May 27 - May 31, Saubhik Sarkar
May 31
1.arXiv:1207.2490 (replaced) [pdf, ps, other]
Superconductivity and the Pseudogap in the two-dimensional Hubbard model
Emanuel Gull, Olivier Parcollet, Andrew J. Millis
Recently developed numerical methods have enabled the explicit construction of the superconducting state of the Hubbard model of strongly correlated electrons in parameter regimes where the model also exhibits a pseudogap and a Mott insulating phase. $d_{x^2-y^2}$ symmetry superconductivity is found to occur in proximity to the Mott insulator, but separated from it by a pseudogapped nonsuperconducting phase. The superconducting transition temperature and order parameter amplitude are found to be maximal at the onset of the normal-state pseudogap. The emergence of superconductivity from the normal state pseudogap leads to a decrease in the excitation gap. All of these features are consistent with the observed behavior of the copper-oxide superconductors.
May 30
1. arXiv:1305.6880 [pdf, other]
Entanglement growth in quench dynamics with variable range interactions
J. Schachenmayer, B. P. Lanyon, C. F. Roos, A. J. Daley
Studying entanglement growth in quantum dynamics provides both insight into the underlying microscopic processes, and information about the complexity of the quantum states, which is related to the efficiency of simulations on classical computers. Recently, experiments with trapped ions, polar molecules, and Rydberg excitations have provided new opportunities to observe dynamics with long range interactions. We explore non-equilibrium coherent dynamics after a quantum quench in such systems, identifying qualitatively different behavior as the exponent of algebraically decaying spin-spin interactions in a transverse Ising chain is varied. Computing the build-up of bipartite entanglement as well as mutual information between distant spins, we identify linear growth of entanglement entropy corresponding to propagation of quasiparticles for shorter range interactions, with the maximum rate of growth occurring when the Hamiltonian parameters match those for the quantum phase transition. Counter-intuitively, the growth of bipartite entanglement for long-range interactions is only logarithmic for most regimes, i.e., substantially slower than for shorter range interactions. Experiments with trapped ions allow for the realization of this system with tunable interaction range, and we show that the different phenomena are robust for finite systems sizes and in the presence of noise. These results can act as a direct guide for the generation of large-scale entanglement in such experiments, towards a regime where the entanglement growth can render existing classical simulations inefficient.
2. arXiv:1305.6687 [pdf, ps, other]
Quantum phases of strongly interacting Rydberg atoms in triangular lattices
Jing Qian, Lu Zhou, Weiping Zhang
We present a theoretical study on the system of laser-driven strongly interacting Rydberg atoms trapped in a two-dimensional triangular lattice, in which the dipole-dipole interactions between Rydberg states result in exotic quantum phases. By using the mean-field theory, we investigate the steady state solutions and analyze their dynamical stabilities. We find that in the strong-interaction limit, the dynamics of the system is chaotic and exhibits random oscillations under appropriate laser detunings. Lyapunov exponent criterion is introduced to confirm the existence of this chaotic behavior. In addition, we present a full quantum calculation based on a six-atom model, and find that the system exhibits some bi-antiferromagnetic properties in every triangular cell when the one-photon detuning is exactly resonant or blue-shifted.
3.arXiv:1305.6891 (cross-list from nucl-th) [pdf, other]
Quantum Friction: Cooling Quantum Systems with Unitary Time Evolution
Aurel Bulgac, Michael McNeil Forbes, Kenneth J. Roche, Gabriel Wlazłowski
We introduce a type of quantum dissipation -- local quantum friction -- by adding to the Hamiltonian a local potential that breaks time-reversal invariance so as to cool the system. Unlike the Kossakowski-Lindblad master equation, local quantum friction directly effects unitary evolution of the wavefunctions rather than the density matrix: it may thus be used to cool fermionic many-body systems with thousands of wavefunctions that must remain orthogonal. In addition to providing an efficient way to simulate quantum dissipation and non-equilibrium dynamics, local quantum friction coupled with adiabatic state preparation significantly speeds up many-body simulations, making the solution of the time-dependent Schr\"odinger equation significantly simpler than the solution of its stationary counterpart.
4. arXiv:1305.6852 (cross-list from cond-mat.str-el) [pdf, other]
Vortex Attachment via Matrix Products: Application to Flat Spin-Orbit Bands
V.W. Scarola
A formalism is introduced to implement basis-independent vortex attachment in ansatz wavefunctions defined using matrix products. Wavefunctions describing strongly interacting two-dimensional helical fermions in the presence of spin-orbit coupling are constructed as an application. A comparison between exact diagonalization of a model motivated by experiments on ultracold atomic gases and Laughlin-like ansatz states is made.
May 29
1. arXiv:1305.6598 [pdf, other]
Microscopic observation of magnon bound states and their dynamics
Takeshi Fukuhara, Peter Schauß, Manuel Endres, Sebastian Hild, Marc Cheneau, Immanuel Bloch, Christian Gross
More than eighty years ago, H. Bethe pointed out the existence of bound states of elementary spin waves in one-dimensional quantum magnets. To date, identifying signatures of such magnon bound states has remained a subject of intense theoretical research while their detection has proved challenging for experiments. Ultracold atoms offer an ideal setting to reveal such bound states by tracking the spin dynamics after a local quantum quench with single-spin and single-site resolution. Here we report on the direct observation of two-magnon bound states using in-situ correlation measurements in a one-dimensional Heisenberg spin chain realized with ultracold bosonic atoms in an optical lattice. We observe the quantum walk of free and bound magnon states through time-resolved measurements of the two spin impurities. The increased effective mass of the compound magnon state results in slower spin dynamics as compared to single magnon excitations. In our measurements, we also determine the decay time of bound magnons, which is most likely limited by scattering on thermal fluctuations in the system. Our results open a new pathway for studying fundamental properties of quantum magnets and, more generally, properties of interacting impurities in quantum many-body systems.
