April2012

flat =Apr 30 - May 4, Bin Wang=

=May 4=

Many-particle Systems in One Dimension in the Harmonic Approximation
== [|J. R. Armstrong], [|N. T. Zinner] , [|D. V. Fedorov] , [|A. S. Jensen]  == We consider energetics and structural properties of a many particle system in one dimension with pairwise contact interactions confined in a parabolic external potential. To render the problem analytically solvable, we use the harmonic approximation scheme at the level of the Hamiltonian. We investigate the scaling with particle number of the ground state energies for systems consisting of identical bosons or fermions. We then proceed to focus on bosonic systems and make a detailed comparison to known exact results in the absence of the parabolic external trap for three-body systems. We also consider the thermodynamics of the harmonic model which turns out to be similar for bosons and fermions due to the lack of degeneracy in one dimension.

[|K. B. Gubbels], [|H. T. C. Stoof]
We consider imbalanced Fermi gases with strong attractive interactions, for which Cooper-pair formation plays an important role. The two-component mixtures consist either of identical fermionic atoms in two different hyperfine states, or of two different atomic species both occupying only a single hyperfine state. In both cases, the number of atoms for each component is allowed to be different, which leads to a spin imbalance, or spin polarization. Two different atomic species also lead to a mass imbalance. Imbalanced Fermi gases are relevant to condensed-matter physics, nuclear physics and astroparticle physics. They have been studied intensively in recent years, following their experimental realization in ultracold atomic Fermi gases. The experimental control in such a system allows for a systematic study of the equation of state and the phase diagram as a function of temperature, spin polarization and interaction strength. In this review, we discuss the progress in understanding strongly-interacting imbalanced Fermi gases, where a main goal is to describe the results of the highly controlled experiments. We start by discussing Feshbach resonances, after which we treat the imbalanced Fermi gas in mean-field theory to give an introduction to the relevant physics. We encounter several unusual superfluid phases, including phase separation, gapless Sarma superfluidity, and supersolid phases. To obtain a more quantitative description of the experiments, we review also more sophisticated techniques, such as diagrammatic methods and the renormalization-group theory. We end the review by discussing two theoretical approaches to treat the inhomogeneous imbalanced Fermi gas, namely the Landau-Ginzburg theory and the Bogoliubov-de Gennes approach.

=May 3=

[|Akiyuki Tokuno], [|Thierry Giamarchi]
We compute the integrated doublon production rate in response to a lattice modulation for two component fermions in an optical lattice. We derive a general formula for the integrated intensity, valid in presence of inhomogeneous potentials such as the trap, which gives the integrated intensity in terms of equal time correlation functions only. Such a formula is thus well suited for direct numerical calculations. We show that, in the limit of large repulsion for commensurate fillings, or for temperature ranges for which the hopping is incoherent, the integrated doublon spectrum is directly related to the nearest neighbor spin-spin correlation function. We compute its temperature dependence in this regime using finite temperature quantum Monte Carlo calculation.

== 2. [|arXiv:1205.0496] [ [|pdf], [|other] ] Optical Abelian Lattice Gauge Theories  [|L. Tagliacozzo] , [|A. Celi] , [|A. Zamora] , [|M. Lewenstein]   == We discuss a general framework for the realization of a family of abelian lattice gauge theories, i.e., link models or gauge magnets, in optical lattices. We analyze the properties of these models that make them suitable to quantum simulations. Within this class, we study in detail the phases of a U(1)-invariant lattice gauge theory in 2+1 dimensions originally proposed by Orland. By using exact diagonalization, we extract the low-energy states for small lattices, up to 4x4. We confirm that the model has two phases, with the confined entangled one characterized by strings wrapping around the whole lattice. We explain how to study larger lattices by using either tensor network techniques or digital quantum simulations with Rydberg atoms loaded in optical lattices where we discuss in detail a protocol for the preparation of the ground state. We also comment on the relation between standard compact U(1) LGT and the model considered.

3. [|arXiv:1205.0254] [ [|pdf], [|ps], [|other] ]
== Majorana fermions emerged on orbital ladders without superconductivity  [|Xiaopeng Li], [|Erhai Zhao] , [|W. Vincent Liu]   == We unveil a topological phase of interacting fermions on a two-leg ladder of unequal parity orbitals showing Majorana zero modes, yet requiring no superconductivity. Such an asymmetric ladder is derived from the recent uneven double-well optical lattice experiments by reducing dimensionality. Interesting topology arises from staggered phases of $sp$-orbital quantum tunneling. Experimental signatures are found in density correlations, and phase transitions to trivial band and Mott insulators. Upon recovering two dimensions with coupled ladders, the zero modes form a flat band spectrum, opening the route to strongly correlated states controlled by interactions.

4. [|arXiv:1205.0341] (cross-list from quant-ph) [ [|pdf], [|other] ]
== Quantum Magnetism of Spin-Ladder Compounds with Trapped-Ion Crystals  [|A. Bermudez], [|J. Almeida] , [|K. Ott] , [|H. Kaufmann] , [|S. Ulm] , [|F. Schmidt-Kaler] , [|A. Retzker] , [|M. B. Plenio]   == The quest for experimental platforms that allow for the exploration, and even control, of the interplay of low dimensionality and frustration is a fundamental challenge in several fields of quantum many-body physics, such as quantum magnetism. Here, we propose the use of cold crystals of trapped ions to study a variety of frustrated quantum spin ladders. By optimizing the trap geometry, we show how to tailor the low dimensionality of the models by changing the number of legs of the ladders. Combined with a method for selectively hiding of ions provided by laser addressing, it becomes possible to synthesize stripes of both triangular and Kagome lattices. Besides, the degree of frustration of the phonon-mediated spin interactions can be controlled by shaping the trap frequencies. We support our theoretical considerations by initial experiments with planar ion crystals, where a high and tunable anisotropy of the radial trap frequencies is demonstrated. We take into account an extensive list of possible error sources under typical experimental conditions, and describe explicit regimes that guarantee the validity of our scheme.

= May 2 =

none
= May 1 =

==  1. [|arXiv:1204.6658] [ [|pdf], [|ps] , [|other] ] Evidence of spin liquid with hard-core bosons in a square lattice  [|Y.-H. Chan] , [|L.-M. Duan]   == We show that laser assisted hopping of hard core bosons in a square optical lattice can be described by an antiferromagnetic $J_{1}$-$J_{2}$ XY model with tunable ratio of $J_{2}/J_{1}$. We numerically investigate the phase diagram of the $J_{1}$-$J_{2}$ XY model using both the tensor network algorithm for infinite systems and the exact diagonalization for small clusters and find strong evidence that in the intermediate region around $% J_{2}/J_{1}\sim 0.5$, there is a spin liquid phase with vanishing magnetization and valence bond orders, which interconnects the Neel state on the $J_{2}\ll J_{1}$ side and the stripe antiferromagnetic phase on the $% J_{2}\gg J_{1}$ side. This finding opens up the possibility of studying the exotic spin liquid phase in a realistic experimental system using ultracold atoms in an optical lattice.

