February2011

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=Feb 28 - Mar 4, Xiaopeng Li=

Mar 4
1. ** [|arXiv:1103.0557] [ [|pdf], [|ps] , [|other] ]**  **Title:Entropy of isolated quantum systems after a quench**  Author: Lea F. Santos, Anatoli Polkovnikov, Marcos Rigol A diagonal entropy, which depends only on the diagonal elements of the system's density matrix in the energy representation, has been recently introduced as the proper definition of thermodynamic entropy in out-of-equilibrium quantum systems. We study this quantity after an interaction quench in lattice hard-core bosons and spinless fermions, and after a local chemical potential quench in a system of hard-core bosons in a superlattice potential. The former systems have a chaotic regime, where the diagonal entropy becomes equivalent to the equilibrium microcanonical entropy, coinciding with the onset of thermalization. The latter system is integrable. We show that its diagonal entropy is additive and close, but not equal, to the entropy of a generalized Gibbs ensemble (GGE) that accounts for the conserved quantities at integrability. The difference between the two entropies may be attributed to additional correlations present in the system and not captured by the GGE.


 * 2.  [|arXiv:1103.0574] [ [|pdf], [|other] ] **
 * Title: Magnetic Fluctuations and Correlations in MnSi - Evidence for a Skyrmion Spin Liquid Phase **

 Author: C. Pappas, E. Lelièvre-Berna, P. Bentley, P. Falus, P. Fouquet, B. Farago  We present a comprehensive analysis of high resolution neutron scattering data involving Neutron Spin Echo spectroscopy and Spherical Polarimetry which confirm the first order nature of the helical transition and reveal the existence of a new spin liquid skyrmion phase. Similar to the blue phases of liquid crystals this phase appears in a very narrow temperature range between the low temperature helical and the high temperature paramagnetic phases.

 **Title: Fractional Quantum Hall States at 1/3 and 5/2 Filling: the Density-Matrix Renormalization Group Calculations**  Author: Jize Zhao, D. N. Sheng, F. D. M. Haldane In this paper, the density-matrix renormalization group method is employed to investigate the fractional quantum Hall effect at filling fractions $\nu=1/3$ and 5/2. We first present benchmark results at both filling fractions for large system sizes to show the accuracy as well as the capability of the numerical algorithm. Furthermore, we show that by keeping a large number of basis states, one can also obtain accurate entanglement spectrum at $\nu=5/2$ for large system with electron number up to $N_e=34$, much larger than systems previously studied. Based on a finite-size scaling analysis, we demonstrate that the entanglement gap defined by Li and Haldane is finite in the thermodynamic limit, which characterizes the topological order of the FQHE state.
 * 3.  [|arXiv:1103.0772] [ [|pdf], [|ps] , [|other] ] **

 Mar 3
===** 1. ** [|arXiv:1103.0504] [ [|pdf], [|ps] , [|other] ] === <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; line-height: normal;">Author: Massimiliano Stengel, Pablo Aguado-Puente, Nicola A. Spaldin, Javier Junquera <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: 19px;">Based on recent advances in first-principles theory, we develop a general model of the band offset at metal/ferroelectric interfaces. We show that, depending on the polarization of the film, a pathological regime might occur where the metallic carriers populate the energy bands of the insulator, making it metallic. As the most common approximations of density functional theory are affected by a systematic underestimation of the fundamental band gap of insulators, this scenario is likely to be an artifact of the simulation. We provide a number of rigorous criteria, together with extensive practical examples, to systematically identify this problematic situation in the calculated electronic and structural properties of ferroelectric systems. We discuss our findings in the context of earlier literature studies, where the issues described in this work have often been overlooked. We also discuss formal analogies to the physics of polarity compensation at LaAlO3/SrTiO3 interfaces, and suggest promising avenues for future research.
 * <span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; line-height: normal;">Title: Band alignment at metal/ferroelectric interfaces: insights and artifacts from first principles **

===<span style="font-size: 1.1em; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 5px;">****<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"> 1. ** **<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; font-weight: normal; line-height: normal;"> ** [|arXiv:1103.0224] [ [|pdf], [|other] ]** ** ** === <span style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px;">**<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; line-height: normal;">Title: Spin-to-Orbital Angular Momentum Conversion in Semiconductor Microcavities ** <span style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px;"><span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; line-height: normal;">Author: F. Manni, K. G. Lagoudakis, T. K. Paraïso, R. Cerna, Y. Leger, T. C. H. Liew, I. A. Shelykh, A. V. Kavokin, F. Morier-Genoud, B. Deveaud-Plédran <span style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px;"><span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: 19px;">We experimentally demonstrate a technique for the generation of optical beams carrying orbital angular momentum using a planar semiconductor microcavity. Despite being isotropic systems, the transverse electric - transverse magnetic (TE-TM) polarization splitting featured by semiconductor microcavities allows for the conversion of the circular polarization of an incoming laser beam into the orbital angular momentum of the transmitted light field. The process implies the formation of topological entities, a pair of optical half-vortices, in the intracavity field.

===<span style="font-size: 1.1em; line-height: 19px; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 5px;">****<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"> 1. ** **<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;"> [|arXiv:1102.5687] [ [|pdf], [|other] ] ** ** === <span style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px;"><span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"> **Title**: Atom Loss Maximum in Ultra-cold Fermi Gases <span style="font-size: 13px; font-weight: normal; line-height: 19px; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px;"><span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; line-height: normal;">Author: Shizhong Zhang, Tin-Lun Ho <span style="font-size: 13px; font-weight: normal; line-height: 19px; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px;"><span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: 19px;">Recent experiments on atom loss in ultra-cold Fermi gases all show a maximum at a magnetic field below Feshbach resonance, where the s-wave scattering length is large (close to inter-particle distance) and positive. These experiments have been performed over a wide range of conditions, with temperatures and trap depths spanning over three decades. Different groups have come up with different explanations, among them the emergence of Stoner ferromagnetism. Here, we show that this maximum is a consequence of two major steps. The first is the establishment of a population of shallow dimers, which is the combined effect of dimer formation through three-body recombination, and the dissociation of shallow dimers back to atoms through collisions. The dissociation process will be temperature dependent, and is affected by Pauli blocking at low temperatures. The second is the relaxation of shallow dimers into tightly bound dimers through atom-dimer and dimer-dimer collisions. We have constructed a simple set of rate equations describing these processes. Remarkably, even with only a few parameters, these equations reproduce the loss rate observed in all recent experiments, despite their widely different experimental conditions. Our studies show that the location of the maximum loss rate depends crucially on experimental parameters such as trap depth and temperature. These extrinsic characters show that this maximum is not a reliable probe of the nature of the underlying quantum states. The physics of our equations also explains some general trends found in current experiments.

