May 2014
May 5-May 9, Zhenyu zhou, May 19-May 23, jiyao chen, May 26-May 30, Ahmet Keles
May 26-May 30 Ahmet Keles
[1] arXiv:1405.7730 [pdf, other]
Surface transport coefficients for three-dimensional topological superconductors
Hong-Yi Xie, Yang-Zhi Chou, Matthew S. Foster
We provide evidence that surface transport coefficients of three-dimensional (3D) topological superconductors are universal at low temperature, being determined only by the bulk topological invariant. We model the effects of disorder and interactions upon the gapless Majorana surface bands. For a system with conserved spin and no interactions, it is known that the zero temperature spin conductivity is unmodified by non-magnetic disorder. We show that the Hartree and Fock interaction contributions to the surface spin conductivity vanish in every disorder realization. We also perform a large winding number expansion, using the Wess-Zumino-Novikov-Witten Finkel'stein nonlinear sigma model. We find that the Altshuler-Aronov interaction corrections to the spin and thermal conductances are suppressed. We offer a simple physical interpretation: For a bulk winding number $\nu$, there are $|\nu|$ "colors" of surface Majorana fermions. Carrier scattering off of self-consistent potential fluctuations is the physical origin of Altshuler-Aronov corrections; however, non-zero density accumulations of color or spin are forbidden by time-reversal symmetry. Our results suggest that 3D topological superconductors are a closer analog of the two-dimensional quantum Hall effect than 3D topological insulators.
[2] arXiv:1405.7689 [pdf, other]
A simple field theory representation of symmetry-protected-topological-invariants, group cohomology and beyond - for Dummies
Juven Wang, Zheng-Cheng Gu, Xiao-Gang Wen
Since the symmetry protected topological (SPT) states have no intrinsic topological orders, one cannot probe its bulk via fractionalized excitations. However, the partition function from path integral with various symmetry twists are universal SPT invariants which allow us to fully characterize SPT states. In this work, we will use gauge fields to represent those symmetry twists in the closed spacetime of any dimensions and arbitrary topology. This allows us to express the universal SPT invariants in terms of a continuous field theory action. We show that SPT invariants of pure gauge actions describe SPT states predicted by group cohomology, while the mixed gauge-gravity actions describe the SPT states beyond-group-cohomology, recently observed by Kapustin. We find some new examples of mixed gauge-gravity actions for U(1) SPT states in 3+1D and 4+1D via Stiefel-Whitney class and gravitational Chern-Simons term. Field theory representations of SPT invariants not only serve as a tool to derive group cohomology and beyond, but also guide us to design physical probes for various SPT states.
[3] arXiv:1405.7595 [pdf, other]
Quantum correlations and spatial localization in one-dimensional ultracold bosonic mixtures
M. A. García-March, B. Juliá-Díaz, G. E. Astrakharchik, Th. Busch, J. Boronat, A. Polls
We present the complete phase diagram for one-dimensional binary mixtures of bosonic ultracold atomic gases in a harmonic trap. We obtain exact results with direct numerical diagonalization for small number of atoms, which permits us to quantify quantum many-body correlations. The quantum Monte Carlo method is used to calculate energies and density profiles for larger system sizes. We study the system properties for a wide range of interaction parameters. For the extreme values of these parameters, different correlation limits can be identified, where the correlations are either weak or strong. We investigate in detail how the correlation evolve between the limits. For balanced mixtures in the number of atoms in each species, the transition between the different limits involves sophisticated changes in the one- and two-body correlations. Particularly, we quantify the entanglement between the two components by means of the von Neumann entropy. We show that the limits equally exist when the number of atoms is increased, for balanced mixtures. Also, the changes in the correlations along the transitions among these limits are qualitatively similar. We also show that, for imbalanced mixtures, the same limits with similar transitions exist. Finally, for strongly imbalanced systems, only two limits survive, i.e., a miscible limit and a phase-separated one, resembling those expected with a mean-field approach.