[|Tomoki Ozawa], [|Gordon Baym]
The striped state of ultracold bosons with Rashba spin-orbit coupling in a homogeneous infinite system has, as we show, a constant particle flow, which in a finite-size system would accumulate particles at the boundaries; it is thus not a physical steady state of the system. We propose, as a variational ansatz, a condensate wave function for a weakly trapped system which behaves similarly to the striped state near the center, but does not have particle flow at the boundaries. This state has a line of unquantized coreless vortices. We show, by minimizing the total energy, that our modified striped state has lower energy than the conventional striped state and it is thus a physically appropriate starting point to analyze striped states in finite systems.

Dynamic freezing of strongly correlated ultracold bosons
== [|S. Mondal], [|D. Pekker] , [|K. Sengupta]  == We study the non-equilibrium dynamics of ultracold bosons in an optical lattice with a time dependent hopping amplitude J(t)=J_0 +\delta J \cos(\omega t) which takes the system from a superfluid phase near the Mott-superfluid transition (J= J_0+\delta J) to a Mott phase (J=J_0-\delta J) and back through a quantum critical point (J=J_c) and demonstrate dynamic freezing of the boson wavefunction at specific values of \omega. At these values, the wavefunction overlap F (defect density P=1-F) approaches unity (zero). We provide a qualitative explanation of the freezing phenomenon, show it's robustness against quantum fluctuations and the presence of a trap, compute residual energy and superfluid order parameter for such dynamics, and suggest experiments to test our theory.

= Apr 30 =

Dynamics of single neutral impurity atoms immersed in an ultracold gas
== [|Nicolas Spethmann], [|Farina Kindermann] , [|Shincy John] , [|Claudia Weber] , [|Dieter Meschede] , [|Artur Widera]  == We report on controlled doping of an ultracold Rb gas with single neutral Cs impurity atoms. We study elastic as well as inelastic collisions between the impurity atom and atoms of the Rb gas with single atom resolution. Following the thermalization of initially laser cooled impurity atoms to the temperature of the ultracold Rb gas allows us to estimate a lower limit of the elastic interspecies s-wave scattering length. Inelastic interactions are restricted to three-body recombination, which we observe event-by-event with atomic resolution and determine the Rb-Rb-Cs three-body loss rate. Our results pave the way for a coherently interacting hybrid system of individually controllable impurities in a quantum many-body system.

= Apr 23 - Apr 27, Johannes Schachenmayer =

= Apr 27 =

**1. [|arXiv:1204.5896] [ [|pdf], [|other] ] ** Dynamical effects of exchange symmetry breaking in mixtures of interacting bosons [|Malte C. Tichy], [|Jacob F. Sherson] , [|Klaus Mølmer] In a double-well potential, a Bose-Einstein condensate exhibits Josephson oscillations or self-trapping, depending on its initial preparation and on the ratio of inter-particle interaction to inter-well tunneling. Here, we elucidate the role of the exchange symmetry for the dynamics with a mixture of two distinguishable species with identical physical properties, i.e. which are governed by an isospecific interaction and external potential. In the mean-field limit, the spatial population imbalance of the mixture can be described by the dynamics of a single species in an effective potential with modified properties or, equivalently, with an effective total particle number. The oscillation behavior can be tuned by populating the second species while maintaining the spatial population imbalance and all other parameters constant. In the corresponding many-body approach, the single-species description approximates the full counting statistics well also outside the realm of spin-coherent states. The method is extended to general Bose-Hubbard systems and to their classical mean-field limits, which suggests an effective single-species description of multicomponent Bose gases with weakly an-isospecific interactions.

**2. [|arXiv:1204.5889] [ [|pdf], [|ps] , [|other] ] ** Robust non-Markovianity in ultracold gases [|Pinja Haikka], [|Suzanne McEndoo] , [|Gabriele De Chiara] , [|Massimo Palma] , [|Sabrina Maniscalco] We study the effect of thermal fluctuations on a probe qubit interacting with a Bose-Einstein condensed (BEC) reservoir. The zero-temperature case was studied in [Haikka P et al 2011 Phys. Rev. A 84 031602], where we proposed a method to probe the effects of dimensionality and scattering length of a BEC based on its behavior as an environment. Here we show that the sensitivity of the probe qubit is remarkably robust against thermal noise. We give an intuitive explanation for the thermal resilience, showing that it is due to the unique choice of the probe qubit architecture of our model.

**3. [|arXiv:1204.5827] [ [|pdf], [|ps] , [|other] ] ** Scattering of hole excitations in a one-dimensional spinless quantum liquid [|K. A. Matveev], [|A. V. Andreev] Luttinger liquid theory accounts for the low energy boson excitations of one-dimensional quantum liquids, but disregards the high energy excitations. The most important high energy excitations are holes which have infinite lifetime at zero temperature. At finite temperatures they can be scattered by thermally excited bosons. We describe the interaction of the hole with the bosons by treating it as a mobile impurity in a Luttinger liquid. This approach enables us to evaluate the scattering probability at arbitrary interaction strength. In general, the result is expressed in terms of the hole spectrum, its dependence on the density and momentum of the fluid, and the parameters of the Luttinger liquid Hamiltonian. In the special case of Galilean invariant systems the scattering probability is expressed in terms of only the hole spectrum and its dependence on the fluid density. We apply our results to the problem of equilibration of one-dimensional quantum liquids.

** 4. [|arXiv:1204.6021] [ [|pdf], [|ps] , [|other] ] ** Bose-Hubbard model for universal quantum walk-based computation [|Michael S. Underwood], [|David L. Feder] We present a novel scheme for universal quantum computation based on spinless interacting bosonic quantum walkers on a piecewise-constant graph, described by the two-dimensional Bose-Hubbard model. Arbitrary X and Z rotations are constructed, as well as an entangling two-qubit CPHASE gate and a SWAP gate. Quantum information is encoded in the positions of the walkers on the graph, as in previous quantum walk-based proposals for universal quantum computation, though in contrast to prior schemes this proposal requires a number of vertices only linear in the number of encoded qubits. It allows single-qubit measurements to be performed in a straightforward manner with localized operators, and can make use of existing quantum error correcting codes either directly within the universal gate set provided, or by extending the lattice to a third dimension. We present an intuitive example of a logical encoding to implement the seven-qubit Steane code. Finally, an implementation in terms of ultracold atoms in optical lattices is suggested.

**5. [|arXiv:1204.6019] [ [|pdf], [|ps] , [|other] ] ** Four-coloring model and frustrated superfluidity in the diamond lattice [|Gia-Wei Chern], [|Congjun Wu] We propose a novel four-coloring model which describes "frustrated superfluidity" of p-band bosons in the diamond optical lattice. We show that the combined effects of strong intra-site repulsions, inter-site phase coherence, and anisotropy give rise to a macroscopically degenerate classic ground state in this system. The superfluid phases of the condensate wavefunction on the diamond bonds are mapped to four distinct colors. By further mapping the problem to an antiferromagnetic Potts model on the pyrochlore lattice, we show that both color and orbital angular momentum correlations exhibit power-law decay in the degenerate manifold that is described by an emergent magnetostatic theory with three independent flux fields.

