, Chungwei Lin=

[|1. arXiv:1010.6190]
Heterogeneous Kibble-Zurek mechanism: vortex nucleation during Bose-Einstein condensation Authors: A. del Campo, A. Retzker, M. B. Plenio (Submitted on 29 Oct 2010)

Abstract: The Kibble-Zurek mechanism is applied to the spontaneous formation of vortices in a harmonically trapped thermal gas following a temperature quench through the critical value for Bose-Einstein condensation. While in the homogeneous scenario vortex nucleation is always expected, we show that it can be completely suppressed in the presence of the confinement potential, whenever the speed of the spatial front undergoing condensation is lower than a threshold velocity. Otherwise, the interplay between the geometry and causality leads to different scaling laws for the density of vortices.

[|2. arXiv:1010.6085]
Bound states of a localized magnetic impurity in a superfluid of paired ultracold fermions Authors: Eric Vernier, David Pekker, Martin W. Zwierlein, Eugene Demler (Submitted on 28 Oct 2010)

Abstract: We consider a localized impurity atom that interacts with a cloud of fermions in the paired state. We develop an effective scattering length description of the interaction between an impurity and a fermionic atom using their vacuum scattering length. Treating the pairing of fermions at the mean-field level, we show that the impurity atom acts like a magnetic impurity in the condensed matter context, and leads to the formation of a pair of Shiba bound states inside the superconducting gap. In addition, the impurity atom can lead to the formation of deeply bound states below the Fermi sea.

[|3. arXiv:1010.6126]
Topological characterization of periodically-driven quantum systems Authors: Takuya Kitagawa, Erez Berg, Mark Rudner, Eugene Demler (Submitted on 29 Oct 2010)

Abstract: Topological properties of physical systems can lead to robust behaviors that are insensitive to microscopic details. Such topologically robust phenomena are not limited to static systems but can also appear in driven quantum systems. In this paper, we show that the Floquet operators of periodically driven systems can be divided into topologically distinct (homotopy) classes, and give a simple physical interpretation of this classification in terms of the spectra of Floquet operators. Using this picture, we provide an intuitive understanding of the well-known phenomenon of quantized adiabatic pumping. Systems whose Floquet operators belong to the trivial class simulate the dynamics generated by time-independent Hamiltonians, which can be topologically classified according to the schemes developed for static systems. We demonstrate these principles through an example of a periodically driven two--dimensional hexagonal lattice model which exhibits several topological phases. Remarkably, one of these phases supports chiral edge modes even though the bulk is topologically trivial.

[|1. arXiv:1011.0394]
Multi-stability in an optomechanical system with two-component Bose-Einstein condensate Authors: Ying Dong, Jinwu Ye, Han Pu (Submitted on 1 Nov 2010)

Abstract: We investigate a system consisting of a two-component Bose-Einstein condensate interacting dispersively with a Fabry-Perot optical cavity where the two components of the condensate are resonantly coupled to each other by another classical field. The key feature of this system is that the atomic motional degrees of freedom and the internal pseudo-spin degrees of freedom are coupled to the cavity field simultaneously, hence an effective spin-orbital coupling within the condensate is induced by the cavity. The interplay among the atomic center- of-mass motion, the atomic collective spin and the cavity field leads to a strong nonlinearity, resulting in multi- stable behavior in both matter wave and light wave at the few-photon level.

[|2. arXiv:1011.0388]
Repulsively-bound exciton-biexciton states in high-spin fermions in optical lattices Authors: A. Argüelles, L. Santos (Submitted on 1 Nov 2010 (v1), last revised 2 Nov 2010 (this version, v2))

Abstract: We show that the interplay between spin-changing collisions and quadratic Zeeman coupling provides a novel mechanism for the formation of repulsively bound composites in high-spin fermions, which we illustrate by considering spin flips in an initially polarized hard-core 1D Mott insulator of spin-3/2 fermions. We show that after the flips the dynamics is characterized by the creation of two types of exciton-biexciton composites. We analyze the conditions for the existence of these bound states, and discuss their intriguing properties. In particular we show that the effective mass and stability of the composites depends non-trivially on spin-changing collisions, on the quadratic Zeeman effect and on the initial exciton localization. Finally, we show that the composites may remain stable against inelastic collisions, opening the possibility of novel quantum composite phases.

[|3. arXiv:1011.0334]
Novel orbital selective phase transition induced by different magnetic states: A dynamical cluster approximation study Authors: Hunpyo Lee, Yu-Zhong Zhang, 2 Harald O. Jeschke, Roser Valentí (Submitted on 1 Nov 2010)

Abstract: By considering the dynamical cluster approximation combined with the continuous time quantum Monte Carlo algorithm, we analyze the behavior of a degenerate two-orbital anisotropic Hubbard model at half filling where both orbitals have equal bandwidths and one orbital is constrained to be paramagnetic (PM) (PM orbital), while the second one is allowed to have an antiferromagnetic (AF) solution (AF orbital). As the interaction increases, novel orbital selective phase transitions induced by different magnetic states in different orbitals appear regardless of the strength of the Ising Hund's rule coupling $J_z$. Moreover, the PM orbital undergoes a transition from a Fermi liquid (FL) to a Mott insulator through an intermediate non-FL phase while the AF orbital shows a transition from a FL to an AF insulator through an intermediate AF metallic phase. Finally, the phase diagram of the model is presented and possible applications of the model to some aspects of the physics of iron pnictides are discussed.