<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"> Feb 28
<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"><span style="font-family: arial,helvetica,sans-serif; font-size: 13px; line-height: 19px;"> ===<span style="font-size: 1.1em; margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 5px;">**<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;">**<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"> 1. ** **<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; line-height: normal;"> [|arXiv:1102.5325] [ [|pdf], [|other] ] ** ** === <span style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px;"><span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;">**<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; line-height: normal;">Title: Phase Diagram of a Frustrated Quantum Antiferromagnet on the Honeycomb Lattice: Magnetic Order versus Valence-Bond Crystal formation ** <span style="margin-bottom: 0px; margin-left: 0px; margin-right: 0px; margin-top: 0px; padding-bottom: 0px; padding-left: 0px; padding-right: 0px; padding-top: 0px;"><span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif;"><span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; line-height: normal;">Author: A.F. Albuquerque, D. Schwandt, B. Hetényi, S. Capponi, M. Mambrini, A.M. Läuchli

<span style="font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We present a comprehensive computational study of the phase diagram of the frustrated S=1/2 Heisenberg antiferromagnet on the honeycomb lattice, with second-nearest (J_2) and third-neighbor (J_3) couplings. Using a combination of exact diagonalizations of the original spin model, of the Hamiltonian projected into the nearest neighbor short range valence bond basis, and of an effective quantum dimer model, as well as a self-consistent cluster mean-field theory, we determine the boundaries of several magnetically ordered phases in the the region J_2,J_3\in [0,1], and find a sizable magnetically disordered region in between. We characterize this magnetically disordered phase as a plaquette valence bond crystal phase. Furthermore we find that a particular parameter-free Gutzwiller projected tight-binding wave function has remarkably accurate energies compared to finite-size extrapolated ED energies along the transition line from conventional N\'eel to plaquette VBC phases, a fact that points to possibly interesting critical behavior across the transition.

=Feb 21 - Feb 25, Zixu Zhang=

Title: Heavy Fermions and Quantum Phase Transitions
Authors: [|Qimiao Si], [|Frank Steglich] Comments: review article, 26 pages, 4 figures Journal-ref: Science 329, 1161 (2010) Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con); High Energy Physics - Theory (hep-th)

Quantum phase transitions arise in many-body systems due to competing interactions that promote rivaling ground states. Recent years have seen the identification of continuous quantum phase transitions, or quantum critical points, in a host of antiferromagnetic heavy-fermion compounds. Studies of the interplay between the various effects have revealed new classes of quantum critical points, and are uncovering a plethora of new quantum phases. At the same time, quantum criticality has provided fresh insights into the electronic, magnetic, and superconducting properties of the heavy-fermion metals. We review these developments, discuss the open issues, and outline some directions for future research.

Title: Fluctuations and phase transitions in Larkin-Ovchinnikov liquid crystal states of population-imbalanced resonant Fermi gas
Authors: [|Leo Radzihovsky] Comments: 56 pages, 21 figures Subjects: Quantum Gases (cond-mat.quant-gas)

Motivated by a realization of imbalanced Feshbach-resonant atomic Fermi gases, we formulate a low-energy theory of the Fulde-Ferrell and the Larkin-Ovchinnikov (LO) states and use it to analyze fluctuations, stability, and phase transitions in these enigmatic finite momentum-paired superfluids. Focusing on the unidirectional LO pair-density wave state, that spontaneously breaks the continuous rotational and translational symmetries, we show that it is characterized by two Goldstone modes, corresponding to a superfluid phase and a smectic phonon. Because of the liquid-crystalline "softness" of the latter, at finite temperature the 3d state is characterized by a vanishing LO order parameter, quasi-Bragg peaks in the structure and momentum distribution functions, and a "charge"-4, paired Cooper-pairs, off-diagonal-long-range order, with a superfluid-stiffness anisotropy that diverges near a transition into a nonsuperfluid state. In addition to conventional integer vortices and dislocations the LO superfluid smectic exhibits composite half-integer vortex-dislocation defects. A proliferation of defects leads to a rich variety of descendant states, such as the "charge"-4 superfluid and Fermi-liquid nematics and topologically ordered nonsuperfluid states, that generically intervene between the LO state and the conventional superfluid and the polarized Fermi-liquid at low and high imbalance, respectively. The fermionic sector of the LO gapless superconductor is also quite unique, exhibiting a Fermi surface of Bogoliubov quasiparticles associated with the Andreev band of states, localized on the array of the LO domain-walls.

Title: Quantum Criticality
Authors: [|Subir Sachdev], [|Bernhard Keimer] Comments: 16 pages, 7 figures Journal-ref: Physics Today vol 64, number 2, page 29, 2011 Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Superconductivity (cond-mat.supr-con); High Energy Physics - Theory (hep-th)

This is a review of the basic theoretical ideas of quantum criticality, and of their connection to numerous experiments on correlated electron compounds. A shortened, modified, and edited version appeared in Physics Today.

Title: BEC phase diagram of a $^{87}$Rb trapped gas in terms of macroscopic thermodynamic parameters
Authors: [|V Romero-Rochin], [|R F Shiozaki], [|M Caracanhas], [|E A L Henn], [|K M F Magalhaes], [|G Roati], [|V S Bagnato] Comments: 11 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We measure the phase diagram of a $^{87}$Rb Bose gas in a harmonic trap in terms of macroscopic parameters obtained from the spatial distribution of atoms. Considering the relevant variables as size of the cloud ${\cal V}$, number of atoms $N$ and temperature $T$, a novel parameter $\Pi = \Pi(N,{\cal V},T)$ is introduced to characterize the overall pressure of the system. We construct the phase diagram ($\Pi$ vs $T$) identifying new features related to Bose-Einstein condensation (BEC) transition in a trapped gas. A thermodynamic description of the phase transition based on purely macroscopic parameters, provide us with properties that do not need the local density approximation. An unexpected consequence of this analysis is the suggestion that BEC appears as a continuous third-order phase transition instead of being a second-order one.