= Apr 26 =

[9] [|arXiv:1204.5674] [ [|pdf], [|ps] , [|other] ] Fragmentation, domain formation and atom number fluctuations of a two-species Bose-Einstein condensate in an optical lattice [|Uttam Shrestha], [|Janne Ruostekoski] We theoretically study the loading of a two-species Bose-Einstein condensate to an optical lattice in a tightly-confined one-dimensional trap. Due to quantum fluctuations the relative inter and intra species phase coherence between the atoms and the on-site atom number fluctuations are reduced in the miscible regime. For the immiscible case the fluctuations are enhanced and the atoms form metastable interleaved spatially separated domains where the domain length and its fluctuations are affected by quantum fluctuations.

= Apr 25 =

**1. [|arXiv:1204.5183] [ [|pdf], [|other] ]** The `Higgs' Amplitude Mode at the Two-Dimensional Superfluid-Mott Insulator Transition [|Manuel Endres], [|Takeshi Fukuhara] , [|David Pekker] , [|Marc Cheneau] , [|Peter Schauß] , [|Christian Gross] , [|Eugene Demler] , [|Stefan Kuhr] , [|Immanuel Bloch] Spontaneous symmetry breaking plays a key role in our understanding of nature. In a relativistic field theory, a broken continuous symmetry leads to the emergence of two types of fundamental excitations: massless Nambu-Goldstone modes and a massive `Higgs' amplitude mode. An excitation of Higgs type is of crucial importance in the standard model of elementary particles and also appears as a fundamental collective mode in quantum many-body systems. Whether such a mode exists in low-dimensional systems as a resonance-like feature or becomes over-damped through coupling to Nambu-Goldstone modes has been a subject of theoretical debate. Here we reveal and study a Higgs mode in a two-dimensional neutral superfluid close to the transition to a Mott insulating phase. We unambiguously identify the mode by observing the expected softening of the onset of spectral response when approaching the quantum critical point. In this regime, our system is described by an effective relativistic field theory with a two-component quantum-field, constituting a minimal model for spontaneous breaking of a continuous symmetry. Additionally, all microscopic parameters of our system are known from first principles and the resolution of our measurement allows us to detect excited states of the many-body system at the level of individual quasiparticles. This allows for an in-depth study of Higgs excitations, which also addresses the consequences of reduced dimensionality and confinement of the system. Our work constitutes a first step in exploring emergent relativistic models with ultracold atomic gases.

The Higgs mode in a two-dimensional superfluid [|L.Pollet], [|N. V. Prokof'ev] We present solid evidence for the existence of a well-defined Higgs amplitude mode in two-dimensional relativistic field theories based on analytically continued results from quantum Monte Carlo simulations of the Bose-Hubbard model in the vicinity of the superfluid-Mott insulator quantum critical point, featuring emergent particle-hole symmetry and Lorentz-invariance. The Higgs boson, seen as a well-defined low-frequency resonance in the spectral density, is quickly pushed to high energies in the superfluid phase and disappears by merging with the broad secondary peak at the characteristic interaction scale. Simulations of a trapped system of ultra-cold $^{87}$Rb atoms demonstrate that the low-frequency resonance feature is lost for typical experimental parameters, while the characteristic frequency for the onset of strong response is preserved.
 * 2. [|arXiv:1204.5190] [ [|pdf], [|other] ]  **

Non-Abelian spin singlet states of two-component Bose gases in artificial gauge fields [|Tobias Graß], [|Bruno Juliá-Díaz] , [|Nuria Barberán] , [|Maciej Lewenstein] We study strongly correlated phases of a pseudo-spin-1/2 Bose gas in an artificial gauge field using the exact diagonalization method. The atoms are confined in two dimensions and interact via a two-body contact potential. For SU(2)-symmetric interactions and Abelian gauge fields, the ground state of the system is a pseudo-spin singlet. In close analogy to the Read-Rezayi (RR) series in spin-polarized systems, a series of incompressible phases is formed at fillings \nu=2k/3. This series resembles the non-Abelian spin singlet (NASS) states, which are the exact zero-energy eigenstates of a (k+1)-body contact interaction. Applying non-Abelian gauge fields, it becomes possible to switch between RR-like and NASS-like states by varying the non-Abelian gauge field strength.
 * 3. [|arXiv:1204.5423] [ [|pdf], [|ps] , [|other] ]  **

**4. [|arXiv:1204.5184] [ [|pdf], [|other] ] ** Probing equilibrium current patterns of ultracold atoms in an optical lattice [|Matthew Killi], [|Arun Paramekanti] Atomic bosons and fermions in an optical lattice can simulate a variety of interesting condensed matter states that support equilibrium current patterns. However, measuring the mass currents in such states of atomic matter is a significant challenge since they do not produce detectable magnetic fields, unlike their electronic counterparts in solids. In this paper, we propose making a nonequilibrium quench of the Hamiltonian that dynamically converts such atomic mass currents into experimentally measurable real-space density patterns. We illustrate how a specific such "unidirec- tional" quench of the optical lattice can be used to uncover checkerboard and stripe current orders in lattice Bose superfluids and Fermi gases, as well as chiral edge currents in an integer quantum Hall state of lattice fermions.



= Apr 24 =

Initial state dependence of the quench dynamics in integrable quantum systems II: Thermal state [|Kai He], [|Marcos Rigol] We study properties of isolated integrable systems after a sudden quench starting from thermal states. We show that, even if the system is initially in thermal equilibrium at finite temperature, the diagonal entropy after a quench remains a fraction of the entropy in thermal equilibrium and in the generalized ensembles introduced to describe integrable systems after relaxation. We also examine the difference between the distribution of conserved quantities in the thermal and generalized ensembles after a quench. A finite size scaling analysis is presented for each quantity, which allows us making predictions for thermodynamically large system sizes.
 * 1 [|arXiv:1204.4739] [ [|pdf], [|ps] , [|other] ]  **

= Apr 23 =

** 1. [|arXiv:1204.4715] [ [|pdf], [|other] ] **

Dynamical crossover between the infinite-volume and empty-lattice limits of ultra-cold fermions in 1D optical lattices
[|Chih-Chun Chien], [|Massimiliano Di Ventra] Unlike typical condensed-matter systems, ultra-cold atoms loaded into optical lattices allow separate control of both the particle number and system size. As a consequence, there are two distinct "thermodynamic" limits that can be defined for these systems: i) "infinite-volume limit" at constant finite density, and ii) "empty-lattice limit" at constant particle number. To probe the difference between these two limits and their crossover, we consider a partially occupied lattice and study the transport of non-interacting fermions and fermions interacting at the mean-field level into the unoccupied region. In the infinite-volume limit, a finite steady-state current emerges. On the other hand, in the empty-lattice limit there is no finite steady-state current. By changing the initial filling, we find a smooth crossover between the two limits. Our predictions may be verified using available experimental tools and demonstrate a fundamental difference between isolated small systems such as ultra-cold atoms and conventional condensed-matter systems.