2.arXiv:1305.6472 [pdf, ps, other]
Quantum Fluctuation Driven First-order Phase Transitions in Optical Lattices
Boyang Liu, Jiangping Hu
We study quantum fluctuation driven first-order phase transitions of a two-species bosonic system in a three-dimensional optical lattice. Using effective potential method we find that the superfluid-Mott insulator phase transition of one type of bosons can be changed from second-order to first-order by the quantum fluctuations of the other type of bosons. The study of the scaling behaviors near the quantum critical point shows that the first-order phase transition has a different universality from the second-order one. We also discuss the observation of this exotic phenomenon in the realistic cold-atom experiments.
May 28
1.arXiv:1305.6072 [pdf, other]
Interplay of disorder and interactions in an optical lattice Hubbard model
S. S. Kondov, W. R. McGehee, B. DeMarco
Open questions abound regarding how the interplay between disorder and inter-particle interactions affects essential properties of electronic solids. For example, a complete understanding of how competing phases and disorder conspire to create inhomogeneity in the high-temperature superconducting cuprates and the role of disorder in materials that exhibit colossal magnetoresistance, such as the manganites, has been elusive despite decades of active research. Fundamental transport issues, such as whether interactions enhance or inhibit localization of metallic phases, also remain unresolved. Here we use ultracold atoms trapped in a disordered optical lattice to realize a minimal model for strongly correlated, disordered electronic solids---the disordered Fermi Hubbard model (DFHM)---and to probe the impact of disorder on the metallic and Mott insulator (MI) phases. We measure a disorder-driven metal--insulator transition, and we observe that interactions raise the critical disorder energy, thereby stabilizing the metallic phase. Introducing disorder in the MI regime creates density fluctuations that are consistent with disruption of the Hubbard gap. Because we utilize fully known and controllable disorder, our work supports new and more stringent tests of theoretical and numerical approaches to understanding the influence of disorder on strongly correlated systems. If lower temperatures can be reached in optical lattices, our methods may enable measurements of the effect of disorder on the analogue of high-temperature superconductivity.
2. arXiv:1305.6007 [pdf, ps, other]
Path Integral Monte Carlo study of ultra cold atoms with dipolar moment in a harmonic trap
Michele Ruggeri
In this work we study a system of Bose particles with finite dipole moment in a harmonic trap; in particular we consider a system in which the direction of the dipoles is fixed by an external field. Our computations are performed using Path Integral Monte Carlo, an exact, finite temperature Quantum Monte Carlo technique. The samplings are performed in the Grand Canonical ensemble, using the Worm algorithm. We evaluate the density of the dipoles inside the trap and the radial pair correlation function, as functions of the tilt angle \Theta\ between the dipoles and the x axis. We compare the behaviour of the quantum system and the classical one. We also show the energy per particle and the superfluid fraction. We explore the region between \Theta=55{\deg} and \Theta=90{\deg}, where the system is stable. Systems of different size are studied. We observe that a small \Theta\ hinders the superfluidity and favours the formation of stripes, while for large \Theta\ concentric shells can be observed. Analogue structures can be observed in the classical system at finite temperature.
3. arXiv:1305.5935 [pdf, other]
Quantum degenerate mixtures of strontium and rubidium atoms
Benjamin Pasquiou, Alex Bayerle, Slava Tzanova, Simon Stellmer, Jacek Szczepkowski, Mark Parigger, Rudolf Grimm, Florian Schreck
We report on the realization of quantum degenerate gas mixtures of the alkaline-earth element strontium with the alkali element rubidium. A key ingredient of our scheme is sympathetic cooling of Rb by Sr atoms that are continuously laser cooled on a narrow linewidth transition. This versatile technique allows us to produce ultracold gas mixtures with a phase-space density of up to 0.06 for both elements. By further evaporative cooling we create double Bose-Einstein condensates of 87Rb with either 88Sr or 84Sr, reaching more than 10^5 condensed atoms per element for the 84Sr-87Rb mixture. These quantum gas mixtures constitute an important step towards the production of a quantum gas of polar, open-shell RbSr molecules.
4. arXiv:1305.5908 [pdf, ps, other]
Topological transitions of interacting bosons in one-dimensional bi-chromatic optical lattices
Xiaolong Deng, Luis Santos
Ultra-cold atoms in 1D bi-chromatic lattices constitute a surprisingly simple system for the study of topological insulators. We show that topological phase transitions constitute a general feature of bosons in 1D bi-chromatic lattices, and that these transitions may occur both as a function of the superlattice strength and due to inter-site interactions. We discuss in addition the topological character of incommensurate density wave phases in quasi-periodic lattices.
May 27
1.arXiv:1305.5623 [pdf, other]
Depletion and fluctuations of a trapped dipolar Bose-Einstein condensate in the roton regime
P. B. Blakie, D. Baillie, R. N. Bisset
We consider the non-condensate density and density fluctuations of a trapped dipolar Bose-Einstein condensate, focusing on the regime where a roton-like excitation spectrum develops. Our results show that a characteristic peak in the non-condensate density occurs at trap center due to the rotons. In this regime we also find that the anomalous density becomes positive and peaked, giving rise to enhanced density fluctuations. We calculate the non-condensate density in momentum space and show that a small momentum halo is associated with the roton excitations.
