Aug 2015

Aug 3-Aug 7 Bo Liu, Aug 10-Aug 14 Haiyuan Zou, Aug 17-Aug 21 Ahmet Keles, Aug 24-Aug 28, Xuguang Yue, Aug 31 -Sep 4, Max

Sep, 4

1. arXiv:1509.00854 [pdf, other]
Formation and dynamics of anti-ferromagnetic correlations in tunable optical lattices
Daniel Greif, Gregor Jotzu, Michael Messer, Rémi Desbuquois, Tilman Esslinger

We report on the observation of anti-ferromagnetic correlations of ultracold fermions in a variety of optical lattice geometries that are well described by the Hubbard model, including dimers, 1D chains, ladders, isolated and coupled honeycomb planes, as well as square and cubic lattices. The dependence of the strength of spin correlations on the specific geometry is experimentally studied by measuring the correlations along different lattice tunneling links, where a redistribution of correlations between the different lattice links is observed. By measuring the correlations in a crossover between distinct geometries, we demonstrate an effective reduction of the dimensionality for our atom numbers and temperatures. We also investigate the formation and redistribution time of spin correlations by dynamically changing the lattice geometry and studying the time-evolution of the system. Timescales ranging from a sudden quench of the lattice geometry to an adiabatic evolution are probed.

2. arXiv:1509.01086 (cross-list from quant-ph) [pdf, ps, other]
Perspective: Quantum Thermodynamics
James Millen, André Xuereb

Classical thermodynamics is unrivalled in its range of applications and relevance to everyday life. It enables a description of complex systems, made up of microscopic particles, in terms of a small number of macroscopic quantities, such as work and entropy. As systems get ever smaller, fluctuations of these quantities become increasingly relevant, prompting the development of stochastic thermodynamics. Recently we have seen a surge of interest in exploring the quantum regime, where the origin of fluctuations is quantum rather than thermal. Many questions, such as the role of entanglement and the emergence of thermalisation, lie wide open. Answering these questions may lead to the development of quantum heat engines and refrigerators, as well as to vitally needed simple descriptions of quantum many-body systems.


Sep, 3

1. arXiv:1509.00696 (cross-list from cond-mat.str-el) [pdf, other]
Sudden expansion and domain-wall melting of strongly interacting bosons in two dimensional optical lattices and on multi-leg ladders
Johannes Hauschild, Frank Pollmann, Fabian Heidrich-Meisner

We numerically investigate the expansion of clouds of hard-core bosons in the two-dimensional square lattice using a matrix-product state based method. This nonequilibrium set-up is induced by quenching the trapping potential to zero and our work is specifically motivated by a recent experiment with interacting bosons in an optical lattice [Ronzheimer et al., Phys. Rev. Lett. 110, 205301 (2013)]. As the anisotropy of the amplitudes Jx and Jy for hopping in different spatial directions is varied from the one- to the two-dimensional case, we observe a crossover from a fast ballistic expansion in the one-dimensional limit Jx≫Jy to much slower dynamics in the isotropic two-dimensional limit Jx=Jy. We further study the dynamics on multi-leg ladders and long cylinders. For these geometries we compare the expansion of a cloud to the melting of a domain wall, which helps us to identify several different regimes of the expansion as a function of time. By studying the dependence of expansion velocities on both the anisotropy Jy/Jx and the number of legs, we observe that the expansion on two-leg ladders, while similar to the two-dimensional case, is slower than on wider ladders. We provide a qualitative explanation for this observation based on an analysis of the rung spectrum.

2. arXiv:1509.00515 (cross-list from quant-ph) [pdf, ps, other]
On the Appearance of Families of Efimov States in the Spinor Three-Body Problem
V. E. Colussi, Chris H. Greene, J. P. D'Incao

Few-body systems with access to multiple internal levels exhibit richness beyond that typically found in their single-level counterparts. One example is that of Efimov states in strongly-correlated spinor three-body systems. In [V. E. Colussi, C. H. Greene, and J. P. D'Incao, Phys. Rev. Lett. {\bf 113}, 045302 (2014)] this problem was analyzed for spinor condensates finding a complex level structure as in an early work [Bulgac and Efimov, Sov. J. Nucl. Phys. 22, 153 (1976)] in nuclear physics, and the impact of Efimov physics on the general form of the scattering observables was worked out. In this paper we discuss the appearance of novel families of Efimov states in the spinor three-body problem.


Sep, 2

1. arXiv:1509.00005 [pdf, ps, other]
Interaction driven exotic quantum phases in spin-orbit coupled spin$-1$ bosons
J. H. Pixley, Stefan S. Natu, I. B. Spielman, S. Das Sarma

We study the interplay among large-spin, spin orbit coupling, and superfluidity for bosons in a two dimensional optical lattice, focusing on the spin-1 spin-orbit coupled system recently realized at the Joint Quantum Institute [Campbell et. al.,arXiv:1501.05984]. We find a rich quantum phase diagram, where, in addition to the conventional phases -superfluid and insulator- contained in the spin-1 Bose-Hubbard model, there are new symmetry broken spin-orbit driven exotic phases where bosons condense at single or multiple points in momentum space, that have no analog in the continuum. In the lattice, increasing the spin-orbit coupling strength induces a robust spin density wave superfluid with broken translation symmetry where a non-zero condensate fraction occurs both at the center and edge of the Brillouin zone. For strong interactions, spin density wave order is suppressed concomitantly with the condensate at zero momentum leading to a continuous transition into a superfluid only at the Brillouin zone edge. We show that the spin density wave superfluid phase survives in a two dimensional harmonic trap, and thus establish that our results are directly applicable to experiments on 87Rb, 7Li, and 41K.