[4] arXiv:1405.6811 [pdf, other]
Ultracold Chemistry and its Reaction Kinetics
Florian Richter, Daniel Becker, Cédric Bény, Torben A. Schulze, Silke Ospelkaus, Tobias J. Osborne
We study the reaction kinetics of chemical processes occurring in the ultracold regime and systematically investigate their dynamics. Quantum entanglement is found to play a key role in activating and driving an ultracold reaction towards a dynamical equilibrium. The analogies with the dynamics of quenched systems, where a similar relaxation of local observables occurs, is also explored.
[5] arXiv:1405.7336 [pdf, other]
Light scattering and dissipative dynamics of many fermionic atoms in an optical lattice
Saubhik Sarkar, Stephan Langer, Johannes Schachenmayer, Andrew J. Daley
We investigate the many-body dissipative dynamics of fermionic atoms in an optical lattice in the presence of incoherent light scattering. Deriving and solving a master equation to describe this process microscopically for many particles, we observe contrasting behaviour in terms of the robustness against this type of heating for different many-body states. In particular, we find that the magnetic correlations exhibited by a two-component gas in the Mott insulating phase should be particularly robust against decoherence from light scattering, because the decoherence in the lowest band is suppressed by a larger factor than the timescales for effective superexchange interactions that drive coherent dynamics. Furthermore, the derived formalism naturally generalizes to analogous states with SU(N) symmetry. In contrast, for typical atomic and laser parameters, two-particle correlation functions describing bound dimers for strong attractive interactions exhibit superradiant effects due to the indistinguishability of off-resonant photons scattered by atoms in different internal states. This leads to rapid decay of correlations describing off-diagonal long-range order for these states. Our predictions should be directly measurable in ongoing experiments, providing a basis for characterising and controlling heating processes in quantum simulation with fermions.
[6] arXiv:1405.7187 [pdf, ps, other]
Chandrasekhar-Clogston limit and critical polarization in a Fermi-Bose superfluid mixture
Tomoki Ozawa, Alessio Recati, Sandro Stringari
We study mixtures of a population-imbalanced strongly-interacting Fermi gas and of a Bose-Einstein condensed gas at zero temperature. In homogeneous space, we find that the Chandrasekhar-Clogston critical polarization for the onset of instability of Fermi superfluidity is enhanced due to the interaction with the bosons. Predictions for the critical polarization are also given in the presence of harmonic trapping. Special focus is given to the case of equal Fermi-Bose and Bose-Bose coupling constants where the density of fermions becomes flat despite the inhomogeneity of the trapping potential. This regime is well suited to investigate the emergence of exotic configurations, such as the occurrence of spin domains or the FFLO phase.
[7] arXiv:1405.7086 [pdf, other]
Ken Wilson: Solving the Strong Interactions
Michael E. Peskin
Ken Wilson's ideas on the renormalization group were shaped by his attempts to build a theory of the strong interactions based on the concepts of quantum field theory. I describe the development of his ideas by reviewing four of Wilson's most important papers. [contribution to the Journal of Statistical Physics Special Issue in Memory of K. G. Wilson]
[8] arXiv:1405.6715 [pdf, ps, other]
Chiral spin superfluidity and spontaneous spin Hall effect of interacting bosons
Xiaopeng Li, Stefan S. Natu, Arun Paramekanti, S. Das Sarma
Recent experiments on ultracold atoms in optical lattices have synthesized a variety of tunable bands with degenerate double-well structures in momentum space. Such degeneracies in the single particle spectrum strongly enhance quantum fluctuations, and may lead to exotic many-body ground states. Here we consider weakly interacting spinor Bose gases in such bands, and discover a universal quantum "order by disorder" phenomenon which selects a novel chiral spin superfluid with remarkable properties such as spontaneous anomalous spin Hall effect and momentum space antiferromagnetism. For bosons in the excited Dirac band of a hexagonal lattice, such a state supports staggered spin loop currents in real space. We show that Bloch oscillations provide a powerful dynamical route to quantum state preparation of such a chiral spin superfluid. Our predictions can be readily tested in spin resolved time-of-flight experiments.