2.arXiv:1305.5598 (cross-list from physics.atom-ph) [pdf, other]
Realizing a lattice spin model with polar molecules
Bo Yan, Steven A. Moses, Bryce Gadway, Jacob P. Covey, Kaden R. A. Hazzard, Ana Maria Rey, Deborah S. Jin, Jun Ye
With the recent production of polar molecules in the quantum regime, long-range dipolar interactions are expected to facilitate the understanding of strongly interacting many-body quantum systems and to realize lattice spin models for exploring quantum magnetism. In atomic systems, where interactions require wave function overlap, effective spin interactions on a lattice can be realized via superexchange; however, the coupling is weak and limited to nearest-neighbor interactions. In contrast, dipolar interactions exist in the absence of tunneling and extend beyond nearest neighbors. This allows coherent spin dynamics to persist even at high entropy and low lattice filling. Effects of dipolar interactions in ultracold molecular gases have so far been limited to the modification of chemical reactions. We now report the observation of dipolar interactions of polar molecules pinned in a 3D optical lattice. We realize a lattice spin model with spin encoded in rotational states, prepared and probed by microwaves. This spin-exchange interaction arises from the resonant exchange of rotational angular momentum between two molecules. We observe clear oscillations in the evolution of the spin coherence in addition to an overall decay. The frequency of these oscillations, the strong dependence of the spin coherence time on the lattice filling, and the effect of a multi-pulse sequence designed to reverse dynamics due to two-body exchange interactions all provide evidence of dipolar interactions. We also demonstrate suppression of loss in weak lattices due to a quantum Zeno mechanism. Measurements of these tunneling-induced losses allow us to independently determine the lattice filling factor. These results comprise an initial exploration of the behavior of many-body spin models with direct, long-range spin interactions and lay the groundwork for future studies of many-body dynamics in spin lattices.
May 20 - May 24, Johannes Schachenmayer
May 24
1. arXiv:1305.5496 [pdf, other]
Sudden expansion of Mott insulators in one dimension
L. Vidmar, S. Langer, I.P. McCulloch, U. Schneider, U. Schollwoeck, F. Heidrich-Meisner
We investigate the sudden expansion dynamics of bosons and fermions in a homogeneous lattice using exact numerical methods. As a main result, we show that in one dimension, both bosonic and fermionic Mott insulators expand with the same velocity, irrespective of the interaction strength. They therefore expand like noninteracting spinless fermions or, equivalently, hard-core bosons (HCB), both of which are integrable systems. Moreover, we investigate the effect of breaking the integrability of HCB: While such a system exhibits ballistic dynamics in one dimension, we obtain strong deviations from this behavior on a two-leg ladder. This is consistent with a recent experiment that studied the dimensional crossover from one to two dimensions.
2. arXiv:1305.5487 [pdf, other]
In situ observation of strongly interacting ferromagnetic domains in a shaken optical lattice
Colin V. Parker, Li-Chung Ha, Cheng Chin
Solid state systems derive their richness from the interplay between interparticle interactions and novel band structures that deviate from those of free particles. Strongly interacting systems, where both of these phenomena are of equal importance, exhibit a variety of theoretically interesting and practically useful phases. Systems of ultracold atoms are rapidly emerging as precise and controllable simulators, and it is precisely in this strongly interacting regime where simulation is the most useful. Here we demonstrate how to hybridize Bloch bands in optical lattices to introduce long-range ferromagnetic order in an itinerant atomic system. We find spontaneously broken symmetry for bosons with a double-well dispersion condensing into one of two distinct minima, which we identify with spin-up and spin-down. The density dynamics following a rapid quench to the ferromagnetic state confirm quantum interference between the two states as the mechanism for symmetry breaking. Unlike spinor condensates, where interaction is driven by small spin-dependent differences in scattering length, our interactions scale with the scattering length itself, leading to domains which equilibrate rapidly and develop sharp boundaries characteristic of a strongly interacting ferromagnet.
3. arXiv:1305.5456 [pdf, ps, other]
Spin orders in the supersolid phases in binary Rydberg-dressed Bose-Einstein condensates
C.-H. Hsueh, Y.-C. Tsai, K.-S. Wu, M.-S. Chang, W. C. Wu
We show that the five possible ordered states in a quantum spin-1/2 system with long-range exchange interactions: Neel, ladder, Peierls, coincidence, and domain states, can be realized in a binary Rydberg-dressed BEC system in the supersolid phase. In such a system, blockade phenomenon is shown to also occur for pairs of different excited-state atoms, which results in similar intra- and inter-species long-range interactions between ground-state atoms. It suggests that a pseudo spin-1/2 system can be possibly formed in the ground state of ultracold rudibium.
4. arXiv:1305.5383 [pdf, ps, other]
Harmonically trapped Fermi gas: Temperature dependence of the Tan contact
Yangqian Yan, D. Blume
Ultracold atomic gases with short-range interactions are characterized by a number of universal species-independent relations. Many of these relations involve the two-body Tan contact. Employing the canonical ensemble, we determine the Tan contact for small harmonically trapped two-component Fermi gases at unitarity over a wide range of temperatures, including the zero and high temperature regimes. A cluster expansion that describes the properties of the N-particle system in terms of those of smaller subsystems is introduced and shown to provide an accurate description of the contact in the high temperature regime. Finite-range corrections are quantified and the role of the Fermi statistics is elucidated by comparing results for Fermi, Bose and Boltzmann statistics.