2. arXiv:1509.00035 (cross-list from cond-mat.mes-hall) [pdf, ps, other]
A quantum resonance catastrophe for transport through an AC driven impurity
Daniel Thuberg, Sebastian A. Reyes, Sebastian Eggert

We consider the quantum transport in a tight-binding chain with a locally applied potential which is oscillating in time. The steady state for such a driven impurity can be calculated exactly for any energy and applied potential using the Floquet formalism. The resulting transmission has a non-trivial, non-monotonic behavior depending on incoming momentum, driving frequency, and the strength of the applied periodic potential. Hence there is an abundance of tuning possibilities, which allows to find resonances of total reflection for any choice of incoming momentum and periodic potential. Remarkably, this implies that even for an arbitrarily small infinitesimal impurity potential it is always possible to find a resonance frequency at which there is a catastrophic breakdown of the transmission T=0. The points of zero transmission are closely related to the phenomenon of Fano resonances at dynamically created bound states in the continuum. The results are relevant for a variety of one-dimensional systems where local AC driving is possible, such as quantum nanodot arrays, ultracold gases in optical lattices, photonic crystals, or molecular electronics.


Sep, 1
1. arXiv:1508.07784 [pdf, ps, other]
Dipolar dark solitons
Krzysztof Pawlowski, Kazimierz Rzazewski

We numerically generate, and then study the basic properties of dark soliton-like excitations in a dipolar gas confined in a quasi one dimensional trap. These excitations, although very similar to dark solitons in a gas with contact interaction, interact with each other and can form bound states. During collisions these dipolar solitons emit phonons, loosing energy, but accelerating. Even after thousands of subsequent collisions they survive as gray solitons and finally reach dynamical equilibrium with background quasiparticles. Finally, in the frame of classical field approximation, we verified, that these solitons appear spontaneously in thermal samples, analogously to the type II excitations in a gas of atoms with contact interaction.
Aug 27
1. arXiv:1508.06285 [pdf, other]
Physics of higher orbital bands in optical lattices: a review
Xiaopeng Li, W. Vincent Liu
Comments: 77 pages, 18 figures, invited review for Rep. Prog. Phys
Subjects: Quantum Gases (cond-mat.quant-gas)

Orbital degree of freedom plays a fundamental role in understanding the unconventional properties in solid state materials. Experimental progress in quantum atomic gases has demonstrated that high orbitals in optical lattices can be used to construct quantum emulators of exotic models beyond natural crystals, where novel many-body states such as complex Bose-Einstein condensation and topological semimetals emerge. A brief introduction of orbital degree of freedom in optical lattices is given and a summary of exotic orbital models and resulting many-body phases is provided. Experimental consequences of the novel phases are also discussed.

2. arXiv:1508.06471 (cross-list from quant-ph) [pdf, other]
Exploring Interacting Quantum Many-Body Systems by Experimentally Creating Continuous Matrix Product States in Superconducting Circuits
C. Eichler, J. Mlynek, J. Butscher, P. Kurpiers, K. Hammerer, T. J. Osborne, A. Wallraff
Comments: 11 pages, 9 figuresSubjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)
Improving the understanding of strongly correlated quantum many body systems such as gases of interacting atoms or electrons is one of the most important challenges in modern condensed matter physics, materials research and chemistry. Enormous progress has been made in the past decades in developing both classical and quantum approaches to calculate, simulate and experimentally probe the properties of such systems. In this work we use a combination of classical and quantum methods to experimentally explore the properties of an interacting quantum gas by creating experimental realizations of continuous matrix product states - a class of states which has proven extremely powerful as a variational ansatz for numerical simulations. By systematically preparing and probing these states using a circuit quantum electrodynamics (cQED) system we experimentally determine a good approximation to the ground-state wave function of the Lieb-Liniger Hamiltonian, which describes an interacting Bose gas in one dimension. Since the simulated Hamiltonian is encoded in the measurement observable rather than the controlled quantum system, this approach has the potential to apply to exotic models involving multicomponent interacting fields. Our findings also hint at the possibility of experimentally exploring general properties of matrix product states and entanglement theory. The scheme presented here is applicable to a broad range of systems exploiting strong and tunable light-matter interactions.