= Apr 16 - Apr 20, Saubhik Sarkar =

**Apr 20**
Compressibility of an Ultracold Fermi Gas with Repulsive Interactions [|Ye-Ryoung Lee], [|Myoung-Sun Heo] , [|Jae-Hoon Choi] , [|Tout T. Wang] , [|Caleb A. Christensen] , [|Timur M. Rvachov] , [|Wolfgang Ketterle]
 * 1. [|arXiv:1204.4229] [ [|pdf], [|ps] , [|other] ]  **

Fermi gases with repulsive interactions are characterized by measuring their compressibility as a function of interaction strength. The compressibility is obtained from in-trap density distributions monitored by phase contrast imaging. For interaction parameters k_F a > 0.25 fast decay of the gas prevents the observation of equilibrium profiles. For smaller interaction parameters, the results are adequately described by first-order perturbation theory. A novel phase contrast imaging method compensates for dispersive distortions of the images.

Ultracold collisions between two light indistinguishable diatomic molecules: elastic and rotational energy transfer in HD+HD [|Renat A. Sultanov], [|Dennis Guster] , [|S. K. Adhikari] A close coupling quantum-mechanical calculation is performed for rotational energy transfer in a HD+HD collision at very low energy, down to the ultracold temperatures: $T \sim 10^{-8}$ K. A global six-dimensional H$_2$-H$_2$ potential energy surface is adopted from a previous work [Boothroyd {\it et al.}, J. Chem. Phys., {\bf 116}, 666 (2002).] State-resolved integral cross sections $\sigma_{ij\rightarrow i'j'}(\varepsilon_{kin})$ of different quantum-mechanical rotational transitions $ij\rightarrow i'j'$ in the HD molecules and corresponding state-resolved thermal rate coefficients $k_{ij\rightarrow i'j'}(T)$ have been computed. Additionally, for comparison, H$_2$+H$_2$ calculations for a few selected rotational transitions have also been performed. The hydrogen and deuterated hydrogen molecules are treated as rigid rotors in this work. A pronounced isotope effect is identified in the cross sections of these collisions at low and ultracold temperatures.
 * 2. [|arXiv:1111.2598] (cross-list from physics.chem-ph) [ [|pdf], [|ps] , [|other] ] **

**Apr 19**
Interaction effects in the spinful time-reversal invariant Hofstadter problem [|Daniel Cocks], [|Peter P. Orth] , [|Stephan Rachel] , [|Michael Buchhold] , [|Karyn Le Hur] , [|Walter Hofstetter] We consider a spinful and time-reversal invariant version of the Hofstadter problem which can be realized in cold atom experiments. In these experiments, an additional staggered potential and a Rashba--type hopping are available. Without interactions, the system exhibits various phases such as topological and normal insulator, metal as well as semi--metal phases with two or even more Dirac cones. Using a combination of real-space dynamical mean-field theory and analytical techniques, we discuss the effect of on-site interactions and determine the corresponding phase diagram. In particular, we investigate the semi--metal to antiferromagnetic insulator transition and the stability of different topological insulator phases in the presence of strong interactions. We compute spectral functions which allow us to study the edge states of the strongly correlated topological phases.
 * 1. [|arXiv:1204.4171] [ [|pdf], [|other] ] **

<span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> 2. [|arXiv:1204.3988] [ [|pdf], [|ps] , [|other] ] Counterflow superfluid of polaron pairs in Bose-Fermi mixtures in optical lattices [|Ippei Danshita], [|L. Mathey] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We study the quantum phases of one-dimensional Bose-Fermi mixtures in optical lattices. Assuming repulsive interparticle interactions, equal mass, and unit total filling, we calculate the ground-state phase diagram by means of both Tomonaga-Luttinger liquid theory and time-evolving block decimation method. We demonstrate the existence of a counterflow superfluid (CFSF) phase of polaron pairs, which are composite particles consisting of a fermion and a bosonic hole, in a broad range of the parameter space. We find that this phase naturally emerges in $^{174}$Yb-$^{173}$Yb mixtures, realized in recent experiments, at low temperatures. 3. [|arXiv:1204.3988] [ [|pdf], [|ps] , [|other] ] Counterflow superfluid of polaron pairs in Bose-Fermi mixtures in optical lattices [|Ippei Danshita], [|L. Mathey] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We study the quantum phases of one-dimensional Bose-Fermi mixtures in optical lattices. Assuming repulsive interparticle interactions, equal mass, and unit total filling, we calculate the ground-state phase diagram by means of both Tomonaga-Luttinger liquid theory and time-evolving block decimation method. We demonstrate the existence of a counterflow superfluid (CFSF) phase of polaron pairs, which are composite particles consisting of a fermion and a bosonic hole, in a broad range of the parameter space. We find that this phase naturally emerges in $^{174}$Yb-$^{173}$Yb mixtures, realized in recent experiments, at low temperatures.

<span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> 4. [|arXiv:1204.3960] [ [|pdf], [|ps] , [|other] ] Quantum dark solitons in the 1D Bose gas and the superfluid velocity [|Jun Sato], [|Rina Kanamoto] , [|Eriko Kaminishi] , [|Tetsuo Deguchi] <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> We give explicit connections of quantum one-hole excited states to classical solitons for the one-dimensional Bose gas with repulsive short-range interactions. We call the quantum states connected to classical solitons the quantum soliton states. We show that the matrix element of the canonical field operator between quantum soliton states with $N-1$ and $N$ particles is given by a dark soliton of the Gross-Pitaevskii equation in the weak coupling case. We suggest that the matrix element corresponds to the order parameter of BEC in the quantum soliton state. The result should be useful in the study of many-body effects in Bose-Einstein condensation and superfluids. For instance, we derive the superfluid velocity for a quantum soliton state.

