May2011

flat

= **May 30 - Jun 3, Chungwei Lin** =

Jun 03
1. [|arXiv:1106.0356] (cross-list from math-ph) [[|pdf], [|ps], [|other]] [|Giuseppe Benfatto], [|Pierluigi Falco], [|Vieri Mastropietro] Comments: 50 pages Subjects: Mathematical Physics (math-ph) ; Strongly Correlated Electrons (cond-mat.str-el) We study the 1D extended Hubbard model with a weak repulsive short-range interaction in the non-half-filled band case, using non-perturbative Renormalization Group methods and Ward Identities. At the critical temperature, T = 0, the response functions have anomalous power-law decay with multiplicative logarithmic corrections. The critical exponents, the susceptibility and the Drude weight verify the universal Luttinger liquid relations. Borel summability and (a weak form of) Spin-Charge separation is established. [|2. arXiv:1106.0430] (cross-list from cond-mat.quant-gas) [[|pdf], [|ps], [|other]] [|A. Korolyuk], [|J. J. Kinnunen], [|P. Törmä] Comments: 14 pages, 8 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con) We consider the density response of a trapped two-component Fermi gas. Combining the Bogoliubov-deGennes method with the random phase approximation allows the study of both collective and single particle excitations. Calculating the density response across a wide range of interactions, we observe a crossover from a weakly interacting pair vibration mode to a strongly interacting Goldstone mode. The crossover is associated with a depressed collective mode frequency and an increased damping rate, in agreement with density response experiments performed in strongly interacting atomic gases. 3. [|arXiv:1106.0402] [[|pdf], [|other]] [|Vivek M. Vyas], [|Sandeep Gautam], [|Prasanta K. Panigrahi] Comments: REVTEX 4, 10 pages, 1 figure Subjects: Quantum Gases (cond-mat.quant-gas) We show that quasi-one dimensional Bose-Einstein condensate under suitable conditions can exhibit a Berezinskii-Kosterlitz-Thouless phase transition. The role played by quantized vortices in two dimensional case, is played in this case by dark solitons. We find that the critical temperature for this transition lies in nano Kelvin range and below, for a wide range of experimentally accessible parameters. It is seen that the high temperature (disordered) phase differs from low temperature (ordered) phase in terms of phase coherence, which can be used as an experimental signature for observing this transition. 4. [|arXiv:1106.0324] [[|pdf], [|ps], [|other]] [|Asaad R. Sakhel] Comments: 8 pages and 8 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) We explore condensate and superfluid depletion in a system of a small number of hard core bosons (N=8 and N=40) trapped inside a combined harmonic optical cubic lattice (HOCL). The condensate fraction is computed for chosen individual lattice sites by separating the one-body density matrix (OBDM) of the whole system into its components at the various lattice sites. Then each "lattice-site" component is diagonalized to find its eigenvalues. The eigenvalues are obtained by a method presented earlier in Ref.[1]. The effects of interference between the condensate at a lattice site and the surrounding condensates at all-neighbor lattice sites is also investigated. The superfluid fraction is calculated for N=40 by using the diffusion formula of Pollock and Ceperley [2]. Our chief result is that the condensate at a lattice site is enhanced by the interference with its neighbors beyond the result when the interference is neglected. The superfluid is depleted with a rise of the repulsion between the bosons, yet at very strong interaction superfluidity is still present. Further, the effects of lattice spacing have also been studied and it was found that a reduction in the lattice dimension, i.e. an increase in the lattice wave vector, increases the local condensate fraction at each lattice site but reduces the superfluid fraction of the whole system.
 * Universal Luttinger Liquid Relations in the 1D Hubbard Model**
 * Density response of a trapped Fermi gas: a crossover from the pair vibration mode to the Goldstone mode**
 * On Berezinskii-Kosterlitz-Thouless Phase Transition in Quasi-One Dimensional Bose-Einstein Condensate**
 * Condensate and superfluid depletion of a few hard core bosons in a cubic optical lattice plus external harmonic confinement**

Jun 02
[|1. arXiv:1106.0001] [[|pdf], [|other]] [|Sean A. Hartnoll], [|Diego M. Hofman], [|Max A. Metlitski], [|Subir Sachdev] Comments: 72 pages, 23 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; High Energy Physics - Theory (hep-th) We study the frequency dependence of the electron self energy and the optical conductivity in a recently developed field theory of the spin density wave quantum phase transition in two dimensional metals. We focus on the interplay between the Fermi surface `hot spots' and the remainder of the `cold' Fermi surface. Scattering of electrons off the fluctuations of the spin density order parameter, \phi, is strongest at the hot spots; we compute the conductivity due to this scattering in a rainbow approximation. We point out the importance of composite operators, built out of products of the primary electron or $\phi$ fields: these have important effects also away from the hot spots. The simplest composite operator, \phi^2, leads to non-Fermi liquid behavior on the entire Fermi surface. We also find an intermediate frequency window in which the cold electrons loose their quasiparticle form due to effectively one-dimensional scattering processes. The latter processes are part of umklapp scattering which leads to singular contributions to the optical conductivity at the lowest frequencies at zero temperature. 2. [|arXiv:1106.0006] (cross-list from cond-mat.quant-gas) [[|pdf], [|ps], [|other]] [|Xue-Feng Zhang], [|Raoul Dillenschneider], [|Yue Yu], [|Sebastian Eggert] Comments: 6 pages, 5 figures. The latest version can be found at [|this http URL] Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph) Hard-core bosons on a triangular lattice with nearest neighbor repulsion are a prototypical example for a system with supersolid behavior on a lattice. We show that in this model the physical origin of the supersolid phase can be understood quantitatively and analytically by constructing quasiparticle excitations of defects that are moving on an ordered background. The location of the solid to supersolid phase transition line is predicted from the effective model for both positive and negative (frustrated) hopping parameters. For positive hopping parameters the calculations agree very accurately with numerical Quantum Monte Carlo simulations. The numerical results indicate that the supersolid to superfluid transition is first order. 3 [|arXiv:1106.0188] [[|pdf], [|other]] [|Tarik Yefsah], [|Rémi Desbuquois], [|Lauriane Chomaz], [|Kenneth J. Günter], [|Jean Dalibard] Comments: 6 pages, 5 figures Subjects: Quantum Gases (cond-mat.quant-gas) Using \emph{in situ} measurements on a quasi two-dimensional, harmonically trapped $^{87}$Rb gas, we infer various equations of state for the equivalent homogeneous fluid. From the dependence of the total atom number and the central density of our clouds with the chemical potential and temperature, we obtain the equations of state for the pressure and the phase-space density. Then using the approximate scale invariance of this two-dimensional system, we determine the entropy per particle. We measure values as low as $0.06\,\kB$ in the strongly degenerate regime, which shows that a 2D Bose gas can constitute an efficient coolant for other quantum fluids. We also explain how to disentangle the various contributions (kinetic, potential, interaction) to the energy of the trapped gas using a time-of-flight method, from which we infer the reduction of density fluctuations in a non fully coherent cloud.
 * Quantum critical response at the onset of spin density wave order in two-dimensional metals**
 * Supersolid phase transitions for hardcore bosons on a triangular lattice**
 * Exploring the thermodynamics of a two-dimensional Bose gas**

Jun 01
[|1. arXiv:1105.6369] (cross-list from physics.atom-ph) [[|pdf], [|other]] [|Y. A. Liao], [|M. Revelle], [|T. Paprotta], [|A. S. C. Rittner], [|Wenhui Li], [|G. B. Partridge], [|R. G. Hulet] Comments: 4 pages, 6 figures Subjects: Atomic Physics (physics.atom-ph) ; Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con) We study the role of particle transport and evaporation on the phase separation of an ultracold, spin-polarized atomic Fermi gas. We show that the previously observed deformation of the superfluid paired core is a result of evaporative depolarization of the superfluid due to a combination of enhanced evaporation at the center of the trap and the inhibition of spin transport at the normal-superfluid phase boundary. These factors contribute to a nonequilibrium jump in the chemical potentials at the phase boundary. Once formed, the deformed state is highly metastable, persisting for times of up to 2 s.
 * Metastability in Spin-Polarized Fermi Gases**