3. arXiv:1508.06362 (cross-list from physics.atom-ph) [pdf, other]
Phase Diagram of a Strongly Interacting Spin-Imbalanced Fermi Gas
Ben A. Olsen, Melissa C. Revelle, Jacob A. Fry, Daniel E. Sheehy, Randall G. Hulet
Comments: 7 pages, 5 figuresSubjects: Atomic Physics (physics.atom-ph); Quantum Gases (cond-mat.quant-gas)
We obtain the phase diagram of spin-imbalanced interacting Fermi gases from measurements of density profiles of 6Li atoms in a harmonic trap. These results agree with, and extend, previous experimental measurements. Measurements of the critical polarization at which the balanced superfluid core vanishes generally agree with previous experimental results and with quantum Monte Carlo (QMC) calculations in the BCS and unitary regimes. We disagree with the QMC results in the BEC regime, however, where the measured critical polarizations are greater than theoretically predicted. We also measure the equation of state in the crossover regime for a gas with equal numbers of the two fermion spin states.


4. arXiv:1508.06606 [pdf, other]

Thermodynamics and dynamics of atomic selforganization in an optical cavity
Stefan Schütz, Simon B. Jäger, Giovanna Morigi
Comments: 21 pages, 15 figuresSubjects: Quantum Physics (quant-ph)

Pattern formation of atoms in high-finesse optical resonators results from the mechanical forces of light associated with superradiant scattering into the cavity mode. It occurs when the laser intensity exceeds a threshold value, such that the pumping processes counteract the losses. We consider atoms driven by a laser and coupling with a mode of a standing-wave cavity and describe their dynamics with a Fokker-Planck equation, in which the atomic motion is semiclassical but the cavity field is a full quantum variable. The asymptotic state of the atoms is a thermal state, whose temperature is solely controlled by the detuning between the laser and the cavity frequency and by the cavity loss rate. From this result we derive the free energy and show that in the thermodynamic limit selforganization is a second-order phase transition. The order parameter is the field inside the resonator, to which one can associate a magnetization in analogy to ferromagnetism, the control field is the laser intensity, however the steady state is intrinsically out-of-equilibrium. In the symmetry-broken phase quantum noise induces jumps of the spatial density between two ordered patterns: We characterize the statistical properties of this temporal behaviour at steady state and show that the thermodynamic properties of the system can be extracted by detecting the light at the cavity output. The results of our analysis are in full agreement with previous studies, extend them by deriving a self-consistent theory which is valid also when the cavity field is in the shot-noise limit, and elucidate the nature of the selforganization transition.





5. arXiv:1508.06509 (cross-list from quant-ph) [pdf, other]
Observation of Floquet states in a strongly driven artificial atom
Chunqing Deng, Jean-Luc Orgiazzi, Feiruo Shen, Sahel Ashhab, Adrian Lupascu
Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Atomic Physics (physics.atom-ph)
We present experiments on the driven dynamics of a two-level superconducting artificial atom. The driving strength reaches 4.78 GHz, significantly exceeding the transition frequency of 2.288 GHz. The observed dynamics is described in terms of quasienergies and quasienergy states, in agreement with Floquet theory. In addition, we observe the role of pulse shaping in the dynamics, as determined by non-adiabatic transitions between Floquet states, and we implement subnanosecond single-qubit operations. These results pave the way to quantum control using strong driving with applications in quantum technologies.




Aug 26
1. arXiv:1508.06051 [pdf, ps, other]
Pairing of fermions with unequal charges in an artificial magnetic field
F. Nur Ünal, M. Ö. Oktel
Subjects: Quantum Gases (cond-mat.quant-gas); Superconductivity (cond-mat.supr-con)

Artificial magnetic fields created for ultra cold systems depend sensitively on the internal structure of the atoms. In a mixture, each component couples to the artificial field with a different charge. This enables the study of Bardeen-Cooper-Schrieffer pairing of fermions with unequal charges. In this paper, we investigate the superconducting (SC) transition of a system formed by such pairs as a function of field strength. We consider a homogeneous two-component Fermi gas of unequal charges but equal densities with attractive interactions. We find that the phase diagram is altered drastically compared to the usual equal charge case. First, for some magnetic fields there is no SC transition and isolated SC phases are formed, reflecting the discrete Landau level (LL) structure. SC phases become reentrant both in magnetic field and temperature. For extremely high fields where both components are confined to their lowest LLs, the effect of the charge imbalance is suppressed. Charge asymmetry reduces the critical temperature even in the low-field semiclassical regime. We discuss a pair breaking mechanism due to the unequal Lorentz forces acting on the components of the Cooper pairs to identify the underlying physics.

2. arXiv:1508.05947 [pdf, other]
Parity effect and few-to-many particle crossover in a mesoscopic Fermi gas
Johannes Hofmann, Alejandro M. Lobos, Victor Galitski
Comments: 5 pages, 1 figureSubjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Superconductivity (cond-mat.supr-con)
We develop a quantitative analytic theory that accurately describes the odd-even effect observed experimentally in a one-dimensional, trapped Fermi gas with a small number of particles [G. Z\"urn et al., Phys. Rev. Lett. 111, 175302 (2013)]. We find that the underlying physics is similar to the parity effect known to exist in ultrasmall mesoscopic superconducting grains and atomic nuclei. However, in contrast to superconducting nanograins, the density fluctuation effect (Hartree correction) dominates over the superconducting pairing fluctuations and leads to a much more pronounced odd-even effect in the mesoscopic, trapped Fermi gas. We calculate the corresponding parity parameter and separation energy using both perturbation theory and a path integral framework, generalized to account for the effects of the trap, pairing fluctuations, and Hartree corrections. Our results are in an excellent quantitative agreement with experimental data and exact diagonalization. Finally, we describe a few-to-many particle crossover between the perturbative mesoscopic regime and non-perturbative many-body physics that the system approaches in the thermodynamic limit.