5. arXiv:1305.5314 [pdf, other]
Quantum simulations of the early universe
Bogdan Opanchuk, Rodney Polkinghorne, Oleksandr Fialko, Joachim Brand, Peter D. Drummond
We describe a procedure whereby a linearly coupled spinor Bose condensate can be used as a physically accessible quantum simulator of the early universe. In particular, we show how to generate a model of an unstable vacuum in a relativistic scalar field theory, which is related to the early models of inflation. There is an unstable vacuum sector whose dynamics correspond to the quantum sine-Gordon equations in one, two or three space dimensions. We give numerical simulations of the expected behavior using a truncated Wigner phase-space method, showing evidence for the dynamical formation of complex spatial clusters. The dependence on coupling strength, condensate size and dimensionality is investigated.
6. arXiv:1305.5280 [pdf, ps, other]
Two-mode effective interaction in a double-well condensate
D. M. Jezek, P. Capuzzi, H. M. Cataldo
We investigate the origin of a disagreement between the two-mode model and the exact Gross-Pitaevskii dynamics applied to double-well systems. In general this model, even in its improved version, predicts a faster dynamics and underestimates the critical population imbalance separating Josephson and self-trapping regimes. We show that the source of this mismatch in the dynamics lies in the value of the on-site interaction energy parameter. Using simplified Thomas-Fermi densities, we find that the on-site energy parameter exhibits a linear dependence on the population imbalance, which is also confirmed by Gross-Pitaevskii simulations. When introducing this dependence in the two-mode equations of motion, we obtain a reduced interaction energy parameter which depends on the dimensionality of the system. The use of this new parameter significantly heals the disagreement in the dynamics and also produces better estimates of the critical imbalance.
7. arXiv:1305.5504 [pdf, other]
Inverse indirect magnetic exchange
Andrej Schwabe, Irakli Titvinidze, Michael Potthoff
Magnetic moments strongly coupled to the spins of conduction electrons in a nanostructure can confine the conduction-electron motion due to scattering at almost localized Kondo singlets. We show that this may lead to local-moment formation in the conduction-electron system and to a novel magnetic exchange coupling mediated by the Kondo singlets. Its distance dependence is oscillatory and induces robust ferro- or antiferromagnetic order in multi-impurity systems.
8. arXiv:1305.5370 [pdf, other]
Quantum chaos in open discrete systems: Lattice scars at the band center
Víctor Fernández-Hurtado, Jordi Mur-Petit, Juan José García-Ripoll, Rafael A. Molina
We study the eigenvalues and eigenfunctions of systems on a discrete bounded lattice (lattice billiards). The statistical properties of the spectra show universal features related to the regular or chaotic properties of the dynamics of their classical continuum counterparts. However, the decay dynamics of the open systems appear very different from the continuum case, its properties being dominated by the states in the band center. We identify a class of states ("lattice scars") that survive for infinite times in dissipative systems and that are degenerate at E=0, the center of the band. We prove that their existence is intimately related to the discrete and bipartite nature of the underlying lattice, and give a formula to determine their number. Finally, we discuss how to observe lattice scars using cold atoms in optical lattices, photonic waveguides, and quantum circuits.
May 23
1. arXiv:1305.5221 [pdf, other]
Fractional Vortices with Non-Abelian Modes in Ultracold Color-flavor Locked Phases
Luca Lepori, Andrea Trombettoni, Walter Vinci
We describe an ultracold fermionic set-up where it is possible to synthesize a phase with symmetry obtained by locking independent invariance groups of the normal state. Labeling as colors two species of the mixture and as flavors the other two, we consider the experimentally realistic situation where the symmetries acting on these doublets are global and we show that a color-flavor locked phase can be obtained. Due to its peculiar symmetry, this phase can host exotic soliton structures, as vortices with semi-integer flux and gapless non-Abelian Goldstone modes localized on them. The scenario proposed displays remarkable similarities to what arises in ultra-dense QCD matter, as in the core of some neutron stars. A discussion about the experimental detection of the non-Abelian fractional vortices is as well provided.
2. arXiv:1305.5192 [pdf, ps, other]
Coexistence of phase transitions and hysteresis near BEC
M. Männel, K. Morawetz, P. Lipavský
Multiple phases occurring in a Bose gas with finite-range interaction are investigated. In the vicinity of the onset of Bose-Einstein condensation (BEC) the chemical potential and the pressure show a van-der-Waals like behavior indicating a first-order phase transition although there is no long-range attraction. Furthermore the equation of state becomes multivalued near the BEC transition. For a Hartree-Fock or Popov (Hartree-Fock-Bogoliubov) approximation such a multivalued region can be avoided by the Maxwell construction. For sufficiently weak interaction the multivalued region can also be removed using a many-body \mbox{T-matrix} approximation. However, for strong interactions there remains a multivalued region even for the \mbox{T-matrix} approximation and after the Maxwell construction, what is interpreted as a density hysteresis. This unified treatment of normal and condensed phases becomes possible due to the recently found scheme to eliminate self-interaction in the \mbox{T-matrix} approximation, which allows to calculate properties below and above the critical temperature.
3. arXiv:1305.5141 [pdf, ps, other]
Quantum chaos in SU_3 models with trapped ion chains
Tobias Graß, Bruno Julia-Diaz, Maciej Lewenstein
A scheme to generate effective long-range spin-spin interactions between three-level ions in a chain is presented, providing a feasible experimental route to the rich physics of well-known $SU_3$ models. In particular, we demonstrate different signatures of quantum chaos which can be controlled and observed in experiments with trapped ions.