2. [|arXiv:1105.6161] [[|pdf], [|ps], [|other]] [|Minh-Tien Tran], [|A. Benlagra], [|C. Pépin], [|Ki-Seok Kim] Subjects: Strongly Correlated Electrons (cond-mat.str-el) The existence of multiple energy scales is regarded as a signature of the Kondo breakdown mechanism for explaining the quantum critical behavior of certain heavy fermion compounds, like YbRh$_{2}$Si$_{2}$. The nature of the intermediate state between the heavy Fermi liquid and the quantum critical region, however, remains elusive. In this study we suggest an incoherent heavy-fermion scenario, where inelastic scattering with novel soft modes of the dynamical exponent $z = 3$ gives rise to non-Fermi liquid physics for thermodynamics and transport despite the formation of the heavy-fermion band. We discuss a crossover from $z = 3$ to $z = 1$ for quantum phase fluctuations.
 * Preformed heavy-electrons at the Quantum Critical Point in heavy fermion compounds**

[|3. arXiv:1105.6136] [[|pdf], [|other]] [|Michael Schecter], [|Dimitri Gangardt], [|Alex Kamenev] Comments: 28 pages, 10 figures Subjects: Quantum Gases (cond-mat.quant-gas) We study dynamics of a mobile impurity moving in a one-dimensional quantum liquid. Such an impurity induces a strong non-linear depletion of the liquid around it. The dispersion relation of the combined object, called depleton, is a periodic function of its momentum with the period 2\pi n, where n is the mean density of the liquid. In the adiabatic approximation a constant external force acting on the impurity leads to the Bloch oscillations of the impurity around a fixed position. Dynamically such oscillations are accompanied by the radiation of energy in the form of phonons. The ensuing energy loss results in the uniform drift of the oscillation center. We derive exact results for the radiation-induced mobility as well as the thermal friction force in terms of the equilibrium dispersion relation of the dressed impurity (depleton). These results show that there is a wide range of external forces where the (drifted) Bloch oscillations exist and may be observed experimentally.
 * Dynamics and Bloch oscillations of mobile impurities in one-dimensional quantum liquids**

May 31
[|1. arXiv:1105.5907] [[|pdf], [|ps], [|other]] [|A. Sterdyniak], [|B.A. Bernevig], [|N. Regnault], [|F. D. M. Haldane] Comments: 16 pages, 19 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) We investigate the non-universal part of the orbital entanglement spectrum (OES) of the nu = 1/3 fractional quantum Hall effect (FQH) ground-state with Coulomb interactions. The non-universal part of the spectrum is the part that is missing in the Laughlin model state OES whose level counting is completely determined by its topological order. We find that the OES levels of the Coulomb interaction ground-state are organized in a hierarchical structure that mimic the excitation-energy structure of the model pseudopotential Hamiltonian which has a Laughlin ground state. These structures can be accurately modeled using Jain's "composite fermion" quasihole-quasiparticle excitation wavefunctions. To emphasize the connection between the entanglement spectrum and the energy spectrum, we also consider the thermodynamical OES of the model pseudopotential Hamiltonian at finite temperature. The observed good match between the thermodynamical OES and the Coulomb OES suggests a relation between the entanglement gap and the true energy gap.
 * Hierarchical structure in the orbital entanglement spectrum in Fractional Quantum Hall systems**

2. [|arXiv:1105.5775] (cross-list from math-ph) [[|pdf], [|ps], [|other]] [|A.A.Ovchinnikov] Comments: LaTex, 11 pages Subjects: Mathematical Physics (math-ph) ; Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th) For various one-dimensional quantum liquids in the framework of the Luttinger model (bosonization) we establish the relations between the coefficients before the power-law asymptotics of the correlators (prefactors) and the formfactors of the corresponding local operators. The derivation of these relations in the framework of the bosonization procedure allows to substantiate the prediction for the formfactors corresponding to the low-lying particle-hole excitations. We also obtain the formulas for the summation over the particle-hole states corresponding to the power-law asymptotics of the correlators.
 * On the universal relations for the prefactors in correlation functions of 1D quantum liquids**

[|3. arXiv:1105.6039] [[|pdf], [|ps], [|other]] [|Jarrett Lancaster], [|Thierry Giamarchi], [|Aditi Mitra] Comments: 10 pages, 3 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) The effect of interactions on a system of fermions that are in a non-equilibrium steady state due to a quantum quench is studied employing the random-phase-approximation (RPA). As a result of the quench, the distribution function of the fermions is highly broadened. This gives rise to an enhanced particle-hole spectrum and over-damped collective modes for attractive interactions between fermions. On the other hand, for repulsive interactions, an undamped mode above the particle-hole continuum survives. The sensitivity of the result on the nature of the non-equilibrium steady state is explored by also considering a quench that produces a current carrying steady-state.
 * A random phase approximation study of one-dimensional fermions after a quantum quench**

[|4. arXiv:1105.5834] [[|pdf], [|other]] [|Waseem S. Bakr], [|Philipp M. Preiss], [|M. Eric Tai], [|Ruichao Ma], [|Jonathan Simon], [|Markus Greiner] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) Interaction blockade occurs when strong interactions in a confined few-body system prevent a particle from occupying an otherwise accessible quantum state. Blockade phenomena reveal the underlying granular nature of quantum systems and allow the detection and manipulation of the constituent particles, whether they are electrons, spins, atoms, or photons. The diverse applications range from single-electron transistors based on electronic Coulomb blockade to quantum logic gates in Rydberg atoms. We have observed a new kind of interaction blockade in transferring ultracold atoms between orbitals in an optical lattice. In this system, atoms on the same lattice site undergo coherent collisions described by a contact interaction whose strength depends strongly on the orbital wavefunctions of the atoms. We induce coherent orbital excitations by modulating the lattice depth and observe a staircase-type excitation behavior as we cross the interaction-split resonances by tuning the modulation frequency. We demonstrate two applications of the orbital excitation blockade (OEB) phenomenon. First, we use the lattice depth vibration, usually thought of as a heating mechanism in ultracold gas systems, to realize a Maxwell demon for cooling atoms in an optical lattice. Second, we use OEB to obtain site-resolved images of antiferromagnetic Ising chains in an optical lattice and study the evolution of Neel ordering across a quantum phase transition. These applications illustrate the power of OEB in both manipulating and imaging many-body states in optical lattices, with implications for developing a microscopic understanding of strongly-correlated electron systems that can be simulated in optical lattices. In addition, the close analogy between OEB and dipole blockade in Rydberg atoms provides a roadmap for the implementation of two-qubit gates in a quantum computing architecture with natural scalability.
 * Interaction-induced orbital excitation blockade of ultracold atoms in an optical lattice**

5. [|arXiv:1105.5947] (cross-list from quant-ph) [[|pdf], [|other]] [|S. Diehl], [|E. Rico], [|M. A. Baranov], [|P. Zoller] Comments: 15 pages, 5 figures Subjects: Quantum Physics (quant-ph) ; Quantum Gases (cond-mat.quant-gas) Robust edge states and non-Abelian excitations are the trademark of topological states of matter, with promising applications such as "topologically protected" quantum memory and computing. While so far topological phases have been exclusively discussed in a Hamiltonian context, we show that such phases and the associated topological protection and phenomena also emerge in open quantum systems with engineered dissipation. The specific system studied here is a quantum wire of spinless atomic fermions in an optical lattice coupled to a bath. The key feature of the dissipative dynamics described by a Lindblad master equation is the existence of Majorana edge modes, representing a non-local decoherence free subspace. The isolation of the edge states is enforced by a dissipative gap in the p-wave paired bulk of the wire. We describe dissipative non-Abelian braiding operations within the Majorana subspace, and we illustrate the insensitivity to imperfections. Topological protection is granted by a nontrivial winding number of the system density matrix.
 * Topology by Dissipation in Atomic Quantum Wires**