Title: Interacting Preformed Cooper Pairs in Resonant Fermi Gases
Authors: [|K. B. Gubbels], [|H. T. C. Stoof] Comments: 8 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We consider a strongly interacting Fermi gas in its normal phase that has an equal amount of particles in each of its two accessible spin states. Due to the unitarity-limited attractive interaction between particles with different spin, noncondensed Cooper pairs are formed. The starting point in treating preformed pairs is the Nozi\`{e}res-Schmitt-Rink (NSR) theory, which approximates the pairs as being noninteracting. Here, we consider also the effects of the interactions between the Cooper pairs. Starting from the exact bosonic action for the pairs, we calculate the Cooper-pair self-energy by combining the NSR formalism with the Wilsonian renormalization group approach. We compare our findings with the recent experiments by Harikoshi {\it et al.} [Science {\bf 327}, 442 (2010)] and Nascimb\`{e}ne {\it et al.} [Nature {\bf 463}, 1057 (2010)], and find very good agreement.

Title: The ground state of the bose-hubbard model is a supersolid
Authors: [|Philip W Anderson] Subjects: Quantum Gases (cond-mat.quant-gas)

The Bose-Hubbard model is well-defined description of a Bose solid which may be realistic for cold atoms in a periodic optical lattice. We show that contrary to accepted theories it can never have as a ground state a perfect Mott insulator solid and that it has a low-energy spectrum of vortex-like phase fluctuations. Whether the ground state is necessarily commensurate remains an open question.

Title: Superconductivity mediated by the antiferromagnetic spin-wave in chalcogenide iron-base superconductors
Authors: [|G.M. Zhang], [|Z.Y. Lu], [|T. Xiang] Comments: 4 pages, 3 figures Subjects: Superconductivity (cond-mat.supr-con) ; Materials Science (cond-mat.mtrl-sci); Strongly Correlated Electrons (cond-mat.str-el)

The ground state of K$_{0.8+x}$Fe$_{1.6+y}$Se$_2$ and other iron-based selenide superconductors are doped antiferromagnetic semiconductors. There are well defined iron local moments whose energies are separated from those of conduction electrons by a large band gap in these materials. We propose that the low energy physics of this system is governed by a model Hamiltonian of interacting electrons with on-site ferromagnetic exchange interactions and inter-site superexchange interactions. We have derived the effective pairing potential of electrons under the linear spin-wave approximation and shown that the superconductivity can be driven by mediating coherent spin wave excitations in these materials. Our work provides a natural account for the coexistence of superconducting and antiferromagnetic long range orders observed by neutron scattering and other experiments.

Title: Lattice Induced Resonances in One Dimensional Bosonic Systems
Authors: [|Javier von Stecher], [|Victor Gurarie], [|Leo Radzihovsky], [|Ana Maria Rey] Comments: 4 pages, 3 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We study the resonant effects produced when a Feshbach dimer crosses a scattering continuum band of atoms in an optical lattice. We numerically obtain the exact spectrum of two particles in a one-dimensional lattice and develop an effective atom-dimer Hamiltonian that accurately captures resonant effects. The lattice-induced resonances lead to the formation of bound states simultaneously above and below the scattering continuum and significantly modify the curvature of the dimer dispersion relation. The nature of the atom-dimer coupling depends strongly on the parity of the dimer state leading to a novel coupling in the case of negative parity dimers. From the exact solutions we extract the dimer Wannier function from which we quantitatively determine the effective Hamiltonian parameters for a many-body description.

Title: Beware of the pseudogap
Authors: [|Dirk van der Marel] Comments: 2 pages, html Journal-ref: Nature Physics 7, 10 (2011) Subjects: Superconductivity (cond-mat.supr-con) ; Strongly Correlated Electrons (cond-mat.str-el)

In the pseudogap phase of a high-temperature cuprate superconductor, conflicting evidence from different experiments points to a competing state or a precursor-to-superconductivity state. One single experiment now determines that both states exist.

Title: Exact diagonalization: the Bose-Hubbard model as an example
Authors: [|J. M. Zhang], [|R. X. Dong] Journal-ref: Eur. J. Phys. 31, 591 (2010) Subjects: Statistical Mechanics (cond-mat.stat-mech) ; Mathematical Physics (math-ph); Quantum Physics (quant-ph)

We take the Bose-Hubbard model to illustrate exact diagonalization techniques in a pedagogical way. We follow the road of first generating all the basis vectors, then setting up the Hamiltonian matrix with respect to this basis, and finally using the Lanczos algorithm to solve low lying eigenstates and eigenvalues. Emphasis is placed on how to enumerate all the basis vectors and how to use the hashing trick to set up the Hamiltonian matrix or matrices corresponding to other quantities. Although our route is not necessarily the most efficient one in practice, the techniques and ideas introduced are quite general and may find use in many other problems.

Title: Gross-Pitaevskii Equation and Center of Mass Motion in Spin-Orbit Coupled Bose-Einstein Condensates
Authors: [|Yongping Zhang], [|Li Mao], [|Chuanwei Zhang] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We derive the mean-field Gross-Pitaevskii equation for spin-orbit coupled Bose-Einstein condensates by taking account that the pseudospin states of atoms are superpositions of the hyperfine states with different scattering lengths. The ground state phases of the condensate in a harmonic trap are obtained numerically in various parameter regions. We find a new oscillation period in the center of mass motion of the condensate subject to a sudden shift of the harmonic trap. The oscillation period is dependent on the direction of the shift of the harmonic trap, linearly proportional to the spin-orbit coupling strength, and independent on the interaction strength.

Title: SU(2)-invariant spin liquids on the triangular lattice with spinful Majorana excitations
Authors: [|Rudro R. Biswas], [|Liang Fu], [|Chris R. Laumann], [|Subir Sachdev] Comments: 26 pages, 11 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We describe a new class of spin liquids with global SU(2) spin rotation symmetry in spin 1/2 systems on the triangular lattice, which have real Majorana fermion excitations carrying spin S = 1. The simplest translationally-invariant mean-field state on the triangular lattice breaks time-reversal symmetry and is stable to fluctuations. It generically possesses gapless excitations along 3 Fermi lines in the Brillouin zone. These intersect at a single point where the excitations scale with a dynamic exponent z = 3. An external magnetic field has no orbital coupling to the SU(2) spin rotation-invariant fermion bilinears that can give rise to a transverse thermal conductivity, thus leading to the absence of a thermal Hall effect. The Zeeman coupling is found to gap out two-thirds of the z = 3 excitations near the intersection point and this leads to a suppression of the low temperature specific heat, the spin susceptibility and the Wilson ratio. We also compute physical properties in the presence of weak disorder and discuss possible connections to recent experiments on organic insulators.