**Apr 18**

Total Current Blockade in an Ultra-Cold Dipolar Quantum Wire [|Líney Halla Kristinsdóttir], [|Olov Karlström] , [|Johannes Bjerlin] , [|Jonas C. Cremon] , [|Peter Schlagheck] , [|Andreas Wacker] , [|Stephanie M. Reimann] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Cold atom systems offer a great potential for the future design of new mesoscopic quantum systems with properties that are fundamentally different from semiconductor nanostructures, such as quantum dots and quantum wires with electrons. Here, we investigate the analog of a quantum wire using ultra-cold particles, and find a new scenario for the quantum transport: Attractive interactions may lead to a complete suppression of current in the low-bias range, a total current blockade. We demonstrate this effect for the example of ultra-cold quantum gases with dipolar interactions.
 * 1. [|arXiv:1204.2966] (cross-list from cond-mat.mes-hall) [ [|pdf], [|other] ] **

**Apr 17**
Temporal fluctuations in the bosonic Josephson junction as a probe for phase space tomography [|Christine Khripkov], [|Doron Cohen] , [|Amichay Vardi] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We study the long time coherence dynamics of a two-mode Bose-Hubbard model in the Josephson interaction regime, as a function of the relative phase and occupation imbalance of an arbitrary coherent preparation. We find that the variance of the long time fluctuations of the one-body coherence can be factorized as a product of the inverse participation number 1/M that depends only on the preparation, and a semi-classical function C(E) that reflects the phase space characteristics of the pertinent observable. Temporal fluctuations can thus be used as a sensitive probe for phase space tomography of quantum many-body states.
 * 1. [|arXiv:1204.3242] [ [|pdf], [|ps] , [|other] ] **

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">2. [|arXiv:1204.3371] (cross-list from cond-mat.stat-mech) [ [|pdf], [|ps] , [|other] ] Quantum dynamics and entanglement of a 1D Fermi gas released from a trap [|Ettore Vicari] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We investigate the entanglement properties of the nonequilibrium dynamics of one-dimensional noninteracting Fermi gases released from a trap. The gas of N particles is initially in the ground state within hard-wall or harmonic traps, then it expands after dropping the trap. We compute the time dependence of the von Neumann and Renyi entanglement entropies and the particle fluctuations of spatial intervals around the original trap, in the limit of a large number N of particles. The results for these observables apply to one-dimensional gases of impenetrable bosons as well. <span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> We identify different dynamical regimes at small and large times, depending also on the initial condition, whether it is that of a hard-wall or harmonic trap. In particular, we analytically show that the expansion from hard-wall traps is characterized by the asymptotic small-time behavior $S \approx (1/3)\ln(1/t)$ of the von Neumann entanglement entropy, and the relation $S\approx \pi^2 V/3$ where V is the particle variance, which are analogous to the equilibrium behaviors whose leading logarithms are essentially determined by the corresponding conformal field theory with central charge $c=1$. The time dependence of the entanglement entropy of extended regions during the expansion from harmonic traps shows the remarkable property that it can be expressed as a global time-dependent rescaling of the space dependence of the initial equilibrium entanglement entropy.

**Apr 16**
1. [|arXiv:1204.3036] [ [|pdf], [|ps] , [|other] ] Thermalisation of Local Observables in Small Hubbard Lattices [|S. Genway], [|A. F. Ho] , [|D. K. K. Lee] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We present a study of thermalisation of a small isolated Hubbard lattice cluster prepared in a pure state with a well-defined energy. We examine how a two-site subsystem of the lattice thermalises with the rest of the system as its environment. We explore numerically the existence of thermalisation over a range of system parameters, such as the interaction strength, system size and the strength of the coupling between the subsystem and the rest of the lattice. We find thermalisation over a wide range of parameters and that interactions are crucial for efficient thermalisation of small systems. We relate this thermalisation behaviour to the eigenstate thermalisation hypothesis and quantify numerically the extent to which eigenstate thermalisation holds. We also verify our numerical results theoretically with the help of previously established results from random matrix theory for the local density of states, particularly the finite-size scaling for the onset of thermalisation.

<span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> 2. [|arXiv:1204.3009] [ [|pdf], [|other] ] Exact Soliton-like Solutions of the Radial Gross-Pitaevskii Equation [|Lauri Toikka], [|Jarmo Hietarinta] , [|Kalle-Antti Suominen] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We construct exact ring soliton-like solutions of the cylindrically symmetric (i.e., radial) Gross- Pitaevskii equation with a potential, using the similarity transformation method. Depending on the choice of the allowed free functions, the solutions can take the form of stationary dark or bright rings whose time dependence is in the phase dynamics only, or oscillating and bouncing solutions, related to the second Painlev\'e transcendent. In each case the potential can be chosen to be time-independent.

=Apr 9 - Apr 13, Xiaopeng Li=


 * Apr 13**

Measuring entanglement entropy of a generic many-body system with a quantum switch [|Dmitry A. Abanin], [|Eugene Demler] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Entanglement entropy has become an important theoretical concept in condensed matter physics, because it provides a unique tool for characterizing quantum mechanical many-body phases and new kinds of quantum order. However, the experimental measurement of entanglement entropy in a many-body systems is widely believed to be unfeasible, owing to the nonlocal character of this quantity. Here, we propose a general method to measure the entanglement entropy. The method is based on a quantum switch (a two-level system) coupled to a composite system consisting of several copies of the original many-body system. The state of the switch controls how different parts of the composite system connect to each other. We show that, by studying the dynamics of the quantum switch only, the Renyi entanglement entropy of the many-body system can be extracted. We propose a possible design of the quantum switch, which can be realized in cold atomic systems. Our work provides a route towards testing the scaling of entanglement in critical systems, as well as a method for a direct experimental detection of topological order.
 * 1. [|arXiv:1204.2819] [ [|pdf], [|other] ] **

Signatures of Majorana fermions in hybrid superconductor-semiconductor nanowire devices [|V. Mourik], [|K. Zuo] , [|S. M. Frolov] , [|S. R. Plissard] , [|E. P. A. M. Bakkers] , [|L. P. Kouwenhoven] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Majorana fermions are particles identical to their own antiparticles. They have been theoretically predicted to exist in topological superconductors. We report electrical measurements on InSb nanowires contacted with one normal (Au) and one superconducting electrode (NbTiN). Gate voltages vary electron density and define a tunnel barrier between normal and superconducting contacts. In the presence of magnetic fields of order 100 mT we observe bound, mid-gap states at zero bias voltage. These bound states remain fixed to zero bias even when magnetic fields and gate voltages are changed over considerable ranges. Our observations support the hypothesis of Majorana fermions in nanowires coupled to superconductors.
 * 2. [|arXiv:1204.2792] [ [|pdf] ] **

<span style="background-color: #ffffff; display: block; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 16px;"> [|Stefan K. Baur], [|Bernd Fröhlich] , [|Michael Feld] , [|Enrico Vogt] , [|Daniel Pertot] , [|Marco Koschorreck] , [|Michael Köhl] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">The line shape of radio frequency spectra of tightly bound Feshbach molecules in strong transverse confinement can be described by a simple analytic formula that includes final state interactions. By direct comparison to experimental data, we clarify the role of effective range corrections to two-body bound-state energies in lower dimensions.
 * 3. <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|arXiv:1204.2704] [ <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: none;">[|pdf] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: none;">[|other] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">] **
 * Radio frequency spectra of Feshbach molecules in quasi-two dimensional geometries**

Frustrated magnets and quantum paramagnetic phases at finite temperature [|L. Isaev], [|G. Ortiz] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We develop a general framework, which combines exact diagonalization in small clusters with a density matrix variational principle, to study frustrated magnets at finite temperature. This thermodynamic hierarchical mean-field technique is used to describe the phase diagram and magnetization process of the 3D spin-1/2 $J_1$-$J_2$ antiferromagnet on a stacked square lattice. We demonstrate that with temperature the non-magnetic region of this model exhibits a crossover from quantum to classical paramagnet, and show how the corresponding temperature scale can be extracted from thermodynamic measurements. Below this crossover temperature an applied magnetic field induces a variety of phases with non-trivial spin textures. We discuss implications of our results for experiments in frustrated vanadium oxides.
 * 4. [|arXiv:1204.2559] [ [|pdf], [|ps] , [|other] ] **

**Apr 12**
Quantum criticality in spin chains with non-ohmic dissipation [|Iver Bakken Sperstad], [|Einar B. Stiansen] , [|Asle Sudbø] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We investigate the critical behavior of a spin chain coupled to bosonic baths characterized by a spectral density proportional to $\omega^s$, with $s>1$. Varying $s$ changes the effective dimension $d_\text{eff} = d + z$ of the system, where $z$ is the dynamical critical exponent and the number of spatial dimensions $d$ is set to one. We consider two extreme cases of clock models, namely Ising-like and U(1)-symmetric ones, and find the critical exponents using Monte Carlo methods. The dynamical critical exponent and the anomalous scaling dimension $\eta$ are independent of the order parameter symmetry for all values of $s$. The dynamical critical exponent varies continuously from $z \approx 2$ for $s=1$ to $z=1$ for $s=2$, and the anomalous scaling dimension evolves correspondingly from $\eta \gtrsim 0$ to $\eta = 1/4$. The latter exponent values are readily understood from the effective dimensionality of the system being $d_\text{eff} \approx 3$ for $s=1$, while for $s=2$ the anomalous dimension takes the well-known exact value for the 2D Ising and XY models, since then $d_{\rm{eff}}=2$. A noteworthy feature is, however, that $z$ approaches unity and $\eta$ approaches 1/4 for values of $s < 2$, while naive scaling would predict the dissipation to become irrelevant for $s=2$. Instead, we find that $z=1,\eta=1/4$ for $s \approx 1.75$ for both Ising-like and U(1) order parameter symmetry. These results lead us to conjecture that for all site-dissipative $Z_q$ chains, these two exponents are related by the scaling relation $z = \text{max} {(2-\eta)/s, 1}$. We also connect our results to quantum criticality in nondissipative spin chains with long-range spatial interactions.
 * 1. [|arXiv:1204.2538] [ [|pdf], [|other] ] **

== **<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">2. [|arXiv:1204.2537] [ [|pdf], [|ps] , [|other] ] ** == Shiba impurity bound states as a probe of topological superconductivity and Fermion parity changing quantum phase transitions [|Jay D. Sau], [|Eugene Demler] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Spin-orbit coupled superconductors are potentially interesting candidates for realizing topological and potentially non-Abelian states with Majorana Fermions. We argue that time-reversal broken spin-orbit coupled superconductors generically can be characterized as having sub-gap states that are bound to localized non-magnetic impurities. Such bound states, which are referred to as Shiba states, can be detected as sharp resonances in the tunneling spectrum of the spin-orbit coupled superconductors. The Shiba state resonance can be tuned using a gate-voltage or a magnetic field from being at the edge of the gap at zero magnetic fields to crossing zero energy when the Zeeman splitting is tuned into the topological superconducting regime. The zero-crossing signifies a Fermion parity changing first order quantum phase transition, which is characterized by a Pfaffian topological invariant. These zero-crossings of the impurity level can be used to locally characterize the topological superconducting state from tunneling experiments.

3. [|arXiv:1204.2365] [ [|pdf], [|other] ]
Fractional topological phase in one-dimensional flatbands with nontrivial topology [|Huaiming Guo], [|Shun-Qing Shen] , [|Shiping Feng] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">We show the existence of the fractional topological phase (FTP) in a one-dimensional interacting fermion model using exact diagonalization, in which the non-interacting part has flatbands with nontrivial topology. In the presence of the nearest-neighbouring interaction $V_{1}$, the FTP at filling factor $\nu =1/3$ appears, and is characterized by the three-fold degeneracy of the ground states. Although a next-nearest-neighbouring interaction $% V_{2}$ destroys the FTP at $\nu =1/3$, the FTP at $\nu =1/4$ is generated, which is further destroyed by next-next-nearest-neighbouring interaction $V_{3}$. We study the total Berry phase of the low-energy states at both filling factors, and determine the phase diagrams in the $% (V_{1},V_{2})$ and $(V_{2},V_{3})$ planes, respectively. We also present a physical picture of the phase and discuss its existence in the nearly flatband. Within the picture, we argue that the FTP at other filling factors can be generated by introducing proper interactions. The present study may be helpful in understanding the relevant physics in higher dimensions and realized in cold-atom experiments. 4. [|arXiv:1204.2450] (cross-list from gr-qc) [ [|pdf], [|ps] , [|other] ] 4. <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|arXiv:1204.2450] (cross-list from gr-qc) [ <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: none;">[|pdf] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: none;">[|ps] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px; text-decoration: none;">[|other] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">] Entanglement Entropy from a Holographic Viewpoint [|Tadashi Takayanagi] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">The entanglement entropy has been historically studied by many authors in order to obtain quantum mechanical interpretations of the gravitational entropy. The discovery of AdS/CFT correspondence leads to the idea of holographic entanglement entropy, which is a clear solution to this important problem in gravity. In this article, we would like to give a quick survey of recent progresses on the holographic entanglement entropy. We focus on its gravitational aspects, so that it is comprehensible to those who are familiar with general relativity and basics of quantum field theory.

1. [|arXiv:1204.1993] [ [|pdf] ]
Magnetic field induced resistance properties at filling factor 5/2 consistent with non-Abelian e/4 quasiparticles in multiple sized interferometers [|R.L. Willett], [|L.N. Pfeiffer] , [|K.W. West] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Non-Abelian e/4 quasiparticles at 5/2 filling factor in a correlated two-dimensional electron gas have a proposed specific property in an interference measurement of their edge propagation: encircling an even number of localized e/4 quasiparticles allows expression of e/4 Aharonov-Bohm (A-B) oscillations, but suppression of these oscillations will occur with an odd number encircled. This picture is tested explicitly here in multiple interferometers of different areas. The encircled localized e/4 quasiparticle number near 5/2 filling factor is changed by sweeping B-field, and oscillations are observed in resistance near 5/2 of period specific to that device area for each interferometer. The product of the measured interferometric area of each device and the respective 5/2 resistance oscillation period is found to agree with the expected flux quanta addition needed for parity change in the localized e/4 number. This result shows a highly specific non-Abelian property of the quasiparticle excitations at 5/2 filling expressed in multiple interferometers.

<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Apr 10
Spin-Orbit Coupled Degenerate Fermi Gases [|Pengjun Wang], [|Zeng-Qiang Yu], [|Zhengkun Fu], [|Jiao Miao], [|Lianghui Huang], [|Shijie Chai], [|Hui Zhai], [|Jing Zhang]<span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Spin-orbit coupling plays an increasingly important role in the modern condensed matter physics. For instance, it gives birth to topological insulators and topological superconductors. Quantum simulation of spin-orbit coupling using ultracold Fermi gases will offer opportunities to study these new phenomena in a more controllable setting. Here we report the first experimental study of a spin-orbit coupled Fermi gas. We observe spin dephasing in spin dynamics and momentum distribution asymmetry in the equilibrium state as hallmarks of spin-orbit coupling. We also observe evidences of Lifshitz transition where the topology of Fermi surfaces change. This serves as an important first step toward finding Majorana fermions in this system.
 * <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">1. [|arXiv:1204.1887] <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;"> [ <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|pdf] <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|ps] <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">, <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">[|other] <span class="list-identifier" style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">] **

Fractional Quantum Hall Effect in Topological Flat Bands with Chern Number Two [|Yi-Fei Wang], [|Hong Yao] , [|Chang-De Gong] , [|D. N. Sheng] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">Recent theoretical works have demonstrated various robust Abelian and non-Abelian fractional quantum Hall states in lattice models with topological flat bands carrying a Chern number C=1. Here we study hard-core bosons in a three-band triangular-lattice model with the lowest topological flat band of Chern number C=2. We find convincing numerical evidence of bosonic fractional quantum Hall effect at the $\nu=1/3$ filling characterized by three-fold quasi-degeneracy of ground states on a torus, a fractional quantized Chern number for each ground state, a robust spectrum gap, and a gap in quasihole excitation spectrum. More surprisingly for the 1/4 filling, fractional quantum Hall features are also observed, while the topological ground-state degeneracy varies with the particle numbers and shows a strong even-odd effect.
 * 2. [|arXiv:1204.1697] [ [|pdf], [|ps] , [|other] ] **

Apr 9
1. [|arXiv:1204.1434] [ [|pdf], [|ps] , [|other] ] D-wave superconductivity induced by short-range antiferromagnetic correlations in the two-dimensional Kondo lattice model [|Yu Liu], [|Huan Li] , [|Guang-Ming Zhang] , [|L. Yu] <span style="background-color: #ffffff; font-family: 'Lucida Grande',helvetica,arial,verdana,sans-serif; font-size: 14px;">The possible heavy fermion superconductivity is carefully examined in the two-dimensional Kondo lattice model with an antiferromagnetic Heisenberg superexchange between local magnetic moments. In order to establish an effective mean field theory in the limit of the paramagnetic heavy Fermi liquid $J_{K}\gg J_{H}$, we find that the spinon or f-fermion singlet pairing from the local antiferromagnetic short-range correlations can reduces the ground state energy substantially. In the presence of the Kondo screening effect, the Cooper pairs between the conduction electrons is induced. Depending on the ratio of the Heisenberg and the Kondo exchange couplings, the resulting superconducting state is characterized by either a d-wave nodal or d-wave nodeless state, and a continuous phase transition exists between these two states.

=Apr 2 - Apr 6, Zixu Zhang=

Controlling phase separation of a two-component Bose-Einstein condensate by confinement
[|L. Wen], [|W. M. Liu] , [|Yongyong Cai] , [|J. M. Zhang] , [|Jiangping Hu]  Comments: 8 pages, 2 figures Journal-ref: Phys. Rev. A 85, 043602 (2012) Subjects: Quantum Gases (cond-mat.quant-gas) ; Quantum Physics (quant-ph)

We point out that the widely accepted condition g11g22<g122 for phase separation of a two-component Bose-Einstein condensate is insufficient if kinetic energy is taken into account, which competes against the intercomponent interaction and favors phase mixing. Here g11, g22, and g12 are the intra- and intercomponent interaction strengths, respectively. Taking a d-dimensional infinitely deep square well potential of width L as an example, a simple scaling analysis shows that if d=1 (d=3), phase separation will be suppressed as L\rightarrow0 (L\rightarrow\infty) whether the condition g11g22<g122 is satisfied or not. In the intermediate case of d=2, the width L is irrelevant but again phase separation can be partially or even completely suppressed even if g11g22<g122. Moreover, the miscibility-immiscibility transition is turned from a first-order one into a second-order one by the kinetic energy. All these results carry over to d-dimensional harmonic potentials, where the harmonic oscillator length {\xi}ho plays the role of L. Our finding provides a scenario of controlling the miscibility-immiscibility transition of a two-component condensate by changing the confinement, instead of the conventional approach of changing the values of the g's.

Interaction effects on 1D fermionic symmetry protected topological phases
[|Evelyn Tang], [|Xiao-Gang Wen]  Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con)

In free fermion systems with given symmetry and dimension, the possible topological phases are labeled by elements of only three types of Abelian groups, Z_1, Z_2, or Z. For example non-interacting 1D fermionic superconducting phases with S_z spin rotation and time reversal symmetries are classified by Z. With interactions the results are more varied. Here we show that weakly interacting 1D fermionic superconducting phases with S_z spin rotation and time reversal symmetries are classified by Z_4. Using group cohomology, one can show that there are four and only four distinct phases for such 1D superconductors even with strong interactions. Thus all four symmetry protected topological phases of 1D fermionic superconductors with S_z spin rotation and time reversal symmetries can be realized by free fermions. We also show that 1D fermionic superconducting phases with Z_n discrete S_z spin rotation and time reversal symmetries are classified by Z_4 when n=even and Z_2 when n=odd. Again, all these strongly interacting symmetry protected topological phases can be realized by non-interacting fermions.

Andreev Bound states in One Dimensional Topological Superconductor
[|Xiong-Jun Liu] Comments: 4 pages plus appendix; 4 figures Subjects: Superconductivity (cond-mat.supr-con) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We study the Andreev bound states (ABSs) at Josephson junction of the one dimensional topological superconductors with broken spatial inversion symmetry (SIS). Despite the absence of the inversion symmetry, we show a hidden symmetry for the Bogoliubov de Gennes equations around fermi points in addition to the particle-hole symmetry. The hidden symmetry protects that the ABSs carry charges, with the charge value solely depending on fermi velocities. Specifically, when SIS is present, the ABSs are charge neutral, similar as the Majorana fermions. We demonstrate that in the tunneling transport spectroscopy the charge of the ABSs can be measured by the resonant differential tunneling conductance.

Detection of Symmetry Protected Topological Phases in 1D
[|Frank Pollmann], [|Ari M. Turner]  Comments: 12 pages, 9 Figures Subjects: Strongly Correlated Electrons (cond-mat.str-el)

A topological phase is a phase of matter which cannot be characterized by a local order parameter. It has been shown that gapped phases in 1D systems can be completely characterized using tools related to projective representations of the symmetry groups. We show how to determine the matrices of these representations in a simple way in order to distinguish between different phases directly. From these matrices we also point out how to derive several different types of non-local order parameters for time reversal, inversion symmetry and $Z_2 \times Z_2$ symmetry, as well as some more general cases (some of which have been obtained before by other methods). Using these concepts, the ordinary string order for the Haldane phase can be related to a selection rule that changes at the critical point. We furthermore point out an example of a more complicated internal symmetry for which the ordinary string order cannot be applied.