May 30
1. [|arXiv:1105.5387] [[|pdf], [|ps], [|other]] [|Camille Aron], [|Gabriel Kotliar], [|Cedric Weber] Comments: 4 pages, 3 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) We study the steady-state dynamics of the Hubbard model driven out-of-equilibrium by a constant electric field and coupled to a dissipative heat bath. For very strong field, we find a dimensional reduction: the system behaves as an equilibrium Hubbard model in lower dimension. We derive steady-state equations for the dynamical mean field theory in the presence of dissipation. We discuss how the electric field induced dimensional crossover affects the momentum resolved and integrated spectral functions, the energy distribution function, as well as the steady current in the non-linear regime.
 * Dimensional Crossover Driven by Electric Field**

2. [| arXiv:1102.3562] (cross-list from quant-ph) [[|pdf], [|ps], [|other]] [|Davide Rossini], [|Vittorio Giovannetti], [|Rosario Fazio] Comments: 23 pages, 9 figures. Published version, extensive changes Journal-ref: J. Stat. Mech. (2011) P05021 Subjects: Quantum Physics (quant-ph) ; Strongly Correlated Electrons (cond-mat.str-el) We discuss in details a modified variational matrix-product-state algorithm for periodic boundary conditions, based on a recent work by P. Pippan, S.R. White and H.G. Everts, Phys. Rev. B 81, 081103(R) (2010), which enables one to study large systems on a ring (composed of N ~ 10^2 sites). In particular, we introduce a couple of improvements that allow to enhance the algorithm in terms of stability and reliability. We employ such method to compute the stiffness of one-dimensional strongly correlated quantum lattice systems. The accuracy of our calculations is tested in the exactly solvable spin-1/2 Heisenberg chain.
 * Stiffness in 1D Matrix Product States with periodic boundary conditions**

[|3. arXiv:1105.5493] [[|pdf], [|ps], [|other]] [|B. Opanchuk], [|M. Egorov], [|S. Hoffmann], [|A. Sidorov], [|P. D. Drummond] Comments: 4 pages, 2 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph) We develop a theory of quantum noise, fringe contrast and squeezing in a three-dimensional Bose-Einstein condensate atom interferometer with nonlinear losses, using a truncated Wigner simulation. Our results indicate the limits of atom interferometry, showing how quantum noise is modified due to nonlinear losses in mesoscopic atom interferometry.
 * Quantum noise in three-dimensional BEC interferometry**

= **May 23-27, Xiaopeng Li** =

Author: A.A. Burkov, Leon Balents We propose a simple realization of the three-dimensional (3D) Weyl semimetal phase, utilizing a multilayer structure, composed of identical thin films of a magnetically-doped 3D topological insulator (TI), separated by ordinary-insulator spacer layers. We show that the phase diagram of this system contains a Weyl semimetal phase of the simplest possible kind, with only two Dirac nodes of opposite chirality, separated in momentum space, in its bandstructure. This particular type of Weyl semimetal has a finite anomalous Hall conductivity, chiral edge states, and occurs as an intermediate phase between an ordinary insulator and a 3D quantum anomalous Hall insulator with a quantized Hall conductivity, equal to $e^2/h$ per TI layer. We find that the Weyl semimetal has a nonzero DC conductivity at zero temperature and is thus an unusual metallic phase, characterized by a finite anomalous Hall conductivity and topologically-protected edge states.
 * May 27**
 * 1.** [|arXiv:1105.5138] [[|pdf], [|other]]
 * Title:** Weyl Semimetal in a Topological Insulator Multilayer

Author: Rahul Nandkishore, Leonid Levitov The unique sensitivity of optical response to different types of symmetry breaking can be used to detect and identify spontaneously ordered many-body states in bilayer graphene. We predict a strong response at optical frequencies, sensitive to electronic phenomena at low energies, which arises because of nonzero inter-band matrix elements of the electric current operator. In particular, the polar Kerr rotation and reflection anisotropy provide fingerprints of the quantum anomalous Hall state and the nematic state, characterized by spontaneously broken time reversal symmetry and lattice rotation symmetry, respectively. These optical signatures, which undergo a resonant enhancement in the near-infrared regime, lie well within reach of existing experimental techniques.
 * 2.** [|arXiv:1105.5142] [[|pdf], [|ps], [|other]]
 * Title:** Polar Kerr Effect and Time Reversal Symmetry Breaking in Bilayer Graphene

3. [|arXiv:1105.5214] [[|pdf], [|other]] Author: R. Shankar, Ashvin Vishwanath For certain systems, the N-particle ground-state wavefunctions of the bulk happen to be exactly equal to the N-point space-time correlation functions at the edge, in the infrared limit. We show why this had to be so for a class of topological superconductors, beginning with the p+ip state in D=2+1. Varying the chemical potential as a function of Euclidean time between weak and strong pairing states is shown to extract the wavefunction. Then a Euclidean rotation that exchanges time and space and approximate Lorentz invariance lead to the edge connection. We illustrate straightforward extension to other dimensions (eg. 3He- B phase in D=3+1) and to correlated states like fractionalized topological superconductors.
 * Title:** Equality of bulk wave functions and edge correlations in topological superconductors: A spacetime derivation

4. [|arXiv:1105.5289] [[|pdf], [|ps], [|other]] Author: Walter Metzner, Manfred Salmhofer, Carsten Honerkamp, Volker Meden, Kurt Schoenhammer Numerous correlated electron systems exhibit a strongly scale-dependent behavior. Upon lowering the energy scale, collective phenomena, bound states, and new effective degrees of freedom emerge. Typical examples include (i) competing magnetic, charge, and pairing instabilities in two-dimensional electron systems, (ii) the interplay of electronic excitations and order parameter fluctuations near thermal and quantum phase transitions in metals, (iii) correlation effects such as Luttinger liquid behavior and the Kondo effect showing up in linear and non-equilibrium transport through quantum wires and quantum dots. The functional renormalization group is a flexible and unbiased tool for dealing with such scale-dependent behavior. Its starting point is an exact functional flow equation, which yields the gradual evolution from a microscopic model action to the final effective action as a function of a continuously decreasing energy scale. Expanding in powers of the fields one obtains an exact hierarchy of flow equations for vertex functions. Truncations of this hierarchy have led to powerful new approximation schemes. This review is a comprehensive introduction to the functional renormalization group method for interacting Fermi systems. We present a self-contained derivation of the exact flow equations and describe frequently used truncation schemes. Reviewing selected applications we then show how approximations based on the functional renormalization group can be fruitfully used to improve our understanding of correlated fermion systems.
 * Title:**Functional renormalization group approach to correlated fermion systems

Author:T.H. Hansson, A. Karlhede, Masatoshi Sato We propose a topological field theory for a spin-less 2D chiral superconductor that contains fundamental Majorana fields. Due to a fermionic gauge symmetry, the Majorana modes survive as dynamical degrees of freedom only at magnetic vortex cores, and on edges. These modes have the topological properties pertinent to a p-wave superconductor including the non-abelian braiding statistics. We also briefly discuss the connection to the Moore-Read Pfaffian quantum Hall state, and extensions to the spinful case and to 3D topological superconductors.
 * May 26**
 * 1.** [|arXiv:1105.5031] [[|pdf], [|ps], [|other]]
 * Title:**Topological Field Theory for p-wave Superconductors