3. arXiv:1508.06099 [pdf, other]
Quantum Thermodynamics
Sai Vinjanampathy, Janet Anders
Comments: Submitted to Contemporary Physics. Comments are welcomeSubjects: Quantum Physics (quant-ph)

Quantum thermodynamics is an emerging research field aiming to extend standard thermodynamics and non-equilibrium statistical physics to ensembles of sizes well below the thermodynamic limit, in non-equilibrium situations, and with the full inclusion of quantum effects. Fuelled by experimental advances and the potential of future nanoscale applications this research effort is pursued by scientists with different backgrounds, including statistical physics, many-body theory, mesoscopic physics and quantum information theory, who bring various tools and methods to the field. A multitude of theoretical questions are being addressed ranging from issues of thermalisation of quantum systems and various definitions of "work", to the efficiency and power of quantum engines. This overview provides a perspective on a selection of these current trends accessible to postgraduate students and researchers alike.

Aug 25
1. arXiv:1508.05640 [pdf, other]
Quantum properties of light scattered from structured many-body phases of ultracold atoms in quantum optical lattices
Santiago F. Caballero-Benitez, Igor B. Mekhov
Comments: 17 pages, 3 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph); Optics (physics.optics); Quantum Physics (quant-ph)

Quantum trapping potentials for ultracold gases change the landscape of classical properties of scattered light and matter. The atoms in a quantum many-body correlated phase of matter change the properties of light and vice versa. The properties of both light and matter can be tuned by design and depend on the interplay between long-range (nonlocal) interactions mediated by an optical cavity and short-range processes of the atoms. Moreover, the quantum properties of light get significantly altered by this interplay, leading the light to have nonclassical features. Further, these nonclassical features can be designed and optimised.

2. arXiv:1508.05449 [pdf, ps, other]
Quantum versus mean-field collapse in a many-body system
G. E. Astrakharchik, B. A. Malomed
Comments: 10 pages, 4 figuresSubjects: Quantum Gases (cond-mat.quant-gas)
The recent analysis, based on the mean-field approximation (MFA), has predicted that the critical quantum collapse of the bosonic wave function, pulled to the center by the inverse-square potential in the three-dimensional space, is suppressed by the repulsive cubic nonlinearity in the bosonic gas, the collapsing ground state being replaced by a regular one. We demonstrate that a similar stabilization acts in a quantum many-body system, beyond the MFA. While the collapse remains possible, repulsive two-particle interactions give rise to a metastable gaseous state, which is separated by a potential barrier from the collapsing regime. The stability of this state improves with the increase of the number of particles. The results are produced by calculations of the variational energy, with the help of the Monte Carlo method.

3. arXiv:1508.05927 (cross-list from cond-mat.str-el) [pdf, other]
Haldane phase in one-dimensional topological Kondo insulators
Alejandro Mezio, Alejandro M. Lobos, Ariel O. Dobry, Claudio J. Gazza
Comments: 8 pages, 4 figures, 1 appendixSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)
We investigate the groundstate properties of a recently proposed model for a topological Kondo insulator in one dimension (i.e., the p-wave Kondo-Heisenberg lattice model) by means of the Density Matrix Renormalization Group method. The non-standard Kondo interaction in this model is different from the usual (i.e., local) Kondo interaction in that the localized spins couple to the "p-wave" spin density of conduction electrons, inducing a topologically non-trivial insulating groundstate. Based on the analysis of the charge- and spin-excitation gaps, the string order parameter, and the spin profile in the groundstate, we show that, at half-filling and low energies, the system is in the Haldane phase and hosts topologically protected spin-1/2 end-states. Beyond its intrinsic interest as a useful "toy-model" to understand the effects of strong correlations on topological insulators, we show that the p-wave Kondo-Heisenberg model can be implemented in p−band optical lattices loaded with ultra-cold Fermi gases.


4. arXiv:1508.05391 (cross-list from cond-mat.dis-nn) [pdf, other]

Many-Body Localization in a Two-Dimensional Anderson-Hubbard Model
Yevgeny Bar Lev, David R. Reichman
Comments: 5 pages, 3 figuresSubjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech)

We study the real-time dynamics of a two-dimensional Anderson--Hubbard model using nonequilibrium self-consistent perturbation theory within the second-Born approximation. When compared with exact diagonalization performed on small clusters, we demonstrate that for strong disorder this technique approaches the exact result on all available timescales, while for intermediate disorder, in the vicinity of the many-body localization transition, it produces quantitatively accurate results up to nontrivial times. Our method allows for the treatment of system sizes inaccessible by any numerically exact method and for the complete elimination of finite size effects for the largest times considered. We show that for a sufficiently strong disorder the system becomes nonergodic, while for intermediate disorder strengths and for all accessible time scales transport in the system is strictly subdiffusive. We argue that these results are incompatible with a simple percolation picture. Our results are consistent with the heuristic random resistor network model where subdiffusion may be observed for long times until a crossover to diffusion occurs. The prediction of a broad subdiffusive regime in a two-dimensional interacting and disordered system can be directly verified in future cold atoms experiments.









Aug 24
1. arXiv:1508.05321 [pdf, other]
Superfluid phases of fermions with hybridized $s$ and $p$ orbitals
Shaoyu Yin, J.E. Baarsma, M.O.J. Heikkinen, J.-P. Martikainen, P. Törmä
Comments: 13 pages, 11 figures
Subjects: Quantum Gases (cond-mat.quant-gas)

We explore the superfluid phases of a two-component Fermi mixture with hybridized orbitals in optical lattices. We show that there exists a general mapping of this system to the Lieb lattice. By using simple multiband models with hopping between s and p-orbital states, we show that superfluid order parameters can have a π-phase difference between lattice sites, which is distinct from the case with hopping between s-orbitals. If the population imbalance between the two spin species is tuned, the superfluid phase may evolve through various phases due to the interplay between hopping, interactions and imbalance. We show that the rich behavior is observable in experimentally realizable systems.

2. arXiv:1508.05278 [pdf, other]
Topologically non-trivial Hofstadter bands on the kagome lattice
Christoph H. Redder, Götz S. Uhrig
Comments: 8 pages, 11 figuresSubjects: Quantum Gases (cond-mat.quant-gas)
We investigate how the multiple bands of fermions on a crystal lattice evolve if a magnetic field is added which does not increase the number of bands. The kagome lattice is studied as generic example for a lattice with loops of three bonds. Finite Chern numbers occur as non-trivial topological property in presence of the magnetic field. The symmetries and periodicities as function of the applied field are discussed. Strikingly, the dispersions of the edge states depend crucially on the precise shape of the boundary. This suggests that suitable design of the boundaries helps to tune physical properties which may even differ between upper and lower edge. Moreover, we suggest a promising gauge to realize this model in optical lattices.



3. arXiv:1508.05139 [pdf, other]
Topological phases of two-component bosons in species-dependent artificial gauge potentials
Ying-Hai Wu, Tao Shi
Comments: 10 pages, 3 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We study ultracold bosonic atoms with two internal states in artificial gauge potentials whose strengths are different for the two components. A series of topological phases for such systems is proposed based on the composite fermion theory and parton construction. It is found in exact diagonalization that some of the proposed states may be realized for simple contact interaction between bosons. The ground states and low-energy excitations of these states are modeled using trial wave functions. We also construct effective field theories for these states to reveal their properties.




Aug 14
arXiv:1508.03203 [pdf, other]
Weyl Mott Insulator
Takahiro Morimoto, Naoto Nagaosa
Comments: 14 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el)

Relativistic Weyl fermion (WF) often appears in the band structure of three dimensional magnetic materials and acts as a source or sink of the Berry curvature, i.e., the (anti-)monopole. It has been believed that the WFs are stable due to their topological indices except when two Weyl fermions of opposite chiralities annihilate pairwise. Here, we theoretically show for a model including the electron-electron interaction that the Mott gap opens for each WF without violating the topological stability, leading to a topological Mott insulator dubbed {\it Weyl Mott insulator } (WMI). This WMI is characterized by several novel features such as (i) energy gaps in the angle-resolved photo-emission spectroscopy (ARPES) and the optical conductivity, (ii) the nonvanishing Hall conductance, and (iii) the Fermi arc on the surface with the penetration depth diverging as approaching to the momentum at which the Weyl point is projected. Experimental detection of the WMI by distinguishing from conventional Mott insulators is discussed with possible relevance to pyrochlore iridates.


Aug 13
arXiv:1508.02944 [pdf, other]
Topological phases of shaken quantum Ising lattices
Samuel Fernandez-Lorenzo, Juan Jose Garcia-Ripoll, Diego Porras
Comments: 10 pages, 3 figures, comments are welcome
Subjects: Quantum Gases (cond-mat.quant-gas)
The quantum compass model consists of a two-dimensional square spin lattice where the orientation of the spin-spin interactions depends on the spatial direction of the bonds. It has remarkable symmetry properties and the ground state shows topological degeneracy. The implementation of the quantum compass model in quantum simulation setups like ultracold atoms and trapped ions is far from trivial, since spin interactions in those sytems typically are independent of the spatial direction. Ising spin interactions, on the contrary, can be induced and controlled in atomic setups with state-of-the art experimental techniques. In this work, we show how the quantum compass model on a rectangular lattice can be simulated by the use of the photon-assisted tunneling induced by periodic drivings on a quantum Ising spin model. We describe a procedure to adiabatically prepare one of the doubly-degenerate ground states of this model by adiabatically ramping down a transverse magnetic field, with surprising differences depending on the parity of the lattice size. Exact diagonalizations confirm the validity of this approach for small lattices. Specific implementations of this scheme are presented with ultracold atoms in optical lattices in the Mott insulator regime, as well as with Rydberg atoms.