3. arXiv:1305.5097 [pdf, ps, other]
Condensation of quasiparticles and density modulation beyond the superfluid critical velocity
Andras Suto, Peter Szepfalusy
We investigate the effect of a constant external velocity field on the ground state of a bosonic quasiparticle Hamiltonian. Below a critical velocity the ground state is a quasiparticle vacuum, corresponding to a pure superfluid phase at zero temperature. Beyond the critical velocity energy minimization leads to a macroscopic condensation of quasiparticles at a nonzero wave vector k_v parallel to the velocity v. Simultaneously, physical particles also undergo a condensation at k_v and, to a smaller extent, at -k_v. Together with the BEC at k=0, the three entangled condensates give rise to density modulations of wave vectors k_v and 2k_v. For larger |v| our model predicts a bifurcation of k_v with corresponding two pure condensates and no density modulation.
May 22
1. arXiv:1305.4689 [pdf, other]
Free expansion of a quasi-2D Bose-Einstein condensate with quantized vortices
Sang Won Seo, Jae-yoon Choi, Yong-il Shin
We observe concentric density ripples in mechanically perturbed and freely expanding quasi-two-dimensional Bose-Einstein condensates, and show that the ripples develop from quantized vortices in the condensates. Free expansion of a condensate containing a vortex is numerically simulated under the assumption of no atom-atom interactions, and the experimental data are found to be in good quantitative agreement with the numerical results for a singly charged vortex, in particular, including the defocus effect in imaging due to the gravitational freefall of the condensate. In the defocused image, the vortex core becomes magnified and appears to be filled after a certain expansion time. We show that the vortex charge number can be determined from the onset time of the core-filling in the defocused image.
2. arXiv:1305.4678 [pdf, other]
Polytropic equilibrium and normal modes in cold atomic traps
H. Terças, J. T. Mendonça
The compressibility limit of a cold gas confined in a magneto-optical trap due to multiple scattering of light is a long-standing problem. This scattering mechanism induces long-range interactions in the system, which is responsible for the occurrence of plasma-like phenomena. In the present paper, we investigate the importance of the long-range character of the mediated atom-atom interaction in the equilibrium and dynamical features of a magneto-optical trap. Making use of a hydrodynamical formulation, we derive a generalized Lane-Emden equation modeling the polytropic equilibirum of a magneto-optical trap, allowing us to describe the cross-over between the two limiting cases: temperature dominated and multiple-scattering dominated traps. The normal collective modes of the system are also computed.
3. arXiv:1305.4750 [pdf, ps, other]
Ultracold collision in the presence of synthetic spin-orbit coupling
Hao Duan, Li You, Bo Gao
We present an analytic description of ultracold collision between two spin-$\frac{1}{2}$ fermions with isotropic spin-orbit coupling of the Rashba type. We show that regardless of how weak the spin-orbit coupling may be, the ultracold collision at sufficiently low energies is significantly modified, including the ubiquitous Wigner threshold behavior. We further show that the particles are preferably scattered into the lower-energy helicity state due to the break of parity conservation, thus establishing interaction with spin-orbit coupling as one mechanism for the spontaneous emergence of handedness.
May 21
1. arXiv:1305.4594 [pdf, other]
Quench Dynamics in Bose condensates in the Presence of a Bath: Theory and Experiment
A. Rancon, Chen-Lung Hung, Cheng Chin, K. Levin
In this paper we study the transient dynamics of a Bose superfluid subsequent to an interaction quench. Essential for equilibration is a source of dissipation which we include following the approach of Caldeira and Leggett. Here we solve the equations of motion exactly by integrating out an environmental bath. We thereby derive precisely the time dependent density correlation functions with the appropriate analytic and asymptotic properties. The resulting structure factor exhibits the expected damping and thereby differs from that of strict Bogoliubov theory. These damped sound modes, which reflect the physics beyond mean field approaches, are characterized and the structure factors are found to compare favorably with experiment.
2. arXiv:1305.4442 [pdf, other]
Self-trapping dynamics in a 2D optical lattice
Shuming Li, Salvatore R. Manmana, Ana Maria Rey, Rafael Hipolito, Aaron Reinhard, Jean-Félix Riou, Laura A. Zundel, David S. Weiss
In this paper we study the transient dynamics of a Bose superfluid subsequent to an interaction quench. Essential for equilibration is a source of dissipation which we include following the approach of Caldeira and Leggett. Here we solve the equations of motion exactly by integrating out an environmental bath. We thereby derive precisely the time dependent density correlation functions with the appropriate analytic and asymptotic properties. The resulting structure factor exhibits the expected damping and thereby differs from that of strict Bogoliubov theory. These damped sound modes, which reflect the physics beyond mean field approaches, are characterized and the structure factors are found to compare favorably with experiment.
3. arXiv:1305.4411 [pdf, ps, other]
Ground state pressure of quasi-two-dimensional Fermi and Bose gases
Vasiliy Makhalov, Kirill Martiyanov, Andrey Turlapov
Using an ultracold gas of atoms, we have realized a quasi-two-dimensional Fermi system with widely tunable s-wave interactions nearly in a ground state. Pressure and density are measured. The experiment covers physically different regimes: weakly- and strongly-attractive Fermi gases and a Bose gas of tightly-bound pairs of fermions. In the Fermi regime of weak interactions, the pressure is systematically above a Fermi-liquid-theory prediction probably due to mesoscopic effects. In the opposite Bose regime, the pressure agrees with a mean-filed theory of point-like bosons in an unexpectedly wide range. Reported measurements, especially in the challenging strongly-interacting regime, may serve for sensitive testing of theoretical methods applicable across different quantum physics disciplines.
4. arXiv:1305.4285 [pdf, other]
Josephson Effects in a Bose-Einstein Condensate of Magnons
Roberto E. Troncoso, Álvaro S. Núñez
A phenomenological theory is developed, that accounts for the collective dynamics of a Bose-Einstein condensate of magnons. In terms of such description we discuss the nature of spontaneous macroscopic interference between magnon clouds, highlighting the close relation between such effects and the well known Josephson effects. Using those ideas we present a detailed calculation of the Josephson oscillations between two magnon clouds, spatially separated in a magnonic Josephson junction.