Author: J. Dubail, N. Read We study the entanglement in various fully-gapped complex paired states of fermions in two dimensions, focusing on the entanglement spectrum (ES), and using the BCS form of the ground state wavefunction on a cylinder. Certain forms of the pairing functions allow a simple and explicit exact solution for the ES. In the weak-pairing phase of l-wave paired spinless fermions (l odd), the universal low-lying part of the ES consists of |l| chiral Majorana fermion modes [or 2|l| (l even) for spin-singlet states]. For |l|>1, the pseudo-energies of the modes are split in general, but for all l there is a zero--pseudo-energy mode at zero wavevector if the number of modes is odd. This ES agrees with the perturbed conformal field theory of the edge excitations. For more general BCS states, we show how the entanglement gap diverges as a model pairing function is approached.
 * May 25**
 * 1.** [|arXiv:1105.4808] [[|pdf], [|ps], [|other]]
 * Title:**Entanglement spectra of complex paired superfluids

2. [|arXiv:1105.1162] (cross-list from hep-th) [[|pdf], [|ps], [|other]] Title: Holographic Fermi and Non-Fermi Liquids with Transitions in Dilaton Gravity Author: Norihiro Iizuka, Nilay Kundu, Prithvi Narayan, Sandip P. Trivedi We study the two-point function for fermionic operators in a class of strongly coupled systems using the gauge-gravity correspondence. The gravity description includes a gauge field and a dilaton which determines the gauge coupling and the potential energy. Extremal black brane solutions in this system typically have vanishing entropy. By analyzing a charged fermion in these extremal black brane backgrounds we calculate the two-point function of the corresponding boundary fermionic operator. We find that in some region of parameter space it is of Fermi liquid type. Outside this region no well-defined quasi-particles exist, with the excitations acquiring a non-vanishing width at zero frequency. At the transition, the two-point function can exhibit non-Fermi liquid behaviour.

Title: Enhancement of the critical temperature in cuprate superconductors by inhomogeneous doping Authors: Lilach Goren, Ehud Altman We use a renormalized mean field theory to investigate the superconducting properties of underdoped cuprates embedded with overdoped or metallic regions that carry excess dopants. The overdoped regions are considered, within two different models, first as stripes of mesoscopic size larger than the coherence length and then as point impurities. In the former case we compute the temperature dependent superfluid stiffness by solving Bogoliubov de Gennes equations within the slave boson mean field theory. We average over stripes of different orientations to obtain an isotropic result. To compute the superfluid stiffness in the model with point impurities we resort to a diagrammatic expansion in the impurity concentration (to first order) and their strength (up to second order). We find analytic expressions for the disorder averaged superfluid stiffness and the critical temperature. For both types of inhomogeneity we find increased superfluid stiffness, and for a wide range of doping enhancement of Tc relative to a homogeneously underdoped system. Remarkably, in the case of microscopic impurities we find that the maximal Tc can be significantly increased compared to Tc at optimal doping of a pure system.
 * May 24**
 * 1.** [|arXiv:1105.4243] [[|pdf], [|ps], [|other]]

= **May 16-20, Zixu Zhang** =

**May 20**

Title: Asymptotic correlation functions and FFLO signature for the one-dimensional attractive spin-1/2 Fermi gas
Authors: [|J. Y. Lee], [|X. W. Guan] Comments: 2 figures, 18 pages Subjects: Quantum Gases (cond-mat.quant-gas)

We investigate the long distance asymptotics of various correlation functions for the one-dimensional spin-1/2 Fermi gas with attractive interactions using the dressed charge formalism. In the spin polarized phase, these correlation functions exhibit spatial oscillations with a power-law decay whereby their critical exponents are found through conformal field theory. We show that spatial oscillations of the leading terms in the pair correlation function and the spin correlation function solely depend on $\Delta k_F$ and $2\Delta k_F$, respectively. Here $\Delta k_F=\pi(n_{\uparrow}-n_{\downarrow})$ denotes the mismatch between the Fermi surfaces of spin-up and spin-down fermions. Such spatial modulations are characteristics of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. Our key observation is that backscattering among the Fermi points of bound pairs and unpaired fermions results in a one-dimensional analog of the FFLO state and displays a microscopic origin of the FFLO nature. Furthermore, we show that the pair correlation function in momentum space has a peak at the point of mismatch between both Fermi surfaces $k=\Delta k_F$, which has recently been observed in numerous numerical studies.

Title: Superfluid, solid, and supersolid phases of dipolar bosons in a quasi-one-dimensional optical lattice
Authors: [|Jonathan M. Fellows], [|Sam T. Carr] Comments: 4 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el)

We discuss a model of dipolar bosons trapped in a weakly coupled planar array of one-dimensional tubes. We consider the situation where the dipolar moments are aligned by an external field, and find a rich phase diagram as a function of the angle of this field exhibiting quantum phase transitions between solid, superfluid and supersolid phases. In the low energy limit, the model turns out to be identical to one describing quasi-one-dimensional superconductivity in condensed matter systems. This opens the possibility of using bosons as a quantum analogue simulator of electronic systems, a scenario arising from the intricate relation between statistics and interactions in quasi-one-dimensional systems.

Title: Topological Fermi liquids from Coulomb interactions in the doped Honeycomb lattice
Authors: [|Eduardo V. Castro], [|Adolfo G. Grushin], [|Belén Valenzuela], [|María A. H. Vozmediano], [|Alberto Cortijo], [|Fernando de Juan] Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Other Condensed Matter (cond-mat.other)

We get an anomalous Hall metallic state in the Honeycomb lattice with nearest neighbors only arising as a spontaneously broken symmetry state from a local nearest neighbor Coulomb interaction V. The key ingredient is to enlarge the unit cell to host six atoms that permits Kekul\'e distortions and supports self-consistent currents creating non trivial magnetic configurations with total zero flux. We find within a variational mean field approach a metallic phase with broken time reversal symmetry (T) very close in parameter space to a Pomeranchuk instability. Within the T broken region the predominant configuration is an anomalous Hall phase with non zero Hall conductivity, a realization of a topological Fermi liquid. A T broken phase with zero Hall conductivity is stable in a small region of the parameter space for lower values of V.

Title: Hole-doped semiconductor nanowire: A new and experimentally accessible system for Majorana fermions
Authors: [|Li Mao], [|Ming Gong], [|E. Dumitrescu], [|Sumanta Tewari], [|Chuanwei Zhang] Comments: 6 pages, 4 figures, submitted for publication at Apr. 20, 2011 Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Materials Science (cond-mat.mtrl-sci); Superconductivity (cond-mat.supr-con); Quantum Physics (quant-ph) Majorana fermions, quantum particles which are as their own antiparticles, were envisioned by E. Majorana in 1935 to describe neutrinos. Although the realization of Majorana fermions in condensed matter experiments would by itself be an extraordinary development, they have also sparked tremendous recent interest for their potential use in quantum computation. So far most candidate systems for Majorana fermions suffer from one important problem: the required experimental parameters are beyond the capacity of current experiments. Here we show that a hole-doped semiconductor nanowire, under certain external conditions, can support Majorana excitations in a regime of parameters \emph{already within experimental reach} in routine experiments. A hole-doped nanowire, with its fundamentally different underlying physics from its electron-doped counterpart, also leads to many other topical advantages for realizing a topologically non-trivial state. Thus, this system is uniquely suited among all solid state systems for realizing and manipulating Majorana fermions in controllable experiments.