arXiv:1508.02914 [pdf, other]
Unconventional Bose-Einstein Condensations of Two-species Bosons in the $p$-orbital Bands in Optical lattice
Jhih-Shih You, I-Kang Liu, Daw-Wei Wang, Shih-Chuan Gou, Congjun Wu
Comments: 5 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
In the context of the Gross-Pitaevskii theory, we investigate the unconventional Bose-Einstein condensations of two-species mixture with p-wave symmetry in the second band of a bipartite optical lattice. A new imaginary-time propagation method is developed to numerically determine the p-orbital condensation by truncating states in the lowest bands, which is also applicable to even higher orbital bands. Different from the single-species case, the two-species boson mixture exhibits two non-equivalent complex BECs in the intraspecies-interaction-dominating regime, with one breaking time-reversal symmetry while the other not. When the interspecies interaction is tuned across the SU(2) invariant point, the system undergoes a quantum phase transition toward a real-valued checkerboard state characterized by a staggered spin density structure. The effects of lattice asymmetry and strong interaction on the quantum phase transition are addressed. Finally, an experimental scheme for phase measurement is presented.


Aug 12
arXiv:1508.02545 [pdf, ps, other]
Quantum phase transitions and Berezinskii-Kosterlitz-Thouless temperature in a two-dimensional spin-orbit-coupled Fermi gas
Jeroen P.A. Devreese, Jacques Tempere, Carlos A.R. Sá de Melo
Comments: 17 pages, 7 figures, submitted to Physical Review A
Subjects: Quantum Gases (cond-mat.quant-gas)
We study the effect of spin-orbit coupling on both the zero-temperature and non-zero temperature behavior of a two-dimensional (2D) Fermi gas. We include a generic combination of Rashba and Dresselhaus terms into the system Hamiltonian, which allows us to study both the experimentally relevant equal-Rashba-Dresselhaus (ERD) limit and the Rashba-only (RO) limit. At zero temperature, we derive the phase diagram as a function of the two-body binding energy and Zeeman field. In the ERD case, this phase diagram reveals several topologically distinct uniform superfluid phases, classified according to the nodal structure of the quasiparticle excitation energies. Furthermore, we use a momentum dependent SU(2)-rotation to transform the system into a generalized helicity basis, revealing that spin-orbit coupling induces a triplet pairing component of the order parameter. At non-zero temperature, we study the Berezinskii-Kosterlitz-Thouless (BKT) phase transition by including phase fluctuations of the order parameter up to second order. We show that the superfluid density becomes anisotropic due to the presence of spin-orbit coupling (except in the RO case). This leads both to elliptic vortices and antivortices, and to anisotropic sound velocities. The latter prove to be sensitive to quantum phase transitions between topologically distinct phases. We show further that at a fixed non-zero Zeeman field, the BKT critical temperature is increased by the presence of ERD spin-orbit coupling. Subsequently, we demonstrate that the Clogston limit becomes infinite: TBKT remains non-zero at all finite values of the Zeeman field. We conclude by extending the quantum phase transition lines to non-zero temperature, using the nodal structure of the quasiparticle spectrum, thus connecting the BKT critical temperature with the zero-temperature results.


Aug 11

arXiv:1508.01802 [pdf, other]
Exotic Superconductivity Through Bosons in a Dynamical Cluster Approximation
Thomas Bilitewski, Lode Pollet
Comments: 6 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Superconductivity (cond-mat.supr-con)
We study the instabilities towards (exotic) superconductivity of mixtures of spin-1/2 fermions coupled to scalar bosons on a two-dimensional square lattice with the Dynamical-Cluster-Approximation (DCA) using a numerically exact continuous-time Monte-Carlo solver. The Bogoliubov bosons provide an effective phononic bath for the fermions and induce a non-local retarded interaction between the fermions, which can lead to (exotic) superconductivity. Because of the sign problem the biggest clusters we can study are limited to 2×2 in size, but this nevertheless allows us to study the pairing instablilities, and their possible divergence, in the s-, extended s-, p- and d -wave channels as well as the competition with antiferromagnetic fluctuations. At fermionic half-filling we find that d-wave is stable when the mediated interaction by the bosons is of the same order as the bare fermionic repulsion. Its critical temperature can be made as high as the maximum one for s-wave, which opens perspectives for its detection in a cold atom experiment.


arXiv:1508.02197 [pdf, other]
Observation of half-quantum vortices in superfluid 3He
S. Autti, V.V. Dmitriev, V.B. Eltsov, J. Makinen, G.E. Volovik, A.N. Yudin, V.V. Zavjalov
Comments: 11 pages, 6 figures
Subjects: Other Condensed Matter (cond-mat.other); High Energy Physics - Phenomenology (hep-ph)

Recently the new phase of the superfluid 3He has been reported to exist in the aerogel-like structure (nafen) - the so-called polar phase [1]. As distinct from the other phases - the chiral superfluid 3He-A with Weyl nodes and the fully gapped topological 3He-B with Dirac nodes on the surface - the polar phase has Dirac nodal line in bulk and dispersionless band of Andreev-Majorana fermions on the surface [2]. The polar phase can support exotic objects - the half-quantum vortices (HQV) with unpaired Majoranas in their cores [3-5] Originally the HQVs have been predicted to exist in the Weyl superfluid 3He-A [6,7], but still they have not been observed there due to unfavorable combination of spin-orbit and magnetic anisotropy effects on the orientation of the order parameter. In nafen, if the magnetic field is absent or is oriented parallel to nafen strands, the half-quantum vortices win over the conventional single-quantum vortices. We cool down a sample of nafen through transition to the polar phase in rotation up to 2.75\,rad/s without magnetic field. Then the field is switched on, and in the tilted field we observe a satellite peak in the NMR spectrum. Dependence of the satellite on the rotation velocity, temperature and the field orientation is in agreement with our calculations of spin-wave resonances from bound pairs of pinned HQVs connected by solitons. Experiments also demonstrate that the HQVs formed under rotation are strongly pinned by the nafen strands.


Fri, 7
1. arXiv:1508.01275 [pdf, ps, other]
Phase diagrams of Bose-Hubbard model and Haldane-Bose-Hubbard model with complex hopping amplitudes
Yoshihito Kuno, Takashi Nakafuji, Ikuo Ichinose
In this paper, we study Bose-Hubbard models on the square and honeycomb lattices with complex hopping amplitudes, which are feasible by recent experiments of cold atomic gases in optical lattices. To clarify phase diagrams, we use an extended quantum Monte-Carlo simulations (eQMC). For the system on the square lattice, the complex hopping is realized by an artificial magnetic field. We found that vortex-solid states form for certain set of magnetic field, i.e., the magnetic field with the flux quanta per plaquette f=p/q, where p and q are co-prime natural numbers. For the system on the honeycomb lattice, we add the next-nearest neighbor complex hopping. The model is a bosonic analog of the Haldane-Hubbard model. By means of the eQMC, we study the model with both weak and strong on-site repulsions. Numerical study shows that the model has a rich phase diagram. We also found that in the system defined on the honeycomb lattice of the cylinder geometry, an interesting edge state appears.



2. arXiv:1508.01199 (cross-list from hep-th) [pdf, ps, other]
Non-hydrodynamic transport in trapped unitary Fermi gases
Jasmine Brewer, Paul Romatschke
Many strongly coupled fluids are known to share similar hydrodynamic transport properties. In this work we argue that this similarity could extend beyond hydrodynamics to transient dynamics through the presence of non-hydrodynamic modes. We review non-hydrodynamic modes in kinetic theory and gauge/gravity duality and discuss their signatures in trapped Fermi gases close to unitarity. Reanalyzing previously published experimental data, we find hints of non-hydrodynamic modes in cold Fermi gases in two and three dimensions.


Thu, 6
1. arXiv:1508.00958 [pdf, other]
Periodic shedding of vortex dipoles from a moving penetrable obstacle in a Bose-Einstein condensate
Woo Jin Kwon, Sang Won Seo, Yong-il Shin
We investigate vortex shedding from a moving penetrable obstacle in a highly oblate Bose-Einstein condensate. The penetrable obstacle is formed by a repulsive Gaussian laser beam that has the potential barrier height lower than the chemical potential of the condensate. The moving obstacle periodically generates vortex dipoles and the vortex shedding frequency fv linearly increases with the obstacle velocity v as fv=a(v−vc), where vc is a critical velocity. Based on periodic shedding behavior, we demonstrate deterministic generation of a single vortex dipole by applying a short linear sweep of a laser beam. This method will allow further controlled vortex experiments such as dipole-dipole collisions.



2. arXiv:1508.00906 [pdf, other]
Causality and quantum criticality with long-range interactions
Mohammad F. Maghrebi, Zhe-Xuan Gong, Michael Foss-Feig, Alexey V. Gorshkov
Quantum lattice systems with long-range interactions often exhibit drastically different behavior than their short-range counterparts. In particular, because they do not satisfy the conditions for the Lieb-Robinson theorem, they need not have an emergent relativistic structure in the form of a light cone. Adopting a field-theoretic approach, we study the one-dimensional transverse-field Ising model and a fermionic model with long-range interactions, explore their critical and near-critical behavior, and characterize their response to local perturbations. We deduce the dynamic critical exponent, up to the two-loop order within the renormalization group theory, which we then use to characterize the emergent causal behavior. We show that beyond a critical value of the power-law exponent of long-range interactions, the dynamics effectively becomes relativistic. Various other critical exponents describing correlations in the ground state, as well as deviations from a linear causal cone, are deduced for a wide range of the power-law exponent.