May 20
1. arXiv:1305.4097 [pdf, other]
Nonlinear quantum piston for the controlled generation of vortex rings and soliton trains
Florian Pinsker, Natalia G. Berloff, Víctor M. Pérez-García
We propose a simple way to generate nonlinear excitations in a controllable way by managing interactions in Bose-Einstein condensates. Under the action of a quantum analogue of a classical piston the condensed atoms are pushed through the trap generating vortex rings in a fully three- dimensional condensates or soliton trains in quasi-one dimensional scenarios. The vortex rings form due to transverse instability of the shock wave train enhanced and supported by the energy transfer between waves. We elucidate in which sense the self-interactions within the atom cloud define the properties of generated vortex rings and soliton trains. Based on the quantum piston scheme we study the behavior of two component Bose-Einstein condensates and analyze how the presence of an additional superfluid influences the generation of vortex rings or solitons in the other component and vice versa. Finally, we show the dynamical emergence of skyrmions within two component systems in the immiscible regime.
2. arXiv:1305.3995 [pdf, ps, other]
Precursor phenomena of nucleations of quantized vortices in the presence of a uniformly moving obstacle in Bose-Einstein condensates
Masaya Kunimi, Yusuke Kato
We investigate the excitation and the fluctuation of Bose-Einstein condensates in a two-dimensional torus with a uniformly moving Gaussian potential by solving Gross-Pitaevskii equation and the Bogoliubov equation. A scaling law of the energy gap in the finite system is found. This scaling law suggests that dynamical critical phenomena occur in this system. Near the critical velocity, we find that low-energy local density fluctuations are enhanced. These can be regarded as precursor phenomena of the vortex nucleation.
3. arXiv:1305.3925 [pdf, ps, other]
Effective theory of chiral two-dimensional superfluids
Carlos Hoyos, Sergej Moroz, Dam Thanh Son
We construct, to leading orders in the momentum expansion, an effective theory of a chiral (p_x+ip_y) two-dimensional fermionic superfluid at zero temperature that is consistent with nonrelativistic general coordinate invariance. The currents and stress tensor are computed and their linear response to electromagnetic and gravitational sources is calculated. We also consider an isolated vortex in a chiral superfluid and identify the leading chirality effect in the density depletion profile.
4. arXiv:1305.4088 [pdf, other]
Computing with soliton trains in Bose-Einstein condensates
Florian Pinsker
Computing devices can be implemented based on controlled generation of soliton trains in single and multicomponent Bose-Einstein condensates (BEC). Our concepts utilize the phenomenon that the frequency of soliton trains in BEC can be governed by changing interactions within the atom cloud [1]. We use this property to store numbers in terms of those frequencies for a short time until observation. The properties of soliton trains can be changed in an intended way by other components of BEC occupying comparable states or via phase engineering. We elucidate in which sense such an additional degree of freedom can be regarded as a tool for controlled manipulation of data. Finally the outcome of any manipulation made is read out by observing the signature within the density profile.
May 13 - May 17, Li-Jun Lang
May 17
1. arXiv:1305.3626 [pdf, other]
Boosting Majorana zero modes
Torsten Karzig, Gil Refael, Felix von OppenOne-dimensional topological superconductors are known to host Majorana zero modes at domain walls terminating the topological phase. Their nonabelian nature allows for processing quantum information by braiding operations which are insensitive to local perturbations, making Majorana zero modes a promising platform for topological quantum computation. Motivated by the ultimate goal of executing quantum information processing on a finite timescale, we study domain walls moving at a constant velocity. We exploit an effective Lorentz invariance of the Hamiltonian to obtain an exact solution of the associated quasiparticle spectrum and wave functions for arbitrary velocities. Essential features of the solution have a natural interpretation in terms of the familiar relativistic effects of Lorentz contraction and time dilation. We find that the Majorana zero modes remain stable as long as the domain wall moves at subluminal velocities with respect to the effective speed of light of the system. However, the Majorana bound state dissolves into a continuous quasiparticle spectrum once the domain wall propagates at luminal or even superluminal velocities. This relativistic catastrophe implies that there is an upper limit for possible braiding frequencies even in a perfectly clean system with an arbitrarily large topological gap. We also exploit our exact solution to consider domain walls moving past static impurities present in the system.
2. arXiv:1305.3791 [pdf, ps, other]
Searching for New Topological Types of Majorana Fermions from Boundary-bulk Correspondence of Topological Insulators/Superconductors
Y. X. Zhao, Z. D. Wang
Majorana fermions (MFs) are particles that are their own anti-particles and may satisfy non-Abelian statistics. For the past ten years, a kind of MFs have been theoretically predicted in one-dimensional superconducting models without the time-reversal symmetry (TRS), which has recently intrigued a huge experimental interest to search for this type of exotic particles. Here we report our theoretical discoveries, including (i) a general and quantitative relation between topological stabilities of Fermi surfaces (FSs) and topological insulators(TIs)/superconductors(TSCs) is rigorously established; (ii) based on (i), we find four topologically distinct types of MFs in fermionic systems that have at least the particle-hole symmetry (PHS); (iii) apart from the existing one type, three new types of topologically nontrivial gapless edge-modes (MFs) are revealed for the first time in one dimensional finite systems with both T&P symmetries; and (iv) all kinds of candidate models that potentially have MFs are presented, opening a wider door for searching MFs experimentally.