===2. [|arXiv:1101.1054] (cross-list from math-ph) [[|pdf], [|ps], [|other]]===

Title: Clifford modules and symmetries of topological insulators
Authors: [|Gilles Abramovici] (LPS), [|Pavel Kalugin] (LPS) Subjects: Mathematical Physics (math-ph) ; Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph) We complete the classification of symmetry constraints on gapped quadratic fermion hamiltonians proposed by Kitaev. The symmetry group is supposed compact and can include arbitrary unitary or antiunitary operators in the Fock space that conserve the algebra of quadratic observables. We analyze the multiplicity spaces of {\em real} irreducible representations of unitary symmetries in the Nambu space. The joint action of intertwining operators and antiunitary symmetries provides these spaces with the structure of Clifford module: we prove a one-to-one correspondence between the ten Altland-Zirnbauer symmetry classes of fermion systems and the ten Morita equivalence classes of real and complex Clifford algebras. The antiunitary operators, which occur in seven classes, are projectively represented in the Nambu space by unitary "chiral symmetries". The space of gapped symmetric hamiltonians is homotopically equivalent to the product of classifying spaces indexed by the dual object of the group of unitary symmetries.

Title: Local density of states of a quarter-filled one-dimensional Mott insulator with a boundary
Authors: [|Dirk Schuricht] Comments: 13 pages Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study the low-energy limit of a quarter-filled one-dimensional Mott insulator. We analytically determine the local density of states in the presence of a strong impurity potential, which is modeled by a boundary. To this end we calculate the Green function using field theoretical methods. The Fourier transform of the local density of states shows signatures of a pinning of the spin-density wave at the impurity as well as several dispersing features at frequencies above the charge gap. These features can be interpreted as propagating spin and charge degrees of freedom. Their relative strength can be attributed to the "quasi-fermionic" behavior of charge excitations with equal momenta. Furthermore, we discuss the effect of bound states localized at the impurity. Finally, we give an overview of the local density of states in various one-dimensional systems and discuss implications for scanning tunneling microscopy experiments.

Title: Ground-state properties and superfluidity of two- and quasi two-dimensional solid 4He
Authors: [|C. Cazorla], [|G. E. Astrakharchik], [|J. Casulleras], [|J. Boronat] Comments: 16 pages, 8 figures Journal-ref: J. of Phys.: Cond. Matt. 22, 165402 (2010) Subjects: Other Condensed Matter (cond-mat.other)

In a recent study we have reported a new type of trial wave function symmetric under the exchange of particles and which is able to describe a supersolid phase. In this work, we use the diffusion Monte Carlo method and this model wave function to study the properties of solid 4He in two- and quasi two-dimensional geometries. In the purely two-dimensional case, we obtain results for the total ground-state energy and freezing and melting densities which are in good agreement with previous exact Monte Carlo calculations performed with a slightly different interatomic potential model. We calculate the value of the zero-temperature superfluid fraction \rho_{s} / \rho of 2D solid 4He and find that it is negligible in all the considered cases, similarly to what is obtained in the perfect (free of defects) three-dimensional crystal using the same computational approach. Interestingly, by allowing the atoms to move locally in the perpendicular direction to the plane where they are confined to zero-point oscillations (quasi two-dimensional crystal) we observe the emergence of a finite superfluid density that coexists with the periodicity of the system.

Title: Double occupancy as a universal probe for antiferromagnetic correlations and entropy in cold fermions on optical lattices
Authors: [|E. V. Gorelik], [|T. Paiva], [|R. Scalettar], [|A. Klümper], [|N. Blümer] Comments: 4 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el)

We verify signatures of antiferromagnetic (AF) correlations in the double occupancy D [Gorelik et al., PRL 105, 065301 (2010)] and study their dimensional dependence using direct quantum Monte Carlo in dimensions d=2,3 and Bethe Ansatz in d=1. We find quantitative agreement with dynamical mean-field theory (DMFT) in the cubic case and qualitative agreement down to d=1. As a function of entropy s=S/(N k_B), D is nearly universal with respect to d; the minimum in D(s) approaches s=log(2) at strong coupling, as predicted by DMFT. Long-range order appears hardly relevant for the current search of AF signatures in cold fermions. Thus, experimentalists need not achieve s<log(2)/2 and should consider lower dimensions, for which the AF effects are larger.

Title: Orbital Liquid Crystal Phases of Cold Fermions in Optical Lattices
Authors: [|Zixu Zhang], [|Xiaopeng Li], [|W. Vincent Liu] Comments: 4+ pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We study instabilities of spinless fermionic atoms in the p-orbital bands in two dimensional optical lattices at non-integer filling against interactions. Charge-density-wave and orbital-density-wave orders with stripe or checkerboard patterns are found for attractive and repulsive interactions, respectively. The superfluid phase, usually expected of attractively interacting fermions, is strongly suppressed. We also use effective field theory to analyze the possible phase transitions from orbital strip order to liquid crystal like phases and find the phase diagram. The condition of Fermi surface nesting, which is key to the above results, is shown robustly independent of fermion fillings in such p-orbital systems, and the $(2k_F ,\pm2k_F)$ wavevector of density wave oscillation is highly tunable. Such remarkable features show the promise of making those exotic orbital phases, which are of broad interest in condensed matter physics, experimentally realizable with optical lattice gases.

Title: Superconducting ground state of a doped Mott insulator
Authors: [|Zheng-Yu Weng] Comments: 22 pages, 2 figures Subjects: Superconductivity (cond-mat.supr-con)

A d-wave superconducting ground state for a doped Mott insulator is obtained. It is distinguished from a Gutzwiller-projected BCS superconductor by an explicit separation of Cooper pairing and resonating valence bond (RVB) pairing. Such a state satisfies the precise sign structure of the t-J model, just like that a BCS state satisfies the Fermi-Dirac statistics. This new class of wavefunctions can be intrinsically characterized and effectively manipulated by electron fractionalization with neutral spinons and backflow spinons forming a two-component RVB structure. While the former spinon is bosonic, originated from the superexchange correlation, the latter spinon is found to be fermionic, accompanying the hopping of bosonic holons. The low-lying emergent gauge fields associated with such a specific fractionalization are of mutual Chern-Simons type. Corresponding to this superconducting ground state, three types of elementary excitations are identified. Among them a Bogoliubov nodal quasiparticle is conventional, while the other two are neutral excitations of non-BCS type that play crucial roles in higher energy/temperaure regimes. Their unique experimental implications for the cuprates are briefly discussed.

Title: Chern numbers hiding in time-of-flight images
Authors: [|Erhai Zhao], [|Noah Bray-Ali], [|Carl J. Williams], [|I. B. Spielman], [|Indubala I. Satija] Comments: 3 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We present a novel technique for detecting topological invariants -- Chern numbers -- from time-of-flight images of ultra-cold neutral atoms. The Chern number of fermions in a lattice potential depends on magnetic field (for neutral atoms, generated by an artificial magnetic field) and the chemical potential; we find that the Chern numbers leave their fingerprints as oscillations in the atoms' momentum distribution. The signature is strikingly enhanced for anisotropic hopping, where we show the momentum distribution assumes a sinusoidal form with period determined by the Chern number.

**May 16**

Title: Seeing topological order
Authors: [|E. Alba], [|X. Fernandez-Gonzalvo], [|J. Mur-Petit], [|J. K. Pachos], [|J. J. Garcia-Ripoll] Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph)

Different phases of matter can be distinguished by their symmetries. This information is captured by order parameters that summarize the essential properties of the phase. Order parameters are usually defined in terms of local operators that can be measured in the laboratory. Topological insulators are materials with symmetries that depend on the topology of the energy eigenstates of the system. These materials are of interest because they can give rise to robust spin transport effects with potential applications ranging from sensitive detectors to quantum computation. However, direct measurement of topological order has been up to now impossible due to its non-local character. In this work we provide a general methodology to perform a direct measurement of topological order in cold atom systems. As an application we propose the realisation of a characteristic topological model, introduced by Haldane, using optical lattices loaded with fermionic atoms in two internal states. We demonstrate that time-of-flight measurements directly reveal the topological order of the system in the form of momentum space skyrmions.