Wed, 5

1. arXiv:1508.00656 [pdf, other]
Towards quantum-enhanced interferometry with harmonically trapped quantum matter-wave bright solitons
Bettina Gertjerenken, Timothy P. Wiles, Christoph Weiss
We model the dynamics of attractively interacting ultracold bosonic atoms in a quasi-one-dimensional wave-guide with additional harmonic trapping. Initially, we prepare the system in its ground state and then shift the zero of the harmonic trap and switch on an additional narrow scattering potential near the center of the trap. After colliding with the barrier twice, we propose to measure the number of atoms opposite to the initial condition. Quantum-enhanced interferometry with quantum bright solitons allows us to predict detection of an offset of the scattering potential with considerably increased accuracy as compared to single-particle experiments. In a future experimental realization this might lead to measurement of weak forces caused, for example, by small horizontal gradients in the gravitational potential --- with a resolution of several micrometers given essentially by the size of the solitons. Our numerical simulations are based on the rigorously proved effective potential approach developed in [Phys. Rev. Lett. 102, 010403 (2009) and Phys. Rev. Lett. 103, 210402 (2009)], in a parameter regime inaccessible to the mean-field description via the Gross-Pitaevskii equation due to Schr\"odinger-cat states occurring between the two scattering events.



2. arXiv:1508.00578 [pdf, other]
Connecting strongly correlated superfluids by a quantum point contact
Dominik Husmann, Shun Uchino, Sebastian Krinner, Martin Lebrat, Thierry Giamarchi, Tilman Esslinger, Jean-Philippe Brantut
Point contacts provide simple connections between macroscopic particle reservoirs. In electric circuits, strong links between metals, semiconductors or superconductors have applications for fundamental condensed-matter physics as well as quantum information processing. However for complex, strongly correlated materials, links have been largely restricted to weak tunnel junctions. Here we study resonantly interacting Fermi gases connected by a tunable, ballistic quantum point contact, finding a non-linear current-bias relation. At low temperature, our observations agree quantitatively with a theoretical model in which the current originates from multiple Andreev reflections. In a wide contact geometry, the competition between superfluidity and thermally activated transport leads to a conductance minimum. Our system offers a controllable platform for the study of mesoscopic devices based on strongly interacting matter.


Tue, 4
1. arXiv:1508.00472 [pdf, ps, other]
Dynamics of pattern-loaded fermions in bichromatic optical lattices
Matthew Reichl, Erich Mueller
Motivated by experiments in Munich (M. Schreiber et. al. arXiv:1501.05661), we study the dynamics of interacting fermions initially prepared in charge density wave states in bichromatic optical lattices. The experiment sees a marked lack of thermalization, which has been taken as evidence for an interacting generalization of Anderson localization, dubbed "many-body localization." We model the experiments using an interacting Aubry-Andre model and develop a computationally efficient low-density cluster expansion to calculate the even-odd density imbalance as a function of interaction strength and potential strength. Our calculations agree with the experimental results and shed light on the phenomena. We also explore how allowing hopping in transverse directions suppresses the imbalance. The cluster expansion method we develop should have broad applicability to similar problems in non-equilibrium quantum physics.




2. arXiv:1508.00415 (cross-list from hep-lat) [pdf, other]
Complex Langevin simulation in condensed matter physics
Arata Yamamoto, Tomoya Hayata
The complex Langevin method is one hopeful candidate to tackle the sign problem. This method is applicable not only to QCD but also to nonrelativistic field theory, such as condensed matter physics. We present the simulation results of a rotating Bose gas and an imbalanced Fermi-Hubbard model.


Mon, 3

1. arXiv:1507.08944 (cross-list from cond-mat.dis-nn) [pdf, ps, other]
Spin-selective localization of correlated lattice fermions
J. Skolimowski, D. Vollhardt, K. Byczuk
The interplay between local, repulsive interactions and disorder acting only on one spin orientation of lattice fermions ("spin-dependent disorder") is investigated. The nonmagnetic disorder vs. interaction phase diagram is computed using Dynamical Mean-Field Theory in combination with the geometric average over disorder. The latter determines the typical local density of states and is therefore sensitive to Anderson localization. The effect of spin-dependent disorder is found to be very different from that of conventional disorder. In particular, it destabilizes the metallic solution and leads to a novel spin-selective, localized phase at weak interactions and strong disorder.



2. arXiv:1507.08877 (cross-list from cond-mat.other) [pdf, other]
Demixing Effects in Mixtures of Two Bosonic Species
F. Lingua, B. Capogrosso-Sansone, M. Guglielmino, V. Penna
Motivated by recent experiments on two-component systems, we investigate the ground-state phase diagram of a mixture of two bosonic species by means of path-integral quantum Monte Carlo by the two-worm algorithm. The mixture is trapped in a square lattice at half-filling conditions. Various quantum phases are stabilized depending on the interplay between intra- and inter-species interactions. We show that the phase diagram features a demixed superfluid phase and demixed Mott-Insulator phase when the inter-species interaction becomes greater than the intra-species repulsion, and a double-superfluid phase or a supercounterflow otherwise. We also show that demixing, characterized by spatial separation of the two species, can be detected experimentally through the effects of anisotropy revealed by time-of-flight images.