3. arXiv:1305.3872 [pdf, other]
Extracting the Chern number from the dynamics of a Fermi gas: Implementing a quantum Hall bar for cold atoms
Alexandre Dauphin, Nathan Goldman
We propose a scheme to measure the quantized Hall conductivity of an ultracold Fermi gas initially prepared in a topological (Chern) insulating phase, and driven by a constant force. We show that the time evolution of the center of mass, after releasing the cloud, provides a direct and clear signature of the topologically invariant Chern number. We discuss the validity of this scheme, highlighting the importance of driving the system with a sufficiently strong force to displace the cloud over measurable distances while avoiding band-mixing effects. The unusual shapes of the driven atomic cloud are qualitatively discussed in terms of a semi-classical approach.
May 16
1. arXiv:1305.3412 [pdf, ps, other]
Recurrence time in the quantum dynamics of the 1D Bose gas
Eriko Kaminishi, Jun Sato, Tetsuo DeguchiRecurrence time is successfully evaluated for some intially localized quantum states in the one-dimensional (1D) Bose gas with repulsive short range interactions. We suggest that the recurrence time is given typically by several minutes or seconds in experiments of cold atoms trapped in one dimension. It is much shorter than the estimated recurrence time of a generic quantum many-body system, which is usually as long as the age of the universe. We show numerically how the recurrence time depends on the interaction strength. In the free-bosonic and the free-fermionic regimes we evalute the recurrence time rigorously and show that it is proportional to the square of the number of particles. For instance, the result is exact in the impenetrable 1D Bose gas.
2. arXiv:1305.3571 [pdf, ps, other]
Competing orders in the 2D half-filled SU(2N) Hubbard model through the pinning field quantum Monte-Carlo simulations
Da Wang, Yi Li, Zi Cai, Congjun WuWe investigate the ground state magnetic properties of the 2D half-filled SU(2N) Hubbard model in the square lattice by the determinant quantum Monte-Carlo simulations combined with the method of local pinning fields. This method directly gives rise to the values of order parameters instead of their magnitude square, thus it is more accurate at weak orderings than methods using correlation functions. Antiferromagnetic long-range orders are found for both the SU(4) and SU(6) cases at small and intermediate values of $U$. As increasing $U$, the long-range Neel moments first grow and then drop. This is very different from the SU(2) case in which the Neel moments increase monotonically and saturate. In the SU(6) case, a transition to the columnar dimer state is found in the strong interaction regime.
May 15
1. arXiv:1305.2005 [pdf, ps, other]
Probing the structure of entanglement with entanglement moments
Justin H. Wilson, Joe Mitchell, Victor GalitskiWe introduce and define a set of functions on pure bipartite states called entanglement moments. Usual entanglement measures tell you if two systems are entangled, while entanglement moments tell you both if and how two systems are entangled. They are defined with respect to a measurement basis in one system (e.g., a measuring device), and output numbers describing how a system (e.g., a qubit) is entangled with that measurement basis. The moments utilize different distance measures on the Hilbert space of the measured system, and can be generalized to any N-dimensional Hilbert space. As an application, they can distinguish between projective and non-projective measurements. As a particular example, we take the Rabi model's eigenstates and calculate the entanglement moments as well as the full distribution of entanglement.
2. arXiv:1305.3032 [pdf, ps, other]
Universal Superfluid Transition and Transport Properties of Two-Dimensional Dirty Bosons
Giuseppe Carleo, Guilhem Boéris, Markus Holzmann, Laurent Sanchez-Palencia
We study the phase diagram of two-dimensional, interacting bosons in the presence of a correlated disorder in continuous space, using large-scale finite temperature quantum Monte Carlo simulations. We show that the superfluid transition is strongly protected against disorder. It remains of the Berezinskii-Kosterlitz-Thouless type up to disorder strengths comparable to the chemical potential. Moreover, we study the transport properties in the strong disorder regime where a zero-temperature Bose-glass phase is expected. We show that the conductance exhibits a thermally activated behavior and strictly vanishes only at zero temperature. Our results do not show any evidence of a finite-temperature localization transition, and point towards the existence of Bose bad-metal phase as a precursor of the Bose-glass phase.
May 14
1. arXiv:1305.2443 [pdf, ps, other]
Spin dynamics in a spin-orbit coupled Fermi gas
Stefan S. Natu, S. Das SarmaWe study the dynamics of a non-degenerate, harmonically trapped Fermi gas following a sudden ramp of the spin-orbit coupling strength. In the non-interacting limit, we solve the Boltzmann equation in the presence of spin orbit coupling analytically, and derive expressions for the dynamics of an arbitrary initial spin state. In particular we show that for a fully spin polarized initial state, the total magnetization exhibits collapse and revival dynamics in time with a period set by the trapping potential. In real space, this corresponds to oscillations between a fully polarized state and a spin helix. We numerically study the effect of interactions on the dynamics using a collisionless Boltzmann equation.
2. arXiv:1305.2526 [pdf, other]
Quantum simulations of localization effects with dipolar interactions
Gonzalo A. Alvarez, Robin Kaiser, Dieter Suter
Quantum information processing often uses systems with dipolar interactions. We use a nuclear spin-based quantum simulator, to study the spreading of information in such a dipolar-coupled system and how perturbations to the dipolar couplings limit the spreading, leading to localization. In [Phys. Rev. Lett. 104, 230403 (2010)], we found that the system reaches a dynamic equilibrium size, which decreases with the square of the perturbation strength. Here, we study the impact of a disordered Hamiltonian with dipolar 1/r^3 interactions. We show that the expansion of the coherence length of the cluster size of the spins becomes frozen in the presence of large disorder, reminiscent of Anderson localization of non-interacting waves in a disordered potential.