Title: Relating Theories via Renormalization
Authors: [|Leo P. Kadanoff] Comments: This note is partially a summary of a talk given at the workshop "Part and Whole" in Leiden during the period March 22-26, 2010 Subjects: Statistical Mechanics (cond-mat.stat-mech)

The renormalization method is specifically aimed at connecting theories describing physical processes at different length scales and thereby connecting different theories in the physical sciences. The renormalization method used today is the outgrowth of one hundred and fifty years of scientific study of thermal physics and phase transitions. Different phases of matter show qualitatively different behavior separated by abrupt phase transitions. These qualitative differences seem to be present in experimentally observed condensed-matter systems. However, the "extended singularity theorem" in statistical mechanics shows that sharp changes can only occur in infinitely large systems. Abrupt changes from one phase to another are signaled by fluctuations that show correlation over infinitely long distances, and are measured by correlation functions that show algebraic decay as well as various kinds of singularities and infinities in thermodynamic derivatives and in measured system parameters. Renormalization methods were first developed in field theory to get around difficulties caused by apparent divergences at both small and large scales. The renormalization (semi-)group theory of phase transitions was put together by Kenneth G. Wilson in 1971 based upon ideas of scaling and universality developed earlier in the context of phase transitions and of couplings dependent upon spatial scale coming from field theory. Correlations among regions with fluctuations in their order underlie renormalization ideas. Wilson's theory is the first approach to phase transitions to agree with the extended singularity theorem. Some of the history of the study of these correlations and singularities is recounted, along with the history of renormalization and related concepts of scaling and universality. Applications are summarized.

Title: Quantum Spinon Oscillations
Authors: [|Zi Cai], [|Lei Wang], [|X. C. Xie], [|U. Schollwöck], [|X.R. Wang], [|M. Di Ventra], [|Yupeng Wang] Comments: 4.2 pages, 7 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) The full quantum dynamics of a spinon under external magnetic fields is investigated by using the time-evolving block decimation (TEBD) method within the microcanonical picture of transport. We show that the center of the spinon oscillates back and forth in the absence of dissipation. The quantum many-body behavior can be understood in a single-particle picture of transport and Bloch oscillations, where quantum fluctuations induce finite life times. Transport, oscillations and lifetimes can be tuned to some degree separately by external fields. Other nontrivial dynamics such as resonance as well as chaos have also been discussed.

Title: Local atom number fluctuations in quantum gases at finite temperature
Authors: [|M. Klawunn], [|A. Recati], [|L. P. Pitaevskii], [|S. Stringari] Comments: 5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We investigate the number fluctuations in small cells of quantum gases pointing out important deviations from the thermodynamic limit fixed by the isothermal compressibility. Both quantum and thermal fluctuations in weakly as well as highly compressible fluids are considered. For the 2D superfluid Bose gas we find a significant quenching of fluctuations with respect to the thermodynamic limit, in agreement with recent experimental findings. An enhancement of the thermal fluctuations is instead predicted for the 2D dipolar superfluid Bose gas, which becomes dramatic when the size of the sample cell is of the order of the wavelength of the rotonic excitation induced by the interaction.

Title: Ground state properties of the one-dimensional electron liquid
Authors: [|R. M. Lee], [|N. D. Drummond] Comments: 13 pages, 16 figures, 5 tables Subjects: Strongly Correlated Electrons (cond-mat.str-el) We present calculations of the energy, pair correlation function (PCF), static structure factor (SSF), and momentum density (MD) for the one-dimensional electron gas using the quantum Monte Carlo method. We are able to resolve peaks in the SSF at even-integer-multiples of the Fermi wave vector, which grow as the coupling is increased. Our MD results show an increase in the effective Fermi wave vector as the interaction strength is raised in the paramagnetic harmonic wire; this appears to be a result of the vanishing difference between the wave functions of the paramagnetic and ferromagnetic systems. We have extracted the Luttinger liquid exponent from our MDs by fitting to data around the Fermi wave vector, finding good agreement between the exponent of the ferromagnetic infinitely-thin wire and the ferromagnetic harmonic wire.

Title: Coherent light scattering from a two-dimensional Mott insulator
Authors: [|Christof Weitenberg], [|Peter Schauß], [|Takeshi Fukuhara], [|Marc Cheneau], [|Manuel Endres], [|Immanuel Bloch], [|Stefan Kuhr] Comments: 4 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph)

We experimentally demonstrate coherent light scattering from an atomic Mott insulator in a two-dimensional lattice. The far-field diffraction pattern of small clouds of a few hundred atoms was imaged while simultaneously laser cooling the atoms with the probe beams. We describe the position of the diffraction peaks and the scaling of the peak parameters by a simple analytic model. In contrast to Bragg scattering, scattering from a single plane yields diffraction peaks for any incidence angle. We demonstrate the feasibility of detecting spin correlations via light scattering by artificially creating a one-dimensional antiferromagnetic order as a density wave and observing the appearance of additional diffraction peaks.

===7. [|arXiv:1102.3881] (cross-list from math-ph) [[|pdf], [|ps], [|other]] ===

Title: The ground state construction of the two-dimensional Hubbard model on the honeycomb lattice
Authors: [|Alessandro Giuliani] Comments: 61 pages, 8 figures, lecture notes of a mini-course delivered at the Les Houches summer school 2010 "Quantum Theory: from small to large scales" Subjects: Mathematical Physics (math-ph) ; Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el)

In these lectures I consider the half-filled two-dimensional (2D) Hubbard model on the honeycomb lattice and I review the rigorous construction of its ground state properties by making use of constructive fermionic Renormalization Group methods.

=Feb 14 - Feb 18, Chungwei Lin=

T itle: Z2-vortex order of frustrated Heisenberg antiferromagnets in two dimensions
Authors: [|Hikaru Kawamura] Comments: Proceedings of the International Conference on Frustration in Condensed Matter (ICFCM) Subjects: Materials Science (cond-mat.mtrl-sci)

We discuss the recent experimental data on various frustrated quasi-two-dimensional Heisenberg antiferromagnets from the viewpoint of the Z2-vortex order, which include S=3/2 triangular-lattice antiferromagnet NaCrO2, S=1 triangular-lattice antiferromagnet NiGa2S4, S=1/2 organic triangular-lattice antiferromagnets \kappa-(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, and S=1/2 kagome-lattice antiferromagnet volborthite Cu3V2O7(OH)22H2O, etc.

Title: Thermodynamics of the frustrated one-dimensional spin-1/2 Heisenberg ferromagnet in a magnetic field
Authors: [|M. Härtel], [|J. Richter], [|D. Ihle] Comments: 9 pages, 9 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We calculate the low-temperature thermodynamic quantities (magnetization, correlation functions, transverse and longitudinal correlation lengths, spin susceptibility, and specific heat) of the frustrated one-dimensional spin-half J1-J2 Heisenberg ferromagnet, i.e. for J2< 0.25|J1|, in an external magnetic field using a second-order Green-function formalism and full diagonalization of finite systems. We determine power-law relations for the field dependence of the position and the height of the maximum of the uniform susceptibility. Considering the specific heat at low magnetic fields, two maxima in its temperature dependence are found.

Title: Ferromagnetic instability and finite-temperature properties of two-dimensional electron systems with van Hove singularities
Authors: [|A. A. Katanin], [|H. Yamase], [|V. Yu. Irkhin] Comments: 9 pages, 3 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study a ferromagnetic tendency in the two-dimensional Hubbard model near van Hove filling by using a functional renormalization-group method. We compute temperature dependences of magnetic susceptibilities including incommensurate magnetism. The ferromagnetic tendency is found to occur in a dome-shaped region around van Hove filling with an asymmetric property: incommensurate magnetism is favored near the edge of the dome above van Hove filling whereas a first-order-like transition to the ferromagnetic ground state is expected below van Hove filling. The dome-shaped phase diagram is well captured in the Stoner theory by invoking a smaller Coulomb interaction. Triplet p-wave superconductivity tends to develop at low temperatures inside the dome and extends more than the ferromagnetic region above van Hove filling.

Title: Anomalous optical phonons in FeTe pnictides: spin state, magnetic order, and lattice anharmonicity
Authors: [|V. Gnezdilov], [|Yu. Pashkevich], [|P. Lemmens], [|A. Gusev], [|K. Lamonova], [|I. Vitebskiy], [|O. Afanasiev], [|S. Gnatchenko], [|V. Tsurkan], [|J. Deisenhofer], [|A. Loidl] Comments: 17 pages, 3 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Superconductivity (cond-mat.supr-con)

Polarized Raman-scattering spectra of non-superconducting, single-crystalline FeTe are investigated as function of temperature. We have found a relation between the magnitude of ordered magnetic moments and the linewidth of A1g phonons at low temperatures. This relation is attributed to the intermediate spin state (S=1) and the orbital degeneracy of the Fe ions. Spin-phonon coupling constants have been estimated based on microscopic modeling using density-functional theory and analysis of the local spin density. Our observations show the importance of orbital degrees of freedom for the Fe-based superconductors with large ordered magnetic moments, while small magnetic moment of Fe ions in some iron pnictides reflects the low spin state of Fe ions in those systems.

Title: Magnetic and superconducting instabilities in a hybrid model of itinerant/localized electrons for iron pnictides
Authors: [|Yi-Zhuang You], [|Fang Yang], [|Su-Peng Kou], [|Zheng-Yu Weng] Comments: 11 pages, 8 figures Subjects: Superconductivity (cond-mat.supr-con)

We study a unified mechanism for spin-density-wave (SDW) and superconductivity in a minimal model, in which itinerant electrons and local moments coexist as previously proposed for the iron pnictides [EPL, 88, 17010 (2009)]. The phase diagram obtained at the mean field level is in qualitative agreement with the experiment, which shows how the magnetic and superconducting (SC) instabilities are driven by the critial coupling between the itinerant/localized electrons. The spin and charge response functions at the random phase approximation (RPA) level further characterize the dynamical evolution of the system. In particular, the dynamic spin susceptibility displays a Goldstone mode in the SDW phase, which evolves into a gapped resonance-like mode in the superconducting phase. The latter persists all the way into the normal state above Tc, where a strong scattering between the itinerant electrons and local moments is restored, as an essential feature of the model.

Title: Synthetic magnetic field effects on neutral bosonic condensates in quasi three-dimensional anisotropic layered structures
Authors: [|T. A. Zaleski], [|T. P. Polak] Comments: [|this http URL] Journal-ref: Phys. Rev. A 83, 023607 (2011) Subjects: Quantum Gases (cond-mat.quant-gas)

We discuss a system of dilute Bose gas confined in a layered structure of stacked square lattices (slab geometry). A derived phase diagram reveals a non-monotonic dependence of the ratio of tunneling to on-site repulsion on the artificial magnetic field applied to the system. The effect is reduced when more layers are added, which mimics a two- to quasi-three-dimensional geometry crossover. Furthermore, we establish a correspondence between anisotropic infinite (quasi three-dimensional) and isotropic finite (slab geometry) systems that share exactly the same critical values, which can be an important clue for choosing experimental setups that are less demanding, but still leading to the identical results. Finally, we show that the properties of the ideal Bose gas in a three-dimensional optical lattice can be closely mimicked by finite (slab) systems, when the number of two-dimensional layers is larger than ten for isotropic interactions or even less, when the layers are weakly coupled.

Title: Thermodynamics of Trapped Imbalanced Fermi Gases at Unitarity
Authors: [|J. M. Diederix], [|H. T. C. Stoof] Comments: 26 pages, 7 figures, 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)

We present a theory for the low-temperature properties of a resonantly interacting Fermi mixture in a trap, that goes beyond the local-density approximation. The theory corresponds essentially to a Landau-Ginzburg-like approach that includes self-energy effects to account for the strong interactions at unitarity. We show diagrammatically how these self-energy effects arise from fluctuations in the superfluid order parameter. Gradient terms of the order parameter are included to account for inhomogeneities. This approach incorporates the state-of-the-art knowledge of the homogeneous mixture with a population imbalance exactly and gives good agreement with the experimental density profiles of Shin et al. [Nature 451, 689 (2008)]. This allows us to calculate the universal surface tension of the interface between the equal-density superfluid and the partially polarized normal state of the mixture. We also discuss the possibility of a metastable state to explain the deformation of the superfluid core that is seen in the experiment of Partridge et al. [Science 311, 503 (2006)].

Title: Microscopic derivation of the Ginzburg-Landau equations for coexistent states of superconductivity and magnetism
Authors: [|Kazuhiro Kuboki], [|Keiji Yano] Comments: 22 pages, 4 figures Subjects: Superconductivity (cond-mat.supr-con)

Ginzburg-Landau (GL) equations for coexistent states of superconductivity and magnetism are derived microscopically from the extended Hubbard model with on-site repulsive and nearest-neighbor attractive interactions. In the derived GL free energy the cubic term exists which couples spin-singlet and spin-triplet components of superconducting order parameters (SCOP) with magnetization. This term gives rise to a spin-triplet SCOP near the interface between a spin-singlet superconductor and a ferromagnet, consistent with previous theoretical studies based on the Bogoliubov de Gennes method and the quasiclassical Green's function theory. In coexistent states of singlet superconductivity and antiferromagnetism it leads to the occurrence of $\pi$-triplet SCOPs. Using the derived GL free energy the interface states of bilayer systems composed of a ferromagnet and a superconductor are studied numerically, and the materials dependence of these states is discussed.

Title: Bose-glass, superfluid, and rung-Mott phases of hard-core bosons in disordered two-leg ladders
Authors: [|Juan Carrasquilla], [|Federico Becca], [|Michele Fabrizio] Comments: 8 pages, 7 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

By means of Monte Carlo techniques, we study the role of disorder on a system of hard-core bosons in a two-leg ladder with both intra-chain ($t$) and inter-chain ($t^\prime$) hoppings. We find that the phase diagram as a function of the boson density, disorder strength, and $t^\prime/t$ is far from being trivial. This contrasts the case of spin-less fermions where standard localization arguments apply and an Anderson-localized phase pervades the whole phase diagram. A compressible Bose-glass phase always intrudes between the Mott insulator with zero (or one) bosons per site and the superfluid that is stabilized for weak disorder. At half filling, there is a direct transition between a (gapped) rung-Mott insulator and a Bose glass, which is driven by exponentially rare regions where disorder is suppressed. Finally, by doping the rung-Mott insulator, a direct transition to the superfluid is possible only in the clean system, whereas the Mott phase is always surrounded by the a Bose glass when disorder is present. The phase diagram based on our numerical evidence is finally reported.

Title: Time-averaged adiabatic ring potential for ultracold atoms
Authors: [|B. E. Sherlock], [|M. Gildemeister], [|E. Owen], [|E. Nugent], [|C. J. Foot] Subjects: Quantum Gases (cond-mat.quant-gas)

We report the experimental realisation of a versatile ring trap for ultracold atoms. The ring geometry is created by the time-averaged adiabatic potential resulting from the application of an oscillating magnetic bias field to a rf dressed quadrupole trap. Lifetimes for a Bose-Einstein condensate in the ring exceed 11s and the ring radius was continuously varied from 50$\mu$m to 261$\mu$m. An efficient method of loading the ring from a conventional TOP trap is presented together with a rotation scheme which introduces angular momentum into the system. The ring presents an opportunity to study the superfluid properties of a condensate in a multiply connected geometry and also has applications for matter-wave interferometry.

Title: Strong-coupling topological Josephson effect in quantum wires
Authors: [|Flavio S. Nogueira], [|Ilya Eremin] Comments: 4.2 pages, 2 figures Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Superconductivity (cond-mat.supr-con)

the Josephson effect for a setup with two lattice quantum wires featuring Majorana zero energy boundary modes at the tunnel junction. In the weak-coupling, the exact solution reproduces the perturbative result for the energy containing a contribution $\sim \pm\cos(\phi/2)$ relative to the tunneling of paired Majorana fermions. As the tunnel amplitude $g$ grows relative to the hopping amplitude $w$, the gap between the energy levels gradually diminishes until it closes completely at the critical value $g_c=\sqrt{2}w$. At this point the Josephson energies are $E_{m\sigma}=2\sigma\sqrt{2}w\cos(\phi/6-\pi m/3)$, where $m=-1,0,1$ and $\sigma=\pm 1$, which is very different from the result obtained at weak-coupling. It represents the merging of three Bogolyubov states, leading to additional degeneracies and a topologically nontrivial ground state.

Title: Spin dynamics in the strong spin-orbit coupling regime
Authors: [|Xin Liu], [|Xiong-Jun Liu], [|Jairo Sinova] Comments: 8 pages, 9 figures Subjects: Materials Science (cond-mat.mtrl-sci)

We study the spin dynamics in a high-mobility two dimensional electron gas (2DEG) with generic spin-orbit interactions (SOIs). We derive a set of spin dynamic equations which capture the purely exponential to the damped oscillatory spin evolution modes observed in different regimes of SOI strength. Hence we provide a full treatment of the D'yakonov-Perel's mechanism by using the microscopic linear response theory from the weak to the strong SOI limit. We show that the damped oscillatory modes appear when the electron scattering time is larger than half of the spin precession time due to the SOI, in agreement with recent observations. We propose a new way to measure the scattering time and the relative strength of Rashba and linear Dresselhaus SOIs based on these modes and optical grating experiments. We discuss the physical interpretation of each of these modes in the context of Rabi oscillation.

Title: Magnetic dipolar ordering and quantum phase transition in Fe8 molecular magnet
Authors: [|E. Burzurí], [|F. Luis], [|O. Montero], [|J. Campo], [|B. Barbara], [|R. Ballou], [|E. Ressouche], [|S. Maegawa] Comments: 5 pages 4 figures Subjects: Materials Science (cond-mat.mtrl-sci)

We show that a crystal of mesoscopic Fe8 single molecule magnets is an experimental realization of the Quantum Ising Phase Transition (QIPT) model in a transverse field, with dipolar interactions. Quantum annealing has enabled us to explore the QIPT at thermodynamical equilibrium. The phase diagram and critical exponents we obtain are compared to expectations for the mean-field QIPT Universality class.

Title: BEC-BCS crossover in a p+ip-wave pairing Hamiltonian coupled to bosonic molecular pairs
Authors: [|Clare Dunning], [|Phillip S. Isaac], [|Jon Links], [|Shao-You Zhao] Comments: 30 pages Subjects: Quantum Gases (cond-mat.quant-gas) ; Mathematical Physics (math-ph); Exactly Solvable and Integrable Systems (nlin.SI)

We analyse a p+ip-wave pairing BCS Hamiltonian, coupled to a single bosonic degree of freedom representing a molecular condensate, and investigate the nature of the BEC-BCS crossover for this system. For a suitable restriction on the coupling parameters, we show that the model is integrable and we derive the exact solution by the algebraic Bethe ansatz. In this manner we also obtain explicit formulae for correlation functions and compute these for several cases. We find that the crossover between the BEC state and the strong pairing p+ip phase is smooth for this model, with no intermediate quantum phase transition.

Title: p-wave superconductivity on monolayer and bilayer honeycomb lattice
Authors: [|M.V. Milovanović] Comments: 16 pages, 1 figure Subjects: Superconductivity (cond-mat.supr-con) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We derive ground state wave functions of superconducting instabilities on the honeycomb lattice induced by nearest-neighbor attractive interactions. They reflect the Dirac nature of electrons in the low-energy limit. For the order parameter that is the same irrespective of the direction to any of the nearest neighbors we find weak pairing (slowly decaying) behavior in the orbital part of the Cooper pair with no angular dependence. At the neutrality point, in the spin-singlet case, we recover a strong pairing behavior. We also derive ground state wave functions for the superconductivity on the bilayer honeycomb lattice, with strong interlayer coupling, induced by attractive interactions between sites that participate in a low-energy description. Without these interactions, free electrons are described by a Dirac equation with a quadratic dispersion. This unusual feature, similarly to $^3$He - B phase, leads to the description with two kinds of Cooper pairs, with $p_x + i p_y$ and $p_x - i p_y$ pairing, in the presence of the attractive interactions. We discuss the edge modes of such a spin-singlet superconductor and find that it represents a trivial topological superconductor.

Title: Fractional quantum Hall effect in the absence of Landau levels
Authors: [|D. N. Sheng], [|Zheng-Cheng Gu], [|Kai Sun], [|L. Sheng] Comments: 6 pages and 4 figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

It has been well-known that topological phenomena with fractional excitations, i.e., the fractional quantum Hall effect (FQHE)~\cite{Tsui1982} will emerge when electrons move in Landau levels. In this letter, we report the discovery of the FQHE in the absence of Landau levels in an interacting fermion model. The non-interacting part of our Hamiltonian is the recently proposed topologically nontrivial flat band model on the checkerboard lattice~\cite{sun}. In the presence of nearest-neighboring repulsion ($U$), we find that at $1/3$ filling, the Fermi-liquid state is unstable towards FQHE. At $1/5$ filling, however, a next-nearest-neighboring repulsion is needed for the occurrence of the $1/5$ FQHE when $U$ is not too strong. We demonstrate the characteristic features of these novel states and determine the phase diagram correspondingly.

Title: Pairing-gap, pseudo-gap, and no-gap phases in rf spectra of a trapped unitary 6Li gas
Authors: [|P. Pieri], [|A. Perali], [|G. C. Strinati], [|S. Riedl], [|M. J. Wright], [|A. Altmeyer], [|C. Kohstall], [|E. R. Sanchez Guajardo], [|J. Hecker Denschlag], [|R. Grimm] Comments: 6 pages, 7 figures Subjects: Quantum Gases (cond-mat.quant-gas)

Radio frequency spectra of a trapped unitary 6Li gas are reported and analyzed in terms of a theoretical approach that includes both final-state and trap effects. Final-state effects play a crucial role in evidencing two main peaks both above and below the critical temperature Tc as being associated with two distinct phases that reside in different trap regions. These are the pairing-gap and pseudo-gap phases below Tc, which evolve into the pseudo-gap and no-gap phases above Tc. In this way, a long standing puzzle about the interpretation of rf spectra for 6Li in a trap is solved.

Title: Quasiparticle states around a nonmagnetic impurity in electron-doped iron-based superconductors with spin-density-wave order
Authors: [|Tao Zhou], [|Huaixiang Huang], [|Yi Gao], [|Jian-Xin Zhu], [|C. S. Ting] Comments: 11 pages, 18 figures Subjects: Superconductivity (cond-mat.supr-con)

The quasiparticle states around a nonmagnetic impurity in electron-doped iron-based superconductors with spin-density-wave (SDW) order are investigated as a function of doping and impurity scattering strength. In the undoped sample, where a pure SDW state exists, two impurity-induced resonance peaks are observed around the impurity site and they are shifted to higher (lower) energies as the strength of the positive (negative) scattering potential (SP) is increased. For the doped samples where the SDW order and the superconducting order coexist, the main feature is the existence of sharp in-gap resonance peaks whose positions and intensity depend on the strength of the SP and the doping concentration. In all cases, the local density of states exhibits clear $C_2$ symmetry. We also note that in the doped cases, the impurity will divide the system into two sublattices with distinct values of magnetic order. Here we use the band structure of a two-orbital model, which considers the asymmetry of the As atoms above and below the Fe-Fe plane. This model is suitable to study the properties of the surface layers in the iron-pnictides and should be more appropriate to describe the scanning tunneling microscopy experiments.

Title: Chiral spin states in polarized kagome spin systems with spin-orbit coupling
Authors: [|Jia-Wei Mei], [|Evelyn Tang], [|Xiao-Gang Wen] Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study quantum spin systems with a proper combination of geometric frustration, spin-orbit coupling and ferromagnetism. We argue that such a system is likely to be in a chiral spin state, a fractional quantum Hall (FQH) state for bosonic spin degrees of freedom. The energy scale of the bosonic FQH state is of the same order as the spin-orbit coupling and ferromagnetism --- overall much higher than the energy scale of FQH states in semiconductors.

=Feb 7 - Feb 11, Xiaopeng Li=

Feb 11
Author: Bhaskar Kamble, Avinash Singh The correlated motion of electrons in multi-orbital metallic ferromagnets is investigated in terms of a realistic Hubbard model with {\cal N}-fold orbital degeneracy and arbitrary intra- and inter-orbital Coulomb interactions U and J using a Goldstone-mode-preserving non-perturbative scheme. An effective quantum parameter '\hbar'=\frac{U^2+({\cal N}-1)J^2}{(U+({\cal N}-1)J)^2} is obtained which determines, in analogy with 1/S for quantum spin systems and 1/N for the N-orbital Hubbard model, the strength of correlation-induced quantum corrections to magnetic excitations. The rapid suppression of this quantum parameter with Hund's coupling J, especially for large {\cal N}, provides fundamental insight into the phenomenon of strong stabilization of metallic ferromagnetism by orbital degeneracy and Hund's coupling. This approach is illustrated for the case of ferromagnetic iron and the half metallic Heusler alloy Co_2 Mn Si. For realistic values for iron, the calculated spin stiffness and Curie temperature values obtained are in quantitative agreement with measurements. Significantly, the contribution of long wavelength modes is shown to yield a nearly ~25% reduction in the calculated Curie temperature. Finally, an outline is presented for extending the approach to generic multi-band metallic ferromagnets including realistic band-structure features of non-degenerate orbitals and inter-orbital hopping as obtained from LDA calculations.
 * 1. [|arXiv:1102.2115] [[|pdf], [|ps], [|other]] **
 * Title: An effective quantum parameter for strongly correlated metallic ferromagnets**

Author: Onofre Rojas, S. M. de Souza In this article we propose a general transformation for decorated spin models. The advantage of this transformation is to perform a direct mapping of a decorated spin model onto another effective spin thus symplifying algebraic computations by avoiding the proliferation of unnecessary iterative transformations and parameters that might otherwise lead to transcedental equations.
 * 2. [|arXiv:1102.2200] [[|pdf], [|ps], [|other]] **
 * Title: Direct algebraic decoration transformation for spin models**

Feb 10
Authors: Michael Sentef, Philipp Werner, Emanuel Gull, Arno P. Kampf The two-dimensional Hubbard model exhibits superconductivity with d-wave symmetry even at half-filling in the presence of next-nearest neighbor hopping. Using plaquette cluster dynamical mean-field theory with a continuous-time quantum Monte Carlo impurity solver, we reveal the non-Fermi liquid character of the metallic phase in proximity to the superconducting state. Specifically, the low-frequency scattering rate for momenta near (\pi, 0) varies non-monotonously at low temperatures, and the dc conductivity is T-linear at elevated temperatures with an upturn upon cooling. Evidence is provided that pairing fluctuations dominate the normal-conducting state even considerably above the superconducting transition temperature.
 * 1. [|arXiv:1102.1911] [[|pdf], [|ps], [|other]] **
 * Title: Superconductivity and Pairing Fluctuations in the Half-Filled Two-Dimensional Hubbard Model**

Feb 9
Authors: [|L. Huijse] We present a full identification of lattice model properties with their field theoretical counter parts in the continuum limit for a supersymmetric model for itinerant spinless fermions on a one dimensional chain. The continuum limit of this model is described by an $\mathcal{N}=(2,2)$ superconformal field theory (SCFT) with central charge c=1. We identify states and operators in the lattice model with fields in the SCFT and we relate boundary conditions on the lattice to sectors in the field theory. We use the dictionary we develop in this paper, to give a pedagogical explanation of a powerful tool to study supersymmetric models based on spectral flow. Finally, we employ the developed machinery to explain numerically observed properties of the particle density on the open chain presented in Beccaria et al. PRL 94:100401 (2005).
 * 1. [|arXiv:1102.1700] [[|pdf], [|other]] **
 * Title: Detailed analysis of the continuum limit of a supersymmetric lattice model in 1D**

Feb 8
Authors: Masatoshi Sato, Yukio Tanaka, Keiji Yada, Takehito Yokoyama Abstract: A theory of dispersionless Andreev bound states on surfaces of time-reversal invariant unconventional superconductors is presented. The generalized criterion for the dispersionless Andreev bound state is derived from the bulk-edge correspondence, and the chiral spin structure of the dispersionless Andreev bound states is argued from which the Andreev bound state is stabilized. Then we summarize the criterion in a form of index theorems. The index theorems are proved in a general framework to certify the bulk-edge correspondence. As concrete examples, we discuss (i) dxy-wave superconductor (ii) px-wave superconductor, and (iii) noncentrosymmetric superconductors. In the last example, we find a peculiar time-reversal invariant Majorana fermion. The time-reversal invariant Majorana fermion shows an unusual response to the Zeeman magnetic field, which can be used to identify it experimentally.
 * 1. [|arXiv:1102.1322] [[|pdf], [|ps], [|other]] **
 * Title : Topology of Andreev Bound States with Flat Dispersion**


 * 2. [|arXiv:1102.1318] [[|pdf], [|ps], [|other]] **

Authors: [|N. D. Drummond], [|N. R. Cooper], [|R. J. Needs], [|G. V. Shlyapnikov] Motivated by potential realizations in cold-atom or cold-molecule systems, we have performed quantum Monte Carlo simulations of two-component gases of fermions in two dimensions with hard-core interactions. We have determined the gross features of the zero-temperature phase diagram, by investigating the relative stabilities of paramagnetic and ferromagnetic fluids and crystals. We have also examined the effect of including a pairwise, long-range r^(-3) potential between the particles. Our most important conclusion is that there is no region of stability for a ferromagnetic fluid phase, even if the long-range interaction is present. We also present results for the pair-correlation function, static structure factor, and momentum density of two-dimensional hard-core fluids.
 * Title: Quantum Monte Carlo calculation of the zero-temperature phase diagram of the two-component fermionic hard-core gas in two dimensions**

Feb 7
1. [|arXiv:1102.0997] [[|pdf], [|ps], [|other]] Title: Spin Transport in Cold Fermi gases: A Pseudogap Interpretation of Spin Diffusion Experiments at Unitarity Authors: [|Dan Wulin], [|Hao Guo], [|Chih-Chun Chien], [|Kathryn Levin] We address recent spin transport experiments in ultracold unitary Fermi gases. We provide a theoretical understanding for how the measured temperature dependence of the spin diffusivity at low $T$ can disagree with the expected behavior of a Fermi liquid (FL) while the spin susceptiblity(following the experimental protocols) is consistent with a Fermi liquid picture. We show that the experimental protocols for extracting $\chi_s$ are based on a FL presumption; relaxing this leads to consistency within (but not proof of) a pseudogap-based approach. Our tranport calculations yield insight into the measured strong suppression of the spin diffusion constant at lower $T$.