[9] arXiv:1405.6302 [pdf, ps, other]
Spin measurements and control of cold atoms using spin-orbit fields
D. Sokolovski, E. Ya. Sherman
We show that by switching on a spin-orbit interaction in a cold-atom system, experiencing a Zeeman-like coupling to an external field, e.g., in a Bose-Einstein condensate, one can simulate a quantum measurement on a precessing spin. Depending on the realization, the measurement can access both the ergodic and the Zeno regimes, while {the time dependence} of the spin's decoherence may vary from a Gaussian to an inverse fractional power law. Back action of the measurement forms time- and coordinate-dependent profiles of the atoms' density, resulting in its translation, spin-dependent fragmentation, and appearance of interference patterns.
May 12-May 16, Bo Liu,
May 16
1.arXiv:1405.3744 [pdf, other]
Coupled pair approach for strongly-interacting trapped fermionic atoms
C. J. Bradly, B. C. Mulkerin, A. M. Martin, H. M. Quiney
We present a coupled pair approach for studying few-body physics in harmonically trapped ultracold gases. The method is applied to a two-component Fermi system of N particles. A stochastically variational gaussian expansion method is applied, focusing on optimization of the two-body correlations present in the strongly interacting, or unitary, limit. The groundstate energy of the four-, six- and eight-body problem with equal spin populations is calculated with high accuracy and minimal computational effort. We also calculate the structural properties of these systems and discuss their implication for the many-body ultracold gas and other few-body calculations.
May 15
1.arXiv:1405.3514 [pdf, other]
Signatures of the excited-state quantum phase transition in the periodic dynamics of the Lipkin-Meshkov-Glick model
G. Engelhardt, V. M. Bastidas, T. Brandes
We investigate signatures of the excited-state quantum phase transition in the periodic dynamics of the Lipkin-Meshkov-Glick model. With this aim, we use the semiclassical trajectories to calculate quantum mechanical quantities such as the density of states and expectation values of observables in eigenstates of the system. Motivated by this approach, we suggest an experimental protocol, based on the time evolution of the finite-size system, to find singularities in observables, which arise due to the excited-state quantum phase transition. We also discuss the influence of the participation ratio on the deviations of the quantum mechanical finite-size time evolution from the classical one.
May 14
1. arXiv:1405.2979 [pdf, ps, other]
Dynamics of localized waves in 1D random potentials: statistical theory of the coherent forward scattering peak
Kean Loon Lee, Benoît Grémaud, Christian Miniatura
As recently discovered [PRL 109 190601(2012)], Anderson localization in a bulk disordered system triggers the emergence of a coherent forward scattering (CFS) peak in momentum space, which twins the well-known coherent backscattering (CBS) peak observed in weak localization experiments. Going beyond the perturbative regime, we address here the long-time dynamics of the CFS peak in a 1D random system and we relate this novel interference effect to the statistical properties of the eigenfunctions and eigenspectrum of the corresponding random Hamiltonian. Our numerical results show that the dynamics of the CFS peak is governed by the logarithmic level repulsion between localized states, with a time scale that is, with good accuracy, twice the Heisenberg time. This is in perfect agreement with recent findings based on the nonlinear σ-model. In the stationary regime, the width of the CFS peak in momentum space is inversely proportional to the localization length, reflecting the exponential decay of the eigenfunctions in real space, while its height is exactly twice the background, reflecting the Poisson statistical properties of the eigenfunctions. Our results should be easily extended to higher dimensional systems and other symmetry classes.
May 13
1.arXiv:1405.2565 [pdf, other]
Anomalous Hall effect with ultracold gases
O. Dutta, A. Przysiezna, J. Zakrzewski
We study ultracold fermions trapped in a shaken two dimensional triangular lattice. We find that, a combination of interaction induced tunneling and shaking can result in an emergent dice lattice along with controllable staggered magnetic flux and synthetic non-Abelian fields. Moreover, by tuning the staggered flux, we show that one can enter the regime of Quantum Anomalous Hall effect. Our results are reminiscent of Anomalous Hall conductivity in spin-orbit coupled Ferromagnets.
2. arXiv:1405.2723 [pdf, other]
Isospin Correlations in two-partite Hexagonal Optical Lattices
Marta Prada, Eva-Maria Richter, Daniela Pfannkuche
Two-component mixtures in optical lattices reveal a rich variety of different phases. We employ an exact diagonalization method to obtain the relevant correlation functions in hexagonal optical lattices to characterize those phases. We relate the occupation difference of the two species to the magnetic polarization. `Iso'-magnetic correlations disclose the nature of the system, which can be of easy-axis type, bearing phase segregation, or of easy-plane type, corresponding to super-counter-fluidity. In the latter case, the correlations reveal easy-plane segregation, involving a highly-entangled state. We identify striking correlated supersolid phases appearing within the superfluid limit
May 12
1. arXiv:1405.2256 [pdf, other]
Critical Phenomena of Contact near Phase Transitions
Y.-Y. Chen, Y.-Z. Jiang, X.-W. Guan, Qi Zhou
Contact, which measures the two-body correlations at short distances in dilute systems, is a central quantity to rule ultra cold atoms. It builds up universal relations among thermodynamic quantities, such as the large momentum tail, energy, and dynamic structure factor, through the renowned Tan relations. However, a conceptual question remains open as to whether or not Contact signifies phase transitions, which are insensitive to short-range physics. Here we show that, near a continuous classical or quantum phase transition, Contact exhibits a variety of critical phenomena, including scaling laws and critical exponents that are uniquely determined by the universality class of the phase transition, and a constant Contact per particle. We also use a prototypical exactly solvable model to demonstrate these critical phenomena in one-dimensional strongly interacting fermions. Our work establishes an intrinsic connection between the universality of dilute many-body systems and the universal critical phenomena near a phase transition.
May 2
1. arXiv:1405.0230 [pdf, other]
Theory of Bosons in two-leg ladders with large magnetic fields
Ran Wei, Erich J. Mueller
We calculate the ground state of a Bose gas trapped on a two-leg ladder where Raman-induced hopping mimics the effect of a large magnetic field. In the mean-field limit, where there are large numbers of particles per site, this maps onto a uniformly frustrated two-leg ladder classical spin model. The net particle current always vanishes in the ground state, but generically there is a finite "chiral current", corresponding to equal and opposite flow on the two legs. We vary the strength of the hopping across the rungs of the ladder and the interaction between the bosons. We find three phases: (1) A "saturated chiral current phase" (SCCP), where the density is uniform and the chiral current is simply related to the strength of the magnetic field. In this state the only broken symmetry is the condensate phase. (2) A "biased ladder phase" (BLP), where the density is higher on one leg than the other. The fluid velocity is higher on the lower density leg, so the net current is zero. In addition to the condensate phase, this has a broken reflection symmetry. (3) A "modulated density phase" (MDP), where the atomic density is modulated along the ladder. In addition to the condensate phase, this has a second broken symmetry corresponding to translations of the density wave. We further study the fluctuations of the condensate in the BLP, finding a roton-maxon like excitation spectrum. Decreasing the hopping along the rungs softens the spectrum. As the energy of the "roton" reaches to zero, the BLP becomes unstable. We describe the experimental signatures of these phases, including the response to changing the frequency of the Raman transition.
May 1
1. arXiv:1404.7696 [pdf, ps, other]
Counting rule of Nambu-Goldstone modes for internal and spacetime symmetries: Bogoliubov theory approach
Daisuke A. Takahashi, Muneto Nitta
When continuous symmetry is spontaneously broken, there appear Nambu-Goldstone modes (NGMs) with linear and quadratic dispersion relations, which are called type-I and type-II, respectively. We propose a general framework to count these modes including the coefficients of the dispersion relations with applying the Bogoliubov theory. Unlike previous works, our framework is applicable without any modification to the cases when there are additional zero modes, which do not have a symmetry origin, such as quasi-NGMs, and/or when spacetime symmetry is spontaneously broken in the presence of a topological soliton or a vortex. We illustrate our theory in various examples in spinor Bose-Einstein condensates. We also speculate how to interpolate type-II dispersion relations of a quantized vortex and a domain wall in finite systems and non-integer dispersion relations of those in infinite systems.