= **May 9-13, Chungwei lin** =

May 13
1. [|arXiv:1105.2402] [[|pdf], [|ps], [|other]] [|Hong Chul Choi], [|B. I. Min], [|J. H. Shim], [|K. Haule], [|G. Kotliar] Comments: 5 pages main article,3 figures for the main article, 2 page Supplementary information, 2 figures for the Supplementary information. Supplementary movie 1 and 2 are provided on the webpage([|this http URL]) Subjects: Strongly Correlated Electrons (cond-mat.str-el) In Cerium-based heavy electron materials, the 4f electron's magnetic moments bind to the itinerant quasiparticles to form composite heavy quasiparticles at low temperature. The volume of the Fermi surfacein the Brillouin zone incorporates the moments to produce a "large FS" due to the Luttinger theorem. When the 4f electrons are localized free moments, a "small FS" is induced since it contains only broad bands of conduction spd electrons. We have addressed theoretically the evolution of the heavy fermion FS as a function of temperature, using a first principles dynamical mean-field theory (DMFT) approach combined with density functional theory (DFT+DMFT). We focus on the archetypical heavy electrons in CeIrIn5, which is believed to be near a quantum critical point. Upon cooling, both the quantum oscillation frequencies and cyclotron masses show logarithmic scaling behavior (~ ln(T_0/T)) with different characteristic temperatures T_0 = 130 and 50 K, respectively. The resistivity coherence peak observed at T ~ 50 K is the result of the competition between the binding of incoherent 4f electrons to the spd conduction electrons at Fermi level and the formation of coherent 4f electrons.
 * Temperature-dependent Fermi surface evolution in heavy fermion CeIrIn5**

2. [|arXiv:1105.2382] (cross-list from quant-ph) [[|pdf], [|ps], [|other]] [|Aditi Sen De], [|Ujjwal Sen] Comments: 8 pages, 3 figures Subjects: Quantum Physics (quant-ph) ; Strongly Correlated Electrons (cond-mat.str-el) Exactly solvable many-body systems are few and far between, and the utility of approximate methods cannot be overestimated. Entanglement mean field theory is an approximate method to handle such systems. While mean field theories reduce the many-body system to an effective single-body one, entanglement mean field theory reduces it to a two-body system. And in contrast to mean field theories where the self-consistency equations are in terms of single-site physical parameters, those in entanglement mean field theory are in terms of both single- and two-site parameters. Hitherto, the theory has been applied to predict properties of the static states, like ground and thermal states, of many-body systems. Here we give a method to employ it to predict properties of time-evolved states. The predictions are then compared with known results of paradigmatic spin Hamiltonians.
 * Simulating Quantum Dynamics with Entanglement Mean Field Theory**

3. [|arXiv:1105.2446] (cross-list from quant-ph) [[|pdf], [|ps], [|other]] [|Gabriele De Chiara], [|Anna Sanpera] Comments: Submitted to the Special Issue: "Strong correlations in Quantum Gases" in The Journal of Low Temperature Physics Subjects: Quantum Physics (quant-ph) ; Quantum Gases (cond-mat.quant-gas) We propose the use of quantum polarization spectroscopy for detecting multi-particle entanglement of ultracold atoms in optical lattices. This method, based on a light-matter interface employing the quantum Farady effect, allows for the non destructive measurement of spin-spin correlations. We apply it to the specific example of a one dimensional spin chain and reconstruct its phase diagram using the light signal, readily measurable in experiments with ultracold atoms. Interestingly, the same technique can be extended to detect quantum many-body entanglement in such systems.
 * Detection of entanglement in ultracold lattice gases**

4. [|arXiv:1105.2496] [[|pdf], [|ps], [|other]] [|C. Cao], [|E. Elliott], [|H. Wu], [|J. E. Thomas] Comments: 20 pages, 10 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Nuclear Theory (nucl-th) We measure the shear viscosity in a two-component Fermi gas of atoms, tuned to a broad s-wave collisional (Feshbach) resonance. At resonance, the atoms strongly interact and exhibit universal behavior, where the equilibrium thermodynamic properties and the transport coefficients are universal functions of the density $n$ and temperature $T$. We present a new calibration of the temperature as a function of global energy, which is directly measured from the cloud profiles. Using the calibration, the trap-averaged shear viscosity in units of $\hbar\,n$ is determined as a function of the reduced temperature at the trap center, from nearly the ground state to the unitary two-body regime. Low temperature data is obtained from the damping rate of the radial breathing mode, while high temperature data is obtained from hydrodynamic expansion measurements. We also show that the best fit to the high temperature expansion data is obtained for a vanishing bulk viscosity. The measured trap-averaged entropy per particle and shear viscosity are used to estimate the ratio of the shear viscosity to the entropy density, which is compared that conjectured for a perfect fluid.
 * Searching for Perfect Fluids: Quantum Viscosity in a Universal Fermi Gas**

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May 12
1. [|arXiv:1105.2084] [[|pdf], [|ps], [|other]] [|Dagim Tilahun], [|R. A. Duine], [|A. H. MacDonald] Subjects: Quantum Gases (cond-mat.quant-gas) Ultracold atoms in optical lattices undergo a quantum phase transition from a superfluid to a Mott insulator as the lattice potential depth is increased. We describe an approximate theory of interacting bosons in optical lattices which provides a qualitative description of both superfluid and insulator states. The theory is based on a change of variables in which the boson coherent state amplitude is replaced by an effective potential which promotes phase coherence between different number states on each lattice site. It is illustrated here by applying it to uniform and fully frustrated lattice cases, but is simple enough that it can easily be applied to spatially inhomogeneous lattice systems.
 * A Quantum Theory of Cold Bosonic Atoms in Optical Lattices**

2. [|arXiv:1105.2250] [[|pdf], [|other]] [|Zeng-Qiang Yu], [|Hui Zhai] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) In this letter we study both ground state properties and the superfluid transition temperature of a spin-1/2 Fermi gas across a Feshbach resonance with a synthetic spin-orbit coupling, using mean-field theory and exact solution of two-body problem. We show that a strong spin-orbit coupling can significantly enhance the pairing gap at the BCS side. The superfluid transition temperature in the BCS side increases because spin-orbit coupling increases the density-of-state at Fermi energy, while in the BEC sides, the transition temperature also increases because spin-orbit coupling decreases the effective mass of molecule. This leads to the highest critical temperature T_c/T_F ever achieved in fermion superfluids. The universal interaction energy and pair size at resonance are also discussed.
 * Spin-Orbit Coupled Fermi Gases across a Feshbach Resonance**

3. [|arXiv:1105.2259] [[|pdf], [|ps], [|other]] [|Masahiro Sato], [|Hosho Katsura], [|Naoto Nagaosa] Comments: 4+epsilon pages, 2 column version, 3 figures, 2 tables, Revtex Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Statistical Mechanics (cond-mat.stat-mech) We study theoretically the Raman scattering spectra in the one-dimensional quantum spin-1/2 antiferromagnets, a class of systems with strong quantum fluctuation and quantum liquid phases. With the Heisenberg interaction between the nearest-neighbor spins only, no magnetic Raman process exists. Additional interactions such as (i) magnetic anisotropy, (ii) bond dimerization, (iii) longer-range exchange interaction, (iv) tilting of the bond, and (v) staggered gyromagnetic tensor, lead to finite Raman scattering intensities with different behavior of spectra as a function of frequency, temperature, and magnetic field. Therefore, the Raman spectra offers a unique and important tool to study the microscopic Hamiltonian of one-dimensional quantum antiferromagnets. ---
 * Theory of Raman Scattering in One-Dimensional Quantum Antiferromagnets**