Low-energy local density of states of the 1D Hubbard model
[|Stefan A. Soeffing], [|Imke Schneider] , [|Sebastian Eggert]  Comments: 6 pages, 4 figures, more information can be found at [|this http URL] Subjects: Strongly Correlated Electrons (cond-mat.str-el) ; Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

We examine the local density of states (DOS) at low energies numerically and analytically for the Hubbard model in one dimension. The eigenstates represent separate spin and charge excitations with a remarkably rich structure of the local DOS in space and energy. The results predict signatures of strongly correlated excitations in the tunneling probability along finite quantum wires, such as carbon nanotubes, atomic chains or semiconductor wires in scanning tunneling spectroscopy (STS) experiments. However, the detailed signatures can only be partly explained by standard Luttinger liquid theory. In particular, we find that the effective boundary exponent can be negative in finite wires, which leads to an increase of the local DOS near the edges in contrast to the established behavior in the thermodynamic limit.

Effects of Smooth Boundaries on Topological Edge Modes in Optical Lattices
[|Michael Buchhold], [|Daniel Cocks] , [|Walter Hofstetter]  Comments: 12 pages, 10 figures Subjects: Quantum Gases (cond-mat.quant-gas)

Since the experimental realization of synthetic gauge fields for neutral atoms, the simulation of topologically non-trivial phases of matter with ultracold atoms has become a major focus of cold atom experiments. However, several obvious differences exist between cold atom and solid state systems, for instance the finite size of the atomic cloud and the smooth confining potential. In this article we show that sharp boundaries are not required to realize quantum Hall or quantum spin Hall physics in optical lattices and, on the contrary, that edge states which belong to a smooth confinement exhibit additional interesting properties, such as spatially resolved splitting and merging of bulk bands and the emergence of robust auxiliary states in bulk gaps to preserve the topological quantum numbers. In addition, we numerically validate that these states are robust against disorder. Finally, we analyze possible detection methods, with a focus on Bragg spectroscopy, to demonstrate that the edge states can be detected and that Bragg spectroscopy can reveal how topological edge states are connected to the different bulk bands.

Enlarging and cooling the Néel state in an optical lattice
[|Charles J.M. Mathy], [|David A. Huse] , [|Randall G. Hulet]  Comments: 8 pages, 8 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We propose an experimental scheme to favor both the realization and the detection of the N\'eel state in a two-component gas of ultracold fermions in a three-dimensional simple-cubic optical lattice. By adding three compensating Gaussian laser beams to the standard three pairs of retroreflected lattice beams, and adjusting the relative waists and intensities of the beams, one can significantly enhance the size of the N\'eel state in the trap, thus increasing the signal of optical Bragg scattering. Furthermore, the additional beams provide for adjustment of the local chemical potential and the possibility to evaporatively cool the gas while in the lattice. Our proposals are relevant to other attempts to realize many-body quantum phases in optical lattices.

Direct observation of the Fermi surface in an ultracold atomic gas
[|T. E. Drake], [|Y. Sagi] , [|R. Paudel] , [|J. T. Stewart] , [|J. P. Gaebler] , [|D. S. Jin]  Subjects: Quantum Gases (cond-mat.quant-gas) ; Atomic Physics (physics.atom-ph)

The ideal (i.e. noninteracting), homogeneous Fermi gas, with its characteristic sharp Fermi surface in the momentum distribution, is a fundamental concept relevant to the behavior of many systems. With trapped Fermi gases of ultracold atoms, one can realize and probe a nearly ideal Fermi gas, however these systems have a nonuniform density due to the confining potential. We show that the effect of the density variation, which typically washes out any semblance of a Fermi surface step in the momentum distribution, can be mitigated by selectively probing atoms near the center of a trapped gas. With this approach, we have directly measured a Fermi surface in momentum space for a nearly ideal gas, where the average density and temperature of the probed portion of the gas can be determined from the location and sharpness of the Fermi surface.

1. [|arXiv:1203.6653] [ [|pdf], [|other] ]
Frustrated quantum Ising spins simulated by spinless bosons in a tilted lattice: from a quantum liquid to antiferromagnetic order [|Susanne Pielawa], [|Erez Berg] , [|Subir Sachdev]   Comments: 26 pages, 15 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We study spinless bosons in a decorated square lattice with a near-diagonal tilt. The resonant subspace of the tilted Mott insulator is described by an effective Hamiltonian of frustrated quantum Ising spins on a non-bipartite lattice. This generalizes an earlier proposal for the unfrustrated quantum Ising model in one dimension which was realized in a recent experiment on ultracold $^{87}$Rb atoms in an optical lattice. Very close to diagonal tilt, we find a quantum liquid state which is continuously connected to the paramagnet. Frustration can be reduced by increasing the tilt angle away from the diagonal, and the system undergoes a transition to an antiferromagnetically ordered state. Using quantum Monte Carlo simulations and exact diagonalization, we find that for realistic system sizes the antiferromagnetic order appears to be quasi-one-dimensional; however, in the thermodynamic limit the order is two-dimensional.

Quasi-1D atomic gases across wide and narrow confinement-induced-resonances
[|Xiaoling Cui] Comments: 4.5 pages, 4 figures Subjects: Quantum Gases (cond-mat.quant-gas)

We study quasi-one-dimensional atomic gases across wide and narrow confinement-induced-resonances (CIR). We show from Virial expansion that by tuning the magnetic field, the repulsive scattering branch initially prepared at low fields can continuously go across CIR without decay; instead, the decay occurs when approaching non-interacting limit. The interaction properties essentially rely on the resonance width of CIR. Universal thermodynamics is identified for scattering branch right at wide CIR, but is found to be smeared out in narrow CIR due to strong energy-dependence of coupling strength. In wide and narrow CIR, the interaction energy of scattering branch shows different types of strong asymmetry when approaching the decay from opposite sides of magnetic field.

Topological indices, defects and Majorana fermion in chiral superconductors
[|Daichi Asahi], [|Naoto Nagaosa]  Comments: 5 pages, 4 figures Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall) ; Superconductivity (cond-mat.supr-con)

We study theoretically the role of topological invariants to protect the Majorana fermions in a model of two-dimensional chiral superconductors which belong to class D of topological periodic table. A rich phase diagram is revealed. Each phase is characterized by the topological invariants for 2d (Z) and 1d (Z2), which lead to the Majorana fermion at the edge of the dislocation and the core of the vortex. Interference of the Majorana fermions originated from the different topological invariants is studied. The stability of the Majorana fermion with respect to the interlayer coupling, i.e., in 3d, is also examined.