[|4. arXiv:1011.0033]
Confinement-induced p-wave resonances from s-wave interaction Authors: Yusuke Nishida, Shina Tan (Submitted on 29 Oct 2010)

Abstract: We show that a purely s-wave interaction in three dimensions (3D) can induce higher partial-wave resonances in mixed dimensions. We develop two-body scattering theories in all three cases of 0D-3D, 1D-3D, and 2D-3D mixtures and determine the positions of higher partial-wave resonances in terms of the 3D s-wave scattering length assuming a harmonic confinement potential. We also compute the low-energy scattering parameters in the p-wave channel (scattering volume and effective momentum) which are necessary for the low-energy effective theory of the p-wave resonance. We point out that some of the resonances observed in the Florence group experiment [Phys. Rev. Lett. 104, 153202 (2010)] can be interpreted as the p-wave resonances in the 2D-3D mixed dimensions. Our study paves the way for a variety of physics such as Anderson localization of matter wave under p-wave resonant scatterers.

[|1. arXiv:1011.0713]
Integrability breakdown in longitudinaly trapped, one-dimensional bosonic gases Authors: Igor E. Mazets (Submitted on 2 Nov 2010)

Abstract: A system of identical bosons with short-range (contact) interactions is studied. Their motion is confined to one dimension by a tight lateral trapping potential and, additionally, subject to a weak harmonic confinement in the longitudinal direction. Finite delay time associated with penetration of quantum particles through each other in the course of a pairwise one-dimensional collision in the presence of the longitudinal potential makes the system non-integrable and, hence, provides a mechanism for relaxation to thermal equilibrium. To analyse this effect quantitatively in the limit of a non-degenerate gas, we develop a system of kinetic equations and solve it for small-amplitude monopole oscillations of the gas. The obtained damping rate is long enough to be neglected in a realistic cold-atom experiment, and therefore longitudinal trapping does not hinder integrable dynamics of atomic gases in the 1D regime.

[|2. arXiv:1011.0467]
Population imbalance and pairing in the BCS-BEC crossover of three-component ultracold fermions Authors: Tomoki Ozawa, Gordon Baym (Submitted on 1 Nov 2010)

Abstract: We investigate the phase diagram and the BCS-BEC crossover of a homogeneous three-component ultracold Fermi gas with a U(3) invariant attractive interaction. We show that the system at sufficiently low temperatures exhibits population imbalance, as well as fermionic pairing. We describe the crossover in this system, connecting the weakly interacting BCS regime of the partially population-imbalanced fermion pairing state and the BEC limit with three weakly interacting species of molecules, including pairing fluctuations within a t-matrix calculation of the particle self-energies.

[|3. arXiv:1011.0586] Theory of interacting topological superfluids and superconductors Authors: Zhong Wang, Xiao-Liang Qi, Shou-Cheng Zhang (Submitted on 2 Nov 2010)

Abstract: We investigate the three-dimensional, time-reversal invariant topological superconductors with generic interaction by their response to external fields. The first description is a gravitational topological field theory, which gives a $Z_2$ classification of topological superconductors, and predicts a half-quantized thermal Hall effect on the surface. The second description introduces a $s$-wave proximity pairing field on the surface, and the associated topological defects give an integer $Z$ classification of the topological superconductor phases. Generally, charge conserving, time reversal breaking fields probe the $Z_2$ sector, and charge conservation breaking and time reversal breaking fields probe the $Z$ sector of the interacting topological superconductor.

[|1. arXiv:1011.1150]
Strong coupling treatment of the polaronic system consisting of an impurity in a condensate Authors: W. Casteels, T. Van Cauteren, J. Tempere, J. T. Devreese (Submitted on 4 Nov 2010)

Abstract: The strong coupling treatment of the Fr\"ohlich-type polaronic system, based on a canonical transformation and a standard Landau-Pekar type variational wave function, is applied to the polaronic system consisting of an impurity in a condensate. Within this approach the Relaxed Excited States are retrieved as a typical polaronic feature in the energy spectrum. For these states we calculate the corresponding effective mass and the minimal coupling constant required for them to occur. The present approach allows to derive approximate expressions for the transition energies between different Relaxed Excited States in a much simpler way than with the full Mori-Zwanzig approach, and with a good accuracy, which improves with increasing coupling. The transition energies obtained here can be used as the spectroscopic fingerprint for the experimental observation of Relaxed Excited States of impurities in a condensate.