May 11
1. [|arXiv:1105.1787] (cross-list from cond-mat.mes-hall) [[|pdf], [|other]] [|A. Cook], [|M. Franz] Comments: 4 pages + 4 figures. For related work and info visit [|this http URL] Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con) A topological insulator nanoribbon, proximity-coupled to an ordinary bulk s-wave superconductor and subject to a longitudinal applied magnetic field, is shown to realize a one-dimensional topological superconductor with unpaired Majorana fermions localized at both ends. This situation occurs under a wide range of conditions and constitutes what is possibly the most easily accessible physical realization of the elusive Majorana particles in a solid-state system.
 * Majorana Fermions in Proximity-coupled Topological Insulator Nanoribbons**

2. [|arXiv:1105.1793] (cross-list from cond-mat.quant-gas) [[|pdf], [|ps], [|other]] [|Subir Sachdev] Comments: 28 pages, 8 figures; adapted from Chapter 16 of Quantum Phase Transitions, Second Edition (see [|this http URL]); contribution to Lecture Notes in Physics, "BCS-BEC crossover and the Unitary Fermi Gas" edited by W. Zwerger Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el) I give a unified perspective on the properties of a variety of quantum liquids using the theory of quantum phase transitions. A central role is played by a zero density quantum critical point which is argued to control the properties of the dilute gas. An exact renormalization group analysis of such quantum critical points leads to a computation of the universal properties of the dilute Bose gas and the spinful Fermi gas near a Feshbach resonance.
 * Dilute Fermi and Bose Gases**

3. [|arXiv:1105.1778] [[|pdf], [|other]] [|Rajdeep Sensarma], [|David Pekker], [|Ana Maria Rey], [|Mikhail Lukin], [|Eugene Demler] Comments: 5 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Strongly Correlated Electrons (cond-mat.str-el) We theoretically study the relaxation of high energy single particle excitations into molecules in a system of attractive fermions in an optical lattice, both in the superfluid and the normal phase. In a system characterized by an interaction scale $U$ and a tunneling rate $t$, we show that the relaxation rate scales as $\sim Ct\exp(-\alpha U^2/t^2)$ in the large $U/t$ limit. We obtain explicit expressions for the exponent $\alpha$, both in the low temperature superfluid phase and the high temperature phase with pairing but no coherence between the molecules. We find that the relaxation rate decreases both with temperature and deviation of the fermion density from half-filling. We show that quasiparticle and phase degrees of freedom are effectively decoupled within experimental timescales allowing for observation of ordered states even at high total energy of the system.
 * Relaxation of Fermionic Excitations in a Strongly Attractive Fermi Gas in an Optical Lattice**

4. [|arXiv:1105.1786] (cross-list from cond-mat.stat-mech) [[|pdf], [|ps], [|other]] [|Matthew S. Foster], [|Timothy C. Berkelbach], [|David R. Reichman], [|Emil A. Yuzbashyan] Comments: 32 pages, 27 figures Subjects: Statistical Mechanics (cond-mat.stat-mech) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas) We compute the dynamics of localized excitations produced by a quantum quench in the spin 1/2 XXZ chain. Using numerics combining the density matrix renormalization group and exact time evolution, as well as analytical arguments, we show that fractionalization due to interactions in the pre-quench state gives rise to "ultrarelativistic" density waves that travel at the maximum band velocity. The system is initially prepared in the ground state of the chain within the gapless XY phase, which admits a Luttinger liquid (LL) description at low energies and long wavelengths. The Hamiltonian is then suddenly quenched to a band insulator, after which the chain evolves unitarily. Through the gapped dispersion of the insulator spectrum, the post-quench dynamics serve as a "velocity microscope," revealing initial state particle correlations via space time density propagation. We show that the ultrarelativistic wave production is tied to the particular way in which fractionalization evades Pauli-blocking in the zero-temperature initial LL state.
 * Quantum quench spectroscopy of a Luttinger liquid: Ultrarelativistic density wave dynamics due to fractionalization in an XXZ chain**

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May 10
1. [|arXiv:1105.1375] [[|pdf], [|ps], [|other]] [|Jay D. Sau], [|Bin Wang], [|S. Das Sarma] Comments: 4.5 pages, 3 Figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Statistical Mechanics (cond-mat.stat-mech) Mott insulator to superfluid quenches have been used by recent experiments to generate exotic superfluid phases. While the final Hamiltonian following the sudden quench is that of a superfluid, it is not apriori clear how close the final state of the system approaches the ground state of the superfluid Hamiltonian. To understand the nature of the final state we calculate the temporal evolution of the momentum distribution following a Mott insulator to superfluid quench. Using the numerical infinite time-evolving block decimation approach and the analytical rotor model approximation we establish that the one and two dimensional Mott insulators following the quench equilibriate to thermal states with spatially short-ranged coherence peaks in the final momentum distribution and therefore are not strict superfluids. However, in three dimensions we find a divergence in the momentum distribution indicating the emergence of true superfluid order.
 * Quench induced Mott insulator to superfluid quantum phase transition**

2. [|arXiv:1105.1459] [[|pdf], [|ps], [|other]] [|Ryota Watanabe], [|Shunji Tsuchiya], [|Yoji Ohashi] Comments: 6 pages, 6 figures Journal-ref: Proceedings of IEEE-ISUS 2011 Subjects: Quantum Gases (cond-mat.quant-gas) We theoretically investigate the recent photoemission-type experiment on $^{40}$K Fermi gases done by JILA group. Including pairing fluctuations within a strong-coupling $T$-matrix theory, as well as effects of a harmonic trap within the local density approximation, we calculate photoemission spectra in the BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover region. We show that the energy resolution of the current photoemission experiment is enough to detect the pseudogap phenomenon. We also show how the pseudogap in single-particle excitations continuously changes into the superfluid gap, as one decreases the temperature below the superfluid phase transition temperature. Our results would be useful for the study of single-particle properties of ultracold Fermi gases in the BCS-BEC crossover.
 * Theory of Photoemission-type Experiment in the BCS-BEC Crossover Regime of a Superfluid Fermi Gas**

3. [|arXiv:1105.1374] [[|pdf], [|other]] [|E.M. Stoudenmire], [|Steven R. White] Comments: 9 pages, 8 figures, To appear in Annual Reviews of Condensed Matter Physics (2012) [without the abstract] Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Superconductivity (cond-mat.supr-con) The Density Matrix Renormalization Group (DMRG) method scales exponentially in the system width for models in two dimensions, but remains one of the most powerful methods for studying 2D systems with a sign problem. Reviewing past applications of DMRG in 2D demonstrates its success in treating a wide variety of problems, although it remains underutilized in this setting. We present techniques for performing cutting edge 2D DMRG studies including methods for ensuring convergence, extrapolating finite-size data and extracting gaps and excited states. Finally, we compare the current performance of a recently developed tensor network method to 2D DMRG.
 * Studying Two Dimensional Systems With the Density Matrix Renormalization Group**

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May 9
1. [|arXiv:1105.1175] [[|pdf], [|ps], [|other]] [|Emanuel Gull], [|David R. Reichman], [|Andrew J. Millis] Subjects: Strongly Correlated Electrons (cond-mat.str-el) A Monte Carlo sampling of diagrammatic corrections to the non-crossing approximation is shown to provide numerically exact estimates of the long-time dynamics and steady state properties of nonequilibrium quantum impurity models. This `bold' expansion converges uniformly in time and significantly ameliorates the sign problem that has heretofore limited the power of real-time Monte Carlo approaches to strongly interacting real-time quantum problems. The new approach enables the study of previously intractable problems ranging from generic long time nonequilibrium transport characteristics in systems with large onsite repulsion to the direct description of spectral functions on the real frequency axis in Dynamical Mean Field Theory.
 * Numerically Exact Long Time Behavior of Nonequilibrium Quantum Impurity Models**

2. [|arXiv:1105.1314] [[|pdf], [|other]] [|Tommaso Roscilde], [|Cristian Degli Esposti Boschi], [|Marcello Dalmonte] Comments: 4 pages, 4 figures + Supplementary Material Subjects: Strongly Correlated Electrons (cond-mat.str-el) We numerically determine the very rich phase diagram of mass-imbalanced binary mixtures of hardcore bosons (or equivalently -- fermions, or hardcore-Bose/Fermi mixtures) loaded in one-dimensional optical lattices. Focusing on commensurate fillings away from half filling, we find a strong asymmetry between attractive and repulsive interactions. Attraction is found to always lead to pairing, associated with a spin gap, and to pair crystallization for very strong mass imbalance. In the repulsive case the two atomic components remain instead fully gapless over a large parameter range; only a very strong mass imbalance leads to the opening of a spin gap. The spin-gap phase is the precursor of a crystalline phase occurring for an even stronger mass imbalance. The fundamental asymmetry of the phase diagram is at odds with recent theoretical predictions, and can be tested directly via time-of-flight experiments on trapped cold atoms.
 * Pairing, crystallization and string correlations of mass-imbalanced atomic mixtures in one-dimensional optical lattices**

3. [|arXiv:1105.1252] [[|pdf], [|ps], [|other]] [|B. Béri], [|N. R. Cooper] Subjects: Quantum Gases (cond-mat.quant-gas) We describe how optical dressing can be used to generate bandstructures for ultracold atoms with non-trivial Z_2 topological order. Time reversal symmetry is preserved by simple conditions on the optical fields. We first show how to construct optical lattices that give rise to Z_2 topological insulators in two dimensions. We then describe a general method for the construction of three-dimensional Z_2 topological insulators.
 * Z_2 Topological Insulators in Ultracold Atomic Gases**

= **May 2-6, Xiaopeng Li** =

**May 6**
1. [|arXiv:1105.0932] [[|pdf], [|other]] Title : An Optical-Lattice-Based Quantum Simulator For Relativistic Field Theories and Topological Insulators

Author: Leonardo Mazza, Alejandro Bermudez, Nathan Goldman, Matteo Rizzi, Miguel Angel Martin-Delgado, Maciej Lewenstein We present a proposal for a versatile cold-atom-based quantum simulator of relativistic fermionic theories and topological insulators in arbitrary dimensions. The setup consists of a spin-independent optical lattice that traps a collection of hyperfine states of the same alkaline atom, to which the different degrees of freedom of the field theory to be simulated are then mapped. We show that the combination of bi-chromatic optical lattices with Raman transitions can allow the engineering of a spin-dependent tunneling of the atoms between neighboring lattice sites. These assisted-hopping processes can be employed for the quantum simulation of various interesting models, ranging from non-interacting relativistic fermionic theories to topological insulators. We present a toolbox for the realization of different types of relativistic lattice fermions, which can then be exploited to synthesize the majority of phases in the periodic table of topological insulators.

2. [|arXiv:1105.0992] [[|pdf], [|ps], [|other]] Title: Size Effects on Topological Anderson Insulator Author: [|Wei Li], [|Jiadong Zang], [|Yongjin Jiang] We study the size effects on the transport properties in topological Anderson insulator by means of the Landauer-B\"uttiker formalism combined with the nonequilibrium Green function method. Conductances calculated for serval different width of the nanoribbons reveal that there is no longer quantized plateaus for narrow nanoribbons. The local spin polarized current distribution demonstrates that the edge states on the two sides can be coupled, leading to enhancement of backscattering as the width of the nanoribbon decreases, thus destroying the perfect quantization phenomena in the topological Anderson insulator. It is also demonstrated that the main contribution to the nonquantized conductance (with a hump structure) also comes from edge states.

May 5
1. [|arXiv:1105.0904] [[|pdf], [|ps], [|other]] Title: Topological Transitions for Lattice Bosons in a Magnetic Field Author: [|Sebastian D. Huber], [|Netanel H. Lindner] We study the Hall response of the Bose-Hubbard model subjected to a magnetic field. We show that the Hall conductivity is proportional to the particle density plus an integer. The phase diagram is intersected by topological transitions between different integer values. These transitions originate from points in the phase diagram with effective charge conjugation symmetry, and are attributed to degeneracies in the many body spectrum which serve as sources for the Berry curvature. We find that extensive regions in the phase diagram exhibit a negative Hall conductivity, implying that flux flow is reversed in these regions - vortices there flow upstream. We discuss experimental implications of our findings.

**May 4**
1. [|arXiv:1105.0610] [[|pdf], [|ps], [|other]] Title: Localization of Bogoliubov quasiparticles in interacting Bose gases with correlated disorder Author: [|Pierre Lugan] (LCFIO), [|Laurent Sanchez-Palencia] (LCFIO) We study the Anderson localization of Bogoliubov quasiparticles (elementary many-body excitations) in a weakly interacting Bose gas of chemical potential $\mu$ subjected to a disordered potential $V$. We introduce a general mapping (valid for weak inhomogeneous potentials in any dimension) of the Bogoliubov-de Gennes equations onto a single-particle Schr\"odinger-like equation with an effective potential. For disordered potentials, the Schr\"odinger-like equation accounts for the scattering and localization properties of the Bogoliubov quasiparticles. We derive analytically the localization lengths for correlated disordered potentials in the one-dimensional geometry. Our approach relies on a perturbative expansion in $V/\mu$, which we develop up to third order, and we discuss the impact of the various perturbation orders. Our predictions are shown to be in very good agreement with direct numerical calculations. We identify different localization regimes: For low energy, the effective disordered potential exhibits a strong screening by the quasicondensate density background, and localization is suppressed. For high-energy excitations, the effective disordered potential reduces to the bare disordered potential, and the localization properties of quasiparticles are the same as for free particles. The maximum of localization is found at intermediate energy when the quasicondensate healing length is of the order of the disorder correlation length. Possible extensions of our work to higher dimensions are also discussed.

2. [|arXiv:1105.0581] [[|pdf], [|ps], [|other]] Title: Doping the Kagome: a variational Monte-Carlo study Author: [|Siegfried Guertler], [|Hartmut Monien] We study doping on the Kagome lattice by exploring the t-J-model with variational Monte-Carlo. We use a number of Gutzwiller projected spin-liquid and valence bond-crystal states and study their ground-state energy at several sizes. We find that the introducing of mobile impurities drives the system away from the Spin-Liquid state proposed by Ran et al for the undoped system, towards a uniform state with zero-flux. On top of the uniform-state a VBC of the Hastings-type is formed for low doping. The results are compared to exact diagonalization on small clusters. This agrees well.

May 3
1. [|arXiv:1105.0406] [[|pdf], [|ps], [|other]] Title: Fractional quantum Hall effect at zero magnetic field Author: [|Abolhassan Vaezi] In this letter, we discuss the recently proposed fractional quantum Hall effect in the absence of Landau levels. It is shown that the parton construction can explain all properties of 1/3 state, including the effective charge of quasi-particles, their statistics and the many-body ground-state degeneracy. The low energy description of these states has been discussed. We also generalize our model to construct the hierarchical quantum Hall states at filling fractions other than $1/m$.

2. arXiv:1105.0289 [[|pdf], [|ps], [|other]] Title: Metal-Quantum Dot-Topological Superconductor Junction: Kondo correlations and Majorana Bound States Author: [|Anatoly Golub], [|Igor Kuzmenko], [|Yshai Avishai] Electron transport through [normal metal]-[quantum dot]-[topological superconductor] junction is studied and reveals interlacing physics of Kondo correlations with two Majorana fermions bound state residing on the opposite edges of the topological superconductor. When the strength of the Majorana fermion coupling exceeds the temperature $T$, this combination of Kono-Majorana fermion physics can be observed: The usual peak of the temperature dependent zero biased conductance $\sigma(V=0,T)$ splits and the conductance has a {\it dip} at T=0. The height of the conductance side-peaks decreases with magnetic field.