May 13
1. arXiv:1305.2217 [pdf, other]
Fractional quantum Hall effect in a tilted magnetic field
Z. PapicWe discuss the orbital effect of a tilted magnetic field on the quantum Hall effect in parabolic quantum wells. Many-body states realized at the fractional 1/3 and 1/2 filling of the second electronic subband are studied using finite-size exact diagonalization. In both cases, we obtain the phase diagram consisting of a fractional quantum Hall fluid phase that persists for moderate tilts, and eventually undergoes a direct transition to the stripe phase. It is shown that tilting of the field probes the geometrical degree of freedom of fractional quantum Hall fluids, and can be partly related to the effect of band-mass anisotropy.
2. arXiv:1305.2255 [pdf, ps, other]
Minimal Entangled States and Modular Matrix for Fractional Quantum Hall Effect in Topological Flat Bands
W. Zhu, D. N. Sheng, F. D. M. HaldaneWe perform an exact diagonalization study of the topological order in topological flat band models through calculating entanglement entropy and spectra of low energy states. We identify multiple independent minimal entangled states, which form a set of orthogonal basis states for the ground-state manifold. We extract the modular transformation matrices S (U) which contains the information of mutual (self) statistics, quantum dimensions and fusion rule of quasi-particles. Moreover, we demonstrate that these matrices are robust and universal in the whole topological phase against different perturbations until the quantum phase transition takes place.
May 6 - May 10, Jin-Long Yu
May 10
1. arXiv:1305.2152 [pdf, ps, other]Competing superfluid orders in spin-orbit coupled fermionic cold atom optical lattices
Yong Xu, Chunlei Qu, Ming Gong, Chuanwei ZhangThe Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, a superconducting state with non-zero total momentum Cooper pairs in a large magnetic field, was first predicted about 50 years ago, and since then became an important concept in many branches of physics. Despite intensive search in various materials, unambiguous experimental evidence for the FFLO phase is still lacking in experiments. In this Letter, we show that both FF (uniform order parameter with plane-wave phase) and LO phase (spatially varying order parameter amplitude) can be observed using fermionic cold atoms in spin-orbit coupled optical lattices. The increasing spin-orbit coupling enhances the FF phase over the LO phase. The coexistence of superfluid and magnetic orders is also found in the normal BCS phase. The pairing mechanism for different phases is understood by visualizing superfluid pairing densities in different spin-orbit bands. We also discuss the possibility of observing similar physics using spin-orbit coupled superconducting ultra-thin films.
2. arXiv:1305.2018 [pdf, other]
Anderson localization in an interacting fermionic system
Francesco Massel
In the present article, we discuss the role played by the interaction in the Anderson localization problem, for a system of interacting fermions in a one-dimensional disordered lattice, described by the Fermi Hubbard Hamiltonian, in presence of an on-site random potential. We show that, given the proper identification of the elementary excitations of the system described in terms of doublons and unpaired particles, the Anderson localization picture survives. Ensuing a "global quench", we show that the system exhibits a rich localization scenario, which can be ascribed to the nearly-free dynamics of the elementary excitations of the Hubbard Hamiltonian.
3. arXiv:1305.1972 [pdf, ps, other]
Superfluid, staggered state, and Mott insulator of repulsively interacting three-component fermionic atoms in optical lattices
Kensuke Inaba, Sei-ichiro Suga
We review our theoretical analysis of repulsively interacting three-component fermionic atoms in optical lattices. We discuss quantum phase transitions at around half filling with a balanced population by focusing on Mott transitions, staggered ordering, and superfluidity. At half filling (with 3/2 atoms per site), characteristic Mott transitions are induced by the anisotropic interactions, where two-particle repulsions between any two of the three colors have different strengths. At half filling, two types of staggered ordered states appear at low temperatures depending on the anisotropy of the interactions. As the temperature increases, phase transitions occur from the staggered ordered states to the unordered Mott states. Deviating from half filling, an exotic superfluid state appears close to a regime in which the Mott transition occurs. We explain the origin of these phase transitions and present the finite-temperature phase diagrams.
May 9
1. arXiv:1305.1602 [pdf, other]
Ultracold Quantum Gases and Lattice Systems: Quantum Simulation of Lattice Gauge Theories
U.-J. Wiese
Abelian and non-Abelian gauge theories are of central importance in many areas of physics. In condensed matter physics, Abelian U(1) lattice gauge theories arise in the description of certain quantum spin liquids. In quantum information theory, Kitaev's toric code is a Z(2) lattice gauge theory. In particle physics, Quantum Chromodynamics (QCD), the non-Abelian SU(3) gauge theory of the strong interactions between quarks and gluons, is non-perturbatively regularized on a lattice. Quantum link models extend the concept of lattice gauge theories beyond the Wilson formulation, and are well suited for both digital and analog quantum simulation using ultracold atomic gases in optical lattices. Since quantum simulators do not suffer from the notorious sign problem, they open the door to studies of the real-time evolution of strongly coupled quantum systems, which are impossible with classical simulation methods. A plethora of interesting lattice gauge theories suggests itself for quantum simulation, which should allow us to address very challenging problems, ranging from confinement and deconfinement, or chiral symmetry breaking and its restoration at finite baryon density, to color superconductivity and the real-time evolution of heavy-ion collisions, first in simpler model gauge theories and ultimately in QCD.
2. arXiv:1305.1828 [pdf, ps, other]
Dynamical tunneling of a Bose-Einstein condensate
R. K. Shrestha, J. Ni, W. K. Lam, G. S. Summy, S. