Jul 2011

July 25 - 29, Xiaopeng Li
Jul 29
1. arXiv:1107.5703 [pdf, ps, other]
Title: Quantum information transfer between topological and spin qubit systems
Authors: Martin Leijnse, Karsten Flensberg
Comments: 4+ pages, 2 figures
Subjects: Superconductivity (cond-mat.supr-con); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
We propose a method to coherently transfer quantum information, and to create entanglement, between topological qubits and conventional spin qubits. Our suggestion uses gated control to transfer an electron (spin qubit) between a quantum dot and edge Majorana modes in adjacent topological superconductors. Because of the spin polarization of the Majorana modes, the electron transfer translates spin superposition states into superposition states of the Majorana system, and vice versa. Furthermore, we show how a topological superconductor can be used to facilitate long-distance quantum information transfer and entanglement between spatially separated spin qubits.
Jul 28
1. arXiv:1107.5561 [pdf, ps, other]
Title: Many-particle correlations in non-equilibrium Luttinger liquid
Authors: I.V. Protopopov, D.B. Gutman, A.D. Mirlin
(Submitted on 27 Jul 2011)
Abstract: We develop an operator-based approach to the problem of Luttinger liquid conductor in a non-equilibrium stationary state. We show that the coherent-state many-body fermionic density matrix as well as all fermionic correlation functions out of equilibrium are given by one-dimensional functional determinants of the Fredholm type. Thus, the model constitutes a remarkable example of a many-body problem where all the correlation functions can be evaluated exactly. On the basis of the general formalism we investigate four-point correlation functions of the fermions coming out of the Luttinger liquid wire. Obtained correlations in the fermionic distribution functions represent the combined effect of interaction and non-equilibrium conditions.

2. arXiv:1107.5521 [pdf, ps, other]
Title: A constrained theory of non-BCS type superconductivity in gapped Graphene
Authors: Vivek M. Vyas, Prasanta K. Panigrahi
Abstract: We show that gapped Graphene, with a local constraint that current arising from the two valley fermions are exactly equal, shows a non-BCS type superconductivity. Unlike the conventional mechanisms, this superconductivity phenomenon does not require any pairing. We estimate the critical temperature for superconducting-to-normal transition via Berezinskii-Kosterlitz-Thouless mechanism, and find that it is proportional to the gap.

Jul 27
1. arXiv:1107.5272 [pdf, ps, other]
Title: Possible Cooper instabilities in pair Green functions of the two-dimensional Hubbard model
Authors: André LeClair
Abstract: In analogy with ordinary BCS superconductivity, we identify possible pairing instabilities with poles in a certain class of Green functions of the two-dimensional repulsive Hubbard model, which normally signify bound states. Relating the gap to the location of these poles, we find that these instabilities exist in the range of hole doping between 0 and 0.24. The magnitude of the gap can be calculated without introducing an explicit cut-off, and for reasonably large coupling U, the maximum gap is on the order of 0.01 in units of the hopping parameter t.

2. arXiv:1107.5258 [pdf, ps, other]
Title: Diagrammatics for SU(2) invariant matrix product states
Authors: Andreas Fledderjohann, Andreas Klümper, Karl-Heinz Mütter
Abstract: We report on a systematic implementation of su(2) invariance for matrix product states (MPS) with concrete computations cast in a diagrammatic language. As an application we present a variational MPS study of $su(2)$ invariant quantum spin systems. For efficient computations we make systematic use of the su(2) symmetry at all steps of the calculations: (i) the matrix space is set up as a direct sum of irreducible representations, (ii) the local matrices with state-valued entries are set up as superposition of su(2) singlet operators, (iii) products of operators are evaluated algebraically by making use of identities for $3j$ and $6j$ symbols. The remaining numerical computations like the diagonalization of the associated transfer matrix and the minimization of the energy expectation value are done in spaces free of symmetry degeneracies. The energy expectation value is a strict upper bound of the true ground-state energy and yields definite conclusions about the accuracy of DMRG results reported in the literature. Furthermore, we present explicit results with accuracy better than $10^{-4}$ for nearest- and next-nearest neighbour spin correlators and for general dimer-dimer correlators in the thermodynamical limit of the spin-1/2 Heisenberg chain with frustration.

3. arXiv:1107.5233 (cross-list from quant-ph) [pdf, other]Title: Quantum Simulation of Quantum Field Theories in Trapped Ions
Authors: J. Casanova, L. Lamata, I. L. Egusquiza, R. Gerritsma, C. F. Roos, J. J. Garcia-Ripoll, E. Solano
Abstract: We propose the quantum simulation of a fermion and an antifermion field modes interacting via a bosonic field mode, and present a possible implementation with two trapped ions. This quantum platform allows for the scalable add-up of bosonic and fermionic modes, and represents an avenue towards quantum simulations of quantum field theories in perturbative and nonperturbative regimes.


Jul 26
1. arXiv:1107.4858 [pdf]
Title: SUPERSTRIPES Self organization of quantum wires in high Tc superconductors
Authors:A. Bianconi, D. Di Castro, N. L. Saini, G. Bianconi
We report experimental evidence for the phase diagram of doped cuprate superconductors as a function of the micro-strain (e) of the Cu-O bond length, measured by Cu K-edge EXAFS, and hole doping (d). This phase diagram shows a QCP at P(e*,d*) where for the micro-strain e larger than the critical value e* charge-orbital-spin stripes and free carriers co-exist. The superconducting phase occurs in the region of critical fluctuations around this QCP. The critical temperature is function of two variables and Tc shows its maximum at the strain driven QCP. The critical fluctuations near this strain QCP give the self-organization of a metallic superlattice of quantum wires "superstripes" that favors the amplification of the critical temperature.
2. arXiv:1107.4750 [pdf, ps, other]
Title: Tunable Strongly Correlated Band Insulator
Authors: V.V. Cheianov, I.L. Aleiner, V.I. Fal'ko
Abstract: We introduce the notion of the strongly correlated band insulator (SCI), where the lowest energy excitations are collective modes (excitons) rather than the single particles. We construct controllable 1/N expansion for SCI to describe their observables properties. A remarkable example of the SCI is bilayer graphene which is shown to be tunable between the SCI and usual weak coupling regime.

Jul 25
1. arXiv:1107.4403 [pdf, other]
Title: Frequency domain winding number and interaction effect on topological insulators
Authors: Lei Wang, Xi Dai, X. C. Xie
Abstract: We study the effect of interactions on the time reversal invariant topological insulators in four and three spatial dimensions. Their topological indices are expressed by the interacting Green's functions. Under the local self-energy approximation, we find that interaction could induce nontrivial frequency-domain winding numbers and change the topological classes of the system. Our results suggest that the topological phases could be destroyed without developing long range orders. Practical issues on the accurate frequency-momentum integration combined with DMFT and diagrammatic calculations of the interacting Green's functions are also addressed.

2. arXiv:1107.4397 (cross-list from hep-th) [pdf, ps, other]
Holographic Fermionic Fixed Points in d=3
Authors: Joshua L. Davis, Hamid Omid, Gordon W. Semenoff
Abstract: We present a top-down string theory holographic model of strongly interacting relativistic 2+1-dimensional fermions, paying careful attention to the discrete symmetries of parity and time reversal invariance. Our construction is based on probe $D7$-branes in $AdS_5 \times S^5$, stabilized by internal fluxes. We find three solutions, a parity and time reversal invariant conformal field theory which can be viewed as a particular deformation of Coulomb interacting graphene, a parity and time reversal violating but gapless field theory and a system with a parity and time reversal violating charge gap. We show that the Chern-Simons-like electric response function, which is generated perturbatively at one-loop order by parity violating fermions and which is protected by a no-renormalization theorem at orders beyond one loop, indeed appears with the correctly quantized coefficient in the charge gapped theory. In the gapless parity violating solution, the Chern-Simons response function obtains quantum corrections which we compute in the holographic theory.


July 18 - 22, Zixu Zhang

July 22
1. arXiv:1107.4116 [pdf, ps, other]
Title: Equilibration of a spinless Luttinger liquid
Authors:K. A. Matveev, A. V. Andreev
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Gases (cond-mat.quant-gas)

We study how a Luttinger liquid of spinless particles in one dimension approaches thermal equilibrium. Full equilibration requires processes of backscattering of excitations which occur at energies of order of the bandwidth. Such processes are not accounted for by the Luttinger liquid theory. We treat the high-energy excitations as mobile impurities and derive an expression for the equilibration rate in terms of their spectrum. Our results apply at any interaction strength.

2. arXiv:1107.4125 [pdf, ps, other]
Title: Decohering the Fermi liquid: A dual approach to the Mott Transition
Authors:David F. Mross, T. Senthil
Comments: 10 pages, 2 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We present a theoretical approach to describing the Mott transition of electrons on a two dimensional lattice that begins with the low energy effective theory of the Fermi liquid. The approach to the Mott transition must be characterized by the suppression of density and current fluctuations which correspond to specific shape deformations of the Fermi surface. We explore the nature of the Mott insulator and the corresponding Mott transition when these shape fluctuations of the Fermi surface are suppressed without making any a prior assumptions about other Fermi surface shape fluctuations. Building on insights from the theory of the Mott transition of bosons, we implement this suppression by identifying and condensing vortex degrees of freedom in the phase of the low energy electron operator. We show that the resulting Mott insulator is a quantum spin liquid with a spinon fermi surface coupled to a U(1) gauge field which is usually described within a slave particle formulation. Our approach thus provides a coarse-grained treatment of the Mott transition and the proximate spin liquid that is nevertheless equivalent to the standard slave particle construction. This alternate point of view provides further insight into the novel physics of the Mott transition and the spin liquid state that is potentially useful. We describe a generalization that suppresses spin anti-symmetric fluctuations of the Fermi surface that leads to a spin-gapped charge metal previously also discussed in terms of slave particle constructions.

3. arXiv:1107.4136 [pdf, ps, other]
Title: Real-time energy dynamics in spin-1/2 Heisenberg chains
Authors:Stephan Langer, Markus Heyl, Ian P. McCulloch, Fabian Heidrich-Meisner
Comments: 15 pages, 21 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

We study the real-time dynamics of the local energy density in the spin-1/2 XXZ chain starting from initial states with an inhomogeneous profile of bond energies. Numerical simulations of the dynamics of the initial states are carried out using the adaptive time-dependent density matrix renormalization group method. We analyze the time dependence of the spatial variance associated with the local energy density to classify the dynamics as either ballistic or diffusive. Our results are consistent with ballistic behavior both in the massless and the massive phase. We also study the same problem within Luttinger Liquid theory and obtain that energy wave-packets propagate with the sound velocity. We recover this behavior in our numerical simulations in the limit of very weakly perturbed initial states.

4. arXiv:1107.4349 [pdf, other]
Title: Thermal versus Quantum Fluctuations of Optical Lattice Fermions
Authors:V. L. Campo, K. Capelle, C. Hooley, J. Quintanilla, V. W. Scarola
Comments: 5 pages, 4 figures, supplementary material included after bibliography
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)

We show that, for fermionic atoms in a one-dimensional optical lattice, the fraction of atoms in doubly occupied sites is a highly non-monotonic function of temperature. We demonstrate that this property persists even in the presence of realistic harmonic confinement, and that it leads to a suppression of entropy at intermediate temperatures that offers a clear route to adiabatic cooling. Our interpretation of the suppression is that such intermediate temperatures are simultaneously too high for quantum coherence and too low for significant thermal excitation of double occupancy thus offering a clear indicator of the onset of quantum fluctuations.

July 21
1. arXiv:1107.3836 [pdf, other]
Title: Fluctuations of a holographic quantum Hall fluid
Authors:Niko Jokela, Matti Jarvinen, Matthew Lippert
Comments: 20 pages, 8 figures
Subjects: High Energy Physics - Theory (hep-th); Strongly Correlated Electrons (cond-mat.str-el)

We analyze the neutral spectrum of the holographic quantum Hall fluid described by the D2-D8' model. As expected for a quantum Hall state, we find the system to be stable and gapped and that, at least over much of the parameter space, the lowest excitation mode is a magneto-roton. In addition, we find magneto-rotons in higher modes as well. We show that these magneto-rotons are direct consequences of level crossings between vector and scalar modes.

2. arXiv:1107.3839 [pdf, ps, other]
Title: Superfluid density and phase relaxation in superconductors with strong disorder
Authors:G. Seibold, L. Benfatto, C. Castellani, J. Lorenzana
Comments: 4 pages, 2 figures
Subjects: Superconductivity (cond-mat.supr-con); Disordered Systems and Neural Networks (cond-mat.dis-nn); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el)

As a prototype of a disordered superconductor we consider the attractive Hubbard model with on-site disorder. We solve the Bogoljubov-de-Gennes equations on two-dimensional finite clusters at zero temperature and evaluate the electromagnetic response to a vector potential. We find that the standard decoupling between transverse and longitudinal response does not apply in the presence of disorder. Moreover the superfluid density is strongly reduced by the relaxation of the phase of the order parameter already at mean-field level when disorder is large. We also find that the anharmonicity of the phase fluctuations is strongly enhanced by disorder. Beyond mean-field, this provides an enhancement of quantum fluctuations inducing a zero-temperature transition to a non-superconducting phase of disordered preformed pairs. Finally, the connection of our findings with the glassy physics for extreme dirty superconductors is discussed.

3. arXiv:1107.3930 [pdf, other]
Title: Competing instabilities in quench experiments with ultracold Fermi gases near a Feshbach resonance
Authors:David Pekker, Eugene Demler
Comments: Contribution to Le Houches proceedings
Subjects: Quantum Gases (cond-mat.quant-gas)

Tunability of effective two body interactions near Feshbach resonances is a powerful experimental tool in systems of ultracold atoms. It has been used to explore a variety of intriguing phenomena in recent experiments. However not all of the many-body properties of such systems can be understood in terms of effective models with contact interaction given by the scattering length of the two particles in vacuum. For example, when a two component Fermi mixture is quenched to the BEC side of the Feshbach resonance, a positive scattering length suggests that interactions are repulsive and thus collective dynamics should be dominated by the Stoner instability toward a spin polarized ferromagnetic state. On the other hand, existence of low energy two particle bound states suggests a competing instability driven by molecule formation. Compe- tition between spontaneous magnetization and pair formation is determined by the the interplay of two-particle and many-body phenomena. In these lecture notes we summarize our recent theoretical results, which analyzed this competition from the point of view of unstable collective modes. We also comment on the relevance of this theoretical analysis to recent experiments reported in Ref. (Jo, Lee, Choi, Christensen, Kim, Thywissen, Pritchard and Ketterle, 2009).

July 20
1. arXiv:1107.3626 [pdf, other]
Title: BCS-BEC crossover in an optical lattice
Authors:Parag Ghosh
Comments: 21 pages, 12 figures
Subjects: Superconductivity (cond-mat.supr-con); Quantum Gases (cond-mat.quant-gas)

We model fermions with an attractive interaction in an optical lattice with a single-band Hubbard model away from half-filling with on-site attraction $U$ and nearest neighbor hopping $t$. Our goal is to understand the crossover from BCS superfluidity in the weak attraction limit to the BEC of molecules in the strong attraction limit, with particular emphasis on how this crossover in an optical lattice differs from the much better studied continuum problem. We use a large-$N$ theory with Sp(2N) symmetry to study the fluctuations beyond mean field theory. At T=0, we calculate across the crossover various observables, including chemical potential, gap, ground state energy, speed of sound and compressibility. The superfluid density $n_s$ is found to have non-trivial $U/t$ dependence in this lattice system. We show that the transition temperature $T_c$ scales with the energy gap in the weak coupling limit but crosses over to a $t^2/U$ scaling in the BEC limit, where phase fluctuations controlled by $n_s$ determine $T_c$. We also find, quite contrary to our expectations, that in the strong coupling limit, the large-$N$ theory gives qualitatively wrong trends for compressibility. A comparison with a simple Hartree shifted BCS theory, which takes into account both pairing and Hartree shifts, and correctly recovers the atomic limit and the right qualitative trend for compressibility, reveals that the large-$N$ theory on the lattice, although considers a larger number of diagrams, is in fact inferior to the simpler Hartree shifted BCS theory. The failure of the large-$N$ approach is explained by noting (i) the importance of Hartree shift in lattice problems, and (ii) inability of the large-$N$ approach to treat particle-particle and particle-hole channels at equal footing at the saddle point level.

2. arXiv:1107.3750 [pdf, other]
Title: Analytical limits for cold atom Bose gases with tunable interactions
Authors:Bogdan Mihaila, Fred Cooper, John F. Dawson, Chih-Chun Chien, Eddy Timmermans
Comments: 11 pages, 7 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We discuss the equilibrium properties of dilute Bose gases using a non-perturbative formalism based on auxiliary fields related to the normal and anomalous densities. We show analytically that for a dilute Bose gas of weakly-interacting particles at zero temperature, the leading-order auxiliary field (LOAF) approximation leads to well-known analytical results. Close to the critical point the LOAF predictions are the same as those obtained using an effective field theory in the large-N approximation. We also report analyticalapproximations for the LOAF results in the unitarity limit, which compare favorably with our numerical results. LOAF predicts that the equation of state for the Bose gas in the unitarity limit is E / (p V) = 1, unlike the case of the Fermi gas when E / (p V) = 3/2.

3. arXiv:1107.3730 (cross-list from quant-ph) [pdf, ps, other]
Title: Sauter-Schwinger like tunneling in tilted Bose-Hubbard lattices in the Mott phase
Authors:Friedemann Queisser, Patrick Navez, Ralf Schützhold
Comments: 4 pages, 1 figure
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)

We study the Mott phase of the Bose-Hubbard model on a tilted lattice. On the (Gutzwiller) mean-field level, the tilt has no effect -- but quantum fluctuations entail particle-hole pair creation via tunneling. For small potential gradients (long-wavelength limit), we derive a quantitative analogy to the Sauter-Schwinger effect, i.e., electron-positron pair creation out of the vacuum by an electric field. For large tilts, we obtain resonant tunneling related to Bloch oscillations.

July 19
1. arXiv:1107.3162 [pdf, ps, other]
Title: Renormalization of interactions of ultracold atoms in simulated Rashba gauge fields
Authors:Tomoki Ozawa, Gordon Baym
Comments: 4 pages, 2 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

Interactions of ultracold atoms with Rashba spin-orbit coupling, now studied with simulated gauge fields, have non-trivial ultraviolet and infrared behavior. We show that for both physically realizable bosonic and fermionic systems possible ultraviolet logarithmic divergences are absent. We calculate exactly the leading order effective interaction and elucidate the relation between mean-field interactions and free particle scattering lengths, and find infrared logarithmic divergences in the two particle propagator which cause the effective interaction to vanish at zero center-of-mass momentum.

2. Xiv:1107.3316 [pdf, ps, other]
Title: Microscopic and macroscopic theories for the dynamics of polar liquid crystals
Authors:Raphael Wittkowski, Hartmut Löwen, Helmut R. Brand
Comments: 10 pages
Subjects: Soft Condensed Matter (cond-mat.soft)

We derive and analyze the dynamic equations for polar liquid crystals in two spatial dimensions in the framework of classical dynamical density functional theory (DDFT). Translational density variations, polarization, and quadrupolar order are used as order-parameter fields. The results are critically compared with those obtained using the macroscopic approach of time-dependent Ginzburg-Landau (GL) equations for the analogous order-parameter fields. We demonstrate that for both the microscopic DDFT and the macroscopic GL approach the resulting dissipative dynamics can be derived from a dissipation function. We obtain microscopic expressions for all diagonal contributions and for many of the cross-coupling terms emerging from a GL approach. Thus we establish a bridge between molecular correlations and macroscopic modeling for the dissipative dynamics of polar liquid crystals.

July 18
1. arXiv:1107.2915 [pdf, ps, other]
Title: Pseudogap temperature and effects of a harmonic trap in the BCS-BEC crossover regime of an ultracold Fermi gas
Authors:Shunji Tsuchiya, Ryota Watanabe, Yoji Ohashi
Comments: 25 pages, 12 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)

We theoretically investigate excitation properties in the pseudogap regime of a trapped Fermi gas. Using a combined $T$-matrix theory with the local density approximation, we calculate strong-coupling corrections to single-particle local density of states (LDOS), as well as the single-particle local spectral weight (LSW). Starting from the superfluid phase transition temperature $T_{\rm c}$, we clarify how the pseudogap structures in these quantities disappear with increasing the temperature. As in the case of a uniform Fermi gas, LDOS and LSW give different pseudogap temperatures $T^*$ and $T^{}$ at which the pseudogap structures in these quantities completely disappear. Determining $T^*$ and $T^{}$ over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensate) crossover region, we identify the pseudogap regime in the phase diagram with respect to the temperature and the interaction strength. We also show that the so-called back-bending peak recently observed in the photoemission spectra by JILA group may be explained as an effect of pseudogap phenomenon in the trap center. Since strong pairing fluctuations, spatial inhomogeneity, and finite temperatures, are important keys in considering real cold Fermi gases, our results would be useful for clarifying normal state properties of this strongly interacting Fermi system.

2. arXiv:1107.2969 [pdf, other]
Title: Quantum Hall Superfluids in Topological Insulator Thin Films
Authors:Dagim Tilahun, Byounghak Lee, E. M. Hankiewicz, A. H. MacDonald
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Three-dimensional topological insulators have protected Dirac-cone surface states. In this paper we propose magnetic field induced topological insulator thin film ordered states in which coherence is established spontaneously between top and bottom surfaces. We find that the large dielectric constants of these materials increases the layer separation range over which coherence survives and decreases the superfluid sound velocity, but has little influence on superfluid density or charge gap. The coherent state at total Landau-level filling factor $\nu_T=0$ is predicted to be free of edge modes, qualitatively altering its transport phenomenology.


3. arXiv:1107.3077 [pdf, ps, other]
Title: Many-site coherence revivals in the extended Bose-Hubbard model and the Gutzwiller approximation
Authors:Uwe R. Fischer, Bo Xiong
Comments: 7 pages of RevTex4-1, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)

We investigate the collapse and revival of first-order coherence in deep optical lattices when long-range interactions are turned on, and find that the first few revival peaks are strongly attenuated already for moderate values of the nearest-neighbor interaction coupling. It is shown that the conventionally employed Gutzwiller wavefunction, with only onsite-number dependence of the variational amplitudes, leads to incorrect predictions for the collapse and revival oscillations. We provide a modified variant of the Gutzwiller ansatz, reproducing the analytically calculated time dependence of first-order coherence in the limit of zero tunneling.

4. arXiv:1107.3103 [pdf, ps, other]
Title: Spin transport in the XXZ chain at finite temperature and momentum
Authors:Robin Steinigeweg, Wolfram Brenig
Comments: 4 pages, 4 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We investigate the role of momentum for the transport of magnetization in the spin-1/2 Heisenberg chain above the isotropic point at finite temperature and momentum. Using numerical and analytical approaches, we analyze the autocorrelations of density and current and observe a finite region of the Brillouin zone with diffusive dynamics below a cut-off momentum, and a diffusion constant independent of momentum and time, which scales inversely with anisotropy. Lowering the temperature over a wide range, starting from infinity, the diffusion constant is found to increase strongly while the momentum space cut-off for diffusion decreases. Above the cut-off momentum diffusion breaks down completely.

5. arXiv:1107.3107 [pdf, ps, other]
Title: The superfluid-insulator transition in the disordered two-dimensional Bose-Hubbard model
Authors:Fei Lin, Erik S. Sørensen, D. M. Ceperley
Comments: 10 pages, 9 figures
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Strongly Correlated Electrons (cond-mat.str-el)

We investigate the superfluid-insulator transition in the disordered two-dimensional Bose-Hubbard model through quantum Monte Carlo simulations. The Bose-Hubbard model is studied in the presence of site disorder and the quantum critical point between the Bose-glass and superfluid is determined in both the grand canonical ($\mu/U=0.375$ close to $\rho=1$) and canonical ensemble ($\rho=1$ and 0.5). Particular attention is paid to disorder averaging and it is shown that an extremely large number of disorder realizations are needed in order to obtain reliable results. Typically we average over more than $100,000$ disorder realizations. In the grand canonical ensemble we find $Z t_c/U=0.112(1)$ with $\mu/U=0.375$, significantly different from previous studies. When compared to the critical point in the absence of disorder ($Z t_c/U=0.2385$), this result confirms previous findings showing that disorder enlarges the superfluid region. At the critical point, in order to estimate universal features, we compute the dynamic conductivity scaling curves.

July 11 - 15, Chungwei Lin
July 15
1. arXiv:1107.2663 [pdf, ps, other]
In situ measurement of the dynamic structure factor in ultracold quantum gases
Hendrik Weimer, Hans Peter Büchler
Comments: 5 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We propose an experimental setup to efficiently measure the dynamic structure factor of ultracold quantum gases. Our method uses the interaction of the trapped atomic system with two different cavity modes, which are driven by external laser fields. By measuring the output fields of the cavity the dynamic structure factor of the atomic system can be determined. Contrary to previous approaches the atomic system is not destroyed during the measurement process.

2. arXiv:1107.2672 [pdf, other]
Long timescale dynamics of spin textures in a degenerate F=1 $^{87}$ Rb spinor Bose gas
J. Guzman, G.-B. Jo, A. N. Wenz, K. W. Murch, C. K. Thomas, D. M. Stamper-Kurn
Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other); Atomic Physics (physics.atom-ph)
We investigate the long-term dynamics of spin textures prepared by cooling unmagnetized spinor gases of F=1 $^{87}$Rb to quantum degeneracy, observing domain coarsening and a strong dependence of the equilibration dynamics on the quadratic Zeeman shift $q$. For small values of $|q|$, the textures arrive at a configuration independent of the initial spin-state composition, characterized by large length-scale spin domains, and the establishment of easy-axis (negative $q$) or easy-plane (positive $q$) magnetic anisotropy. For larger $|q|$, equilibration is delayed as the spin-state composition of the degenerate spinor gas remains close to its initial value. These observations support the mean-field equilibrium phase diagram predicted for a ferromagnetic spinor Bose-Einstein condensate, but also illustrate that equilibration is achieved under a narrow range of experimental settings, making the F=1 $^{87}$Rb gas more suitable for studies of nonequilibrium quantum dynamics.

July 14
1. arXiv:1107.2613 [pdf, ps, other]
Controlling the pair momentum of the FFLO state in a 3D Fermi gas through a 1D periodic potential
Jeroen P.A. Devreese, Michiel Wouters, Jacques Tempere
Comments: 19 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
The question whether a spin-imbalanced Fermi gas can accommodate the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state has been the subject of intense study. This state, in which Cooper pairs obtain a nonzero momentum, has hitherto eluded experimental observation. Recently, we demonstrated that the FFLO state can be stabilized in a 3D Fermi gas, by adding a 1D periodic potential. Until now it was assumed that the FFLO wave vector always lies parallel to this periodic potential (FFLO-P). In this contribution we show that, surprisingly, the FFLO wave vector can also lie skewed with respect to the potential (FFLO-S). Starting from the partition sum, the saddle-point free energy of the system is derived within the path-integral formalism. Minimizing this free energy allows us to study the different competing ground states of the system. To qualitatively understand the underlying pairing mechanism, we visualize the Fermi surfaces of the spin up and spin down particles. From this visualization, we find that tilting the FFLO wave vector with respect to the direction of the periodic potential, can result in a larger overlap between the pairing bands of both spin species. This skewed FFLO state can provide an additional experimental signature for observing FFLO superfluidity in a 3D Fermi gas.

2. arXiv:1107.2608 (cross-list from cond-mat.supr-con) [pdf, ps, other]
Size of stripe domains in a superconducting ferromagnet
Vu Hung Dao, Sebastien Burdin, Alexandre Buzdin
Comments: 10 pages, 13 figures
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
In a superconducting ferromagnet, the superconducting state appears in the ferromagnetic phase where usually a domain structure has already developed. We study the influence of the superconducting screening currents on a stripe structure with out-of-plane magnetization, in a film of arbitrary thickness. We find that superconductivity always induces a shrinkage of the domains, and there is a critical value of penetration depth below which a mono-domain structure is more stable than the periodic one. Furthermore we investigate the possible different effects of singlet and triplet superconductivity on the domain width, as well as the conditions for the existence of vortices in the domains. The obtained results are then discussed in light of the experimental data of superconducting ferromagnets URhGe, UGe2, and UCoGe.

3. arXiv:1107.2394 [pdf, other]
Exact density functional theory with the density matrix renormalization group
E.M. Stoudenmire, Lucas O. Wagner, Steven R. White, Kieron Burke
Comments: 5 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Atomic and Molecular Clusters (physics.atm-clus); Chemical Physics (physics.chem-ph)
Combining density functional theory with controlled numerical methods for strongly correlated systems could greatly extend the range of both. We use the density matrix renormalization group method to perform Kohn-Sham calculations with the exact density functional for a set of electronic structure models in one dimension. When correlations are strong, the total electronic density converges quickly, but the spin densities do not.

July 13
1. arXiv:1107.2206 [pdf, ps, other]
Non interacting electron gas model of quantum Hall effect
Kang Li, Shuming Long, Jianhua Wang, Yi Yuan
Comments: Revtex, 4 pages
Subjects: Quantum Gases (cond-mat.quant-gas); High Energy Physics - Theory (hep-th)
On the basis of our previous studies on energy levels and wave functions of single electrons in a strong magnetic field, the energy levels and wave functions of non-interacting electron gas system, electron gas Hall surface density and Hall resistivity of electron gas system are calculated first. Then, a comparison is made between non-interaction electron gas model and Laughlin's interaction two dimensional electron gas system. It is found that the former can more quickly explain the integer and the fractional quantum Hall effects without the help of concepts proposed by Laughlin, such as fractional charge, localized state, extended state, etc. After explicit calculation, it is also discovered that quantum Hall effect has the same physical nature as superconducting state. Namely, when $T<20$K (B=150000Gauss), electron's orbital and spin magnetic moments have the same direction as external magnetic field, all electrons will fill the ground state energy levels where electron angular quantum numbers are $0,-1,-2,\text{...}$ In this case, energy degeneracy of the ground state in the system is theoretically infinite and Fermi-Dirac Condensed state will appear in low-temperature electron gas system.

2. arXiv:1107.2195 [pdf, ps, other]
Quantum criticality out of equilibrium in the pseudogap Kondo model
Chung-Hou Chung, Kenneth Yi-Jie Zhang
Comments: 4.1 pages, 2 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We theoretically investigate the non-equilibrium quantum phase transition in a generic setup: the pseudogap Kondo model where a quantum dot couples to two-left (L) and right (R)-voltage-biased fermionic leads with power-law density of states (DOS) with respect to their Fermi levels {\mu}_L/R, {\rho}_c,L(R) ({\omega}) \propto |{\omega} - {\mu}_L(R) |r, and 0 < r < 1. In equilibrium (zero bias voltage) and for 0 < r < 1/2, with increasing Kondo correlations, in the presence of particle-hole symmetry this model exhibits a quantum phase transition from a unscreened local moment (LM) phase to the Kondo phase. Via a controlled frequency-dependent renormalization group (RG) approach, we compute analytically and numerically the non-equilibrium conductance, conduction electron T-matrix and local spin susceptibility at finite bias voltages near criticality. The current-induced decoherence shows distinct nonequilibrium scaling, leading to new universal non-equilibrium quantum critical behaviors in the above observables. Relevance of our results for the experiments is discussed.

3. arXiv:1107.2279 [pdf, ps, other]
Magnetic interactions in iron superconductors revisited
M.J. Calderon, G. Leon, B. Valenzuela, E. Bascones
Comments: 4 pages + supplementary information
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
The magnetic interactions of iron superconductors are studied within the Hartree-Fock and Heisenberg descriptions of the same five orbital model. The experimentally observed (pi,0) order is more stable in the Hartree-Fock approximation than in the Heisenberg one. Due to virtual transitions involving filled orbitals, the first and second nearest neighbor exchange constants J_1 and J_2 of the strong-coupling description become ferromagnetic at large Hund's coupling. Ferromagnetic tendencies are enhanced with electron doping and suppressed with hole doping, the electron-hole asymmetry being less important for small interactions. The Hartree-Fock phase diagram lacks a ferromagnetic phase but includes a double stripe state. In undoped compounds this state is unstable towards phase separation, but it can be stabilized with electron-doping. In both descriptions, (pi,pi) antiferromagnetic order appears on the phase diagram correlated with an unusual orbital reorganization. A non-monotonous dependence of J_1 and J_2 on the Fe-As-Fe angle is found. The implications of our findings for the magnetic state of iron superconductors are discussed.

4. arXiv:0905.4467 (cross-list from cond-mat.supr-con) [pdf]
Fermi surface dichotomy of the superconducting gap and pseudogap in underdoped pnictides
Y.-M. Xu, P. Richard, K. Nakayama, T. Kawahara, Y. Sekiba, T. Qian, M. Neupane, S. Souma, T. Sato, T. Takahashi, H. Luo, H.-H. Wen, G.-F. Chen, N.-L. Wang, Z. Wang, Z. Fang, X. Dai, H. Ding
Comments: Main Manuscript: 19 pages, 3 figures; Supplementary Information: 10 pages, 7 figures
Journal-ref: Nat. Commun. 2, 392 (2011)
Subjects: Superconductivity (cond-mat.supr-con); Strongly Correlated Electrons (cond-mat.str-el)
High-temperature superconductivity in iron-arsenic materials (pnictides) near an antiferromagnetic phase raises the possibility of spin-fluctuation-mediated pairing. However, the interplay between antiferromagnetic fluctuations and superconductivity remains unclear in the underdoped regime, which is closer to the antiferromagnetic phase. Here we report that the superconducting gap of the underdoped pnictides scales linearly with the transition temperature, and that a distinct pseudogap coexisting with the SC gap develops on underdoping. This pseudogap occurs on Fermi surface sheets connected by the antiferromagnetic wavevector, where the superconducting pairing is stronger as well, suggesting that antiferromagnetic fluctuations drive both the pseudogap and superconductivity. Interestingly, we found that the pseudogap and the spectral lineshape vary with the Fermi surface quasi-nesting conditions in a fashion that shares similarities with the nodal-antinodal dichotomous behaviour observed in underdoped copper oxide superconductors.

July 12
1. arXiv:1107.1755 [pdf, ps, other]
Probing Half-odd Topological Number with Cold Atoms in a Non-Abelian Optical Lattice
Feng Mei, Shi-Liang Zhu, Xun-Li Feng, Zhi-Ming Zhang, C. H. Oh
Comments: 5 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We propose an experimental scheme to probe the contribution of a single Dirac cone to the Hall conductivity as half-odd topological number sequence. In our scheme, the quantum anomalous Hall effect as in graphene is simulated with cold atoms trapped in an optical lattice and subjected to a laser-induced non-Abelian gauge field. By tuning the laser intensity to change the gauge flux, the energies of the four Dirac points in the first Brillouin zone are shifted with each other and the contribution of the single Dirac cone to the total atomic Hall conductivity is manifested. We also show such manifestation can be experimentally probed with atomic density profile measurements.

2. arXiv:1107.2024 [pdf, other]
Critical spin transport in Bose gases
R. Kittinaradorn, R.A. Duine, H.T.C. Stoof
Comments: 4 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We consider spin transport in a two-component atomic Bose gas in three dimensions, at temperatures just above the critical temperature for Bose-Einstein condensation. In these systems the spin conductivity is determined by spin drag, i.e., frictional drag between the two spin components due to interactions. We find that in the critical region the temperature dependence of the spin conductivity deviates qualitatively from the Boltzmann result and is fully determined by the critical exponents of the phase transition. We discuss the size of the critical region where these results may be observed experimentally.

3. arXiv:1104.5399 (cross-list from cond-mat.mes-hall) [pdf, ps, other]
Non-equilibrium steady-states for interacting open systems: exact results
Valeriu Moldoveanu, Horia D. Cornean, Claude-Alain Pillet
Comments: To appear in Phys. Rev. B
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Strongly Correlated Electrons (cond-mat.str-el); Mathematical Physics (math-ph)
Under certain conditions we prove the existence of a steady-state transport regime for interacting mesoscopic systems coupled to reservoirs (leads). The partitioning and partition-free scenarios are treated on an equal footing. Our time-dependent scattering approach is {\it exact} and proves, among other things the independence of the steady-state quantities from the initial state of the sample. Closed formulas for the steady-state current amenable for perturbative calculations w.r.t. the interaction strength are also derived. In the partitioning case we calculate the first order correction and recover the mean-field (Hartree-Fock) results.

July 11
1. arXiv:1107.1643 [pdf, ps, other]
Pump-probe coupling of matter wave packets to remote lattice states
Jacob F. Sherson, Sung Jong Park, Poul Pedersen, Nils Winter, Miroslav Gajdacz, Sune Mai, Jan Arlt
Comments: 5 pages, 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
The coherent manipulation of wave packets is an important tool in many areas of physics. We demonstrate the experimental realization of quasi-free wave packets of ultra-cold atoms bound by an external harmonic trap. The wave packets are produced by modulating the intensity of an optical lattice containing a Bose-Einstein condensate. The evolution of these wave packets is monitored in-situ and their reflection on a band gap is observed. In direct analogy with pump-probe spectroscopy, a probe pulse allows for the resonant de-excitation of the wave packet into localized lattice states at a long, controllable distance of more than 100 lattice sites from the main component. This coherent control mechanism for ultra-cold atoms thus enables controlled quantum state preparation, opening exciting perspectives for quantum metrology and simulation.

2. arXiv:1107.1555 [pdf, ps, other]
Renormalization group approach to spinor Bose-Fermi mixtures in a shallow optical lattice
S. Modak, S.-W. Tsai, K. Sengupta
Comments: 8 pages, 8 figs v1
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)
We study a mixture of ultracold spin-half fermionic and spin-one bosonic atoms in a shallow optical lattice where the bosons are coupled to the fermions via both density-density and spin-spin interactions. We consider the parameter regime where the bosons are in a superfluid ground state, integrate them out, and obtain an effective action for the fermions. We carry out a renormalization group analysis of this effective fermionic action at low temperatures, show that the presence of the spinor bosons may lead to a separation of Fermi surfaces of the spin-up and spin-down fermions, and investigate the parameter range where this phenomenon occurs. We also calculate the susceptibilities corresponding to the possible superfluid instabilities of the fermions and obtain their possible broken-symmetry ground states at low temperatures and weak interactions.

July 4 - 8, Xiaopeng Li
July 8
1. arXiv:1107.1245 [pdf, ps, other]
Phase Diagram of the Kane-Mele-Hubbard model
Authors: Christian Griset, Cenke Xu
Comments: 5 pages, 2 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
Motivated by recent numerical results, we study the phase diagram of the Kane-Mele-Hubbard (KHM) model, especially the nature of its quantum critical points. The phase diagram of the Kane-Mele-Hubbard model can be understood by breaking the SO(4) symmetry of our previous work down to U(1)_spin x U(1)_charge x PH symmetry. The vortices of the inplane Neel phase carry charge, and the proliferation of the charged magnetic vortex drives the transition between the inplane Neel phase and the QSH insulator phase; this transition belongs to the 3d XY universality class. The transition between the liquid phase and the inplane Neel phase is an anisotropic O(4) transition, which eventually becomes first order due to quantum fluctuation. The liquid-QSH transition is predicted to be first order based on a 1/N calculation.

2. arXiv:1107.1411 [pdf, ps, other]
Topological insulators on a Mobius Strip
Authors: Langtao Huang, Dung-Hai Lee
Comments: 4 pages, 6 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
We study the two dimensional Chern insulator and spin Hall insulator on a non-orientable Riemann surface, the Mobius strip, where the usual bandstructure topological invariant is not defined. We show that while the flow pattern of edge currents can detect the twist of the Mobius strip in the case of Chern insulator, it can not do so in spin Hall insulator.

July 7
1. arXiv:1107.1223 [pdf, ps, other]
Absence of spontaneous magnetic order of lattice spins coupled to itinerant interacting electrons in one and two dimensions
Authors: Daniel Loss, Fabio L. Pedrocchi, Anthony J. Leggett
Abstract: We extend the Mermin-Wagner theorem to a system of lattice spins which are spin-coupled to itinerant and interacting charge carriers. We use the Bogoliubov inequality to rigorously prove that neither (anti-) ferromagnetic nor helical long-range order is possible in one and two dimensions at any finite temperature. Our proof applies to a wide class of models including any form of electron-electron and single-electron interactions that are independent of spin. In the presence of Rashba or Dresselhaus spin-orbit interactions (SOI) magnetic order is allowed and intimately connected to equilibrium spin currents. However, in the special case when Rashba and Dresselhaus SOIs are tuned to be equal, magnetic order is excluded again. This opens up a new possibility to control magnetism in magnetic semiconductors electrically.

July 6
1. arXiv:1107.0795 [pdf, ps, other]
Title: Theory of optical transitions in graphene nanoribbons
Authors: K. Sasaki, K. Kato, Y. Tokura, K. Oguri, T. Sogawa
Abstract: Matrix elements of electron-light interactions for armchair and zigzag graphene nanoribbons are constructed analytically using a tight-binding model. The changes in wavenumber ($\Delta n$) and pseudospin are the necessary elements if we are to understand the optical selection rule. It is shown that an incident light with a specific polarization and energy, induces an indirect transition ($\Delta n=\pm1$), which results in a characteristic peak in absorption spectra. Such a peak provides evidence that the electron standing wave is formed by multiple reflections at both edges of a ribbon. It is also suggested that the absorption of low-energy light is sensitive to the position of the Fermi energy, direction of light polarization, and irregularities in the edge. The effect of depolarization on the absorption peak is briefly discussed.

2.arXiv:1107.0724 [pdf, other]
Metal-insulator transition in a two-band model for the perovskite nickelates
Authors: SungBin Lee, Ru Chen, Leon Balents
Abstract: Motivated by recent Fermi surface and transport measurements on LaNiO3, we study the Mott Metal-Insulator transitions of perovskite nickelates, with the chemical formula RNiO3, where R is a rare-earth ion. We introduce and study a minimal two-band model, which takes into account only the eg bands. In the weak to intermediate correlation limit, a Hartree-Fock analysis predicts charge and spin order consistent with experiments on R=Pr, Nd, driven by Fermi surface nesting. It also produces an interesting semi-metallic electronic state in the model when an ideal cubic structure is assumed. We also study the model in the strong interaction limit, and find that the charge and magnetic order observed in experiment exist only in the presence of very large Hund's coupling, suggesting that additional physics is required to explain the properties of the more insulating nickelates, R=Eu,Lu,Y. Next, we extend our analysis to slabs of finite thickness. In ultra-thin slabs, quantum confinement effects substantially change the nesting properties and the magnetic ordering of the bulk, driving the material to exhibit highly anisotropic transport properties. However, pure confinement alone does not significantly enhance insulating behavior. Based on these results, we discuss the importance of various physical effects, and propose some experiments.


July 5
1. arXiv:1107.0485 [pdf, ps, other]
Brownian motion in superfluid $^4$He
Authors: Xiao Li, Ran Cheng, Qian Niu
Abstract: We propose to study the Brownian motion of a classical microsphere submerged in superfluid $^4$He using the recent laser technology as a direct investigation of the thermal fluctuation of quasiparticles in the quantum fluid. By calculating the friction coefficient and the strength of the random force as functions of the temperature, we claim that the Full-Width-at-Half-Maximum and the square amplitude of the resonant mode are feasible to be measured directly. Contrary to previous work, it is discovered that the roton contribution is not negligible, and it becomes dominating above 0.76\,K.

July 4
1. arXiv:1107.0006 [pdf, other]
Holey topological thermoelectrics
Authors: Oleg A. Tretiakov, Ar. Abanov, Jairo Sinova
Abstract: We study the thermoelectric properties of three-dimensional topological insulators with many holes (or pores) in the bulk. We show that at high density of these holes the thermoelectric figure of merit ZT can be large due to the contribution of the conducting surfaces and the suppressed phonon thermal conductivity. The maximum efficiency can be tuned by an induced gap in the surface states dispersion through tunneling or external magnetic fields. The large values of ZT, much higher than unity for reasonable parameters, make this system a strong candidate for applications in heat management of nanodevices, especially at low temperatures.

2. arXiv:1106.5913 (cross-list from q-fin.ST) [pdf, ps, other]
Renyi's information transfer between financial time series
Authors: Petr Jizba, Hegen Kleinert,
Abstract: In this paper we quantify the statistical coherence between financial time series by means of Renyi's entropy. With the help of Cambell's coding theorem we show that Renyi's entropy selectively emphasizes only certain sectors of the underlying empirical distribution while strongly suppresses others. This accentuation is controlled with Renyi's parameter q. To tackle the issue of the information flow between time series we formulate the concept of Renyi's transfer entropy as a measure of information that is transferred only between certain parts of underlying distributions. This is particularly pertinent in financial time series where the knowledge of marginal events such as spikes or sudden jumps is of a crucial importance. We apply the Renyian information flow to stock market time series from 11 world stock indices as sampled at a daily rate in the time period 02.01.1990 - 31.12.2009. Corresponding heat maps and net information flows are represented graphically. A detailed discussion of the transfer entropy between DAX and S&P500 indices based on minute tick data gathered in the period from 02.04.2008 to 11.09.2009 is also provided. Our analysis shows that the bivariate information flow between world markets is strongly asymmetric with a distinct information surplus flowing from the Asia-Pacific region both to Europe and the U.S. markets. Important, yet less dramatic excess of information also flows from Europe to the U.S. This is particularly clearly seen from a careful analysis of Renyi information flow between DAX and S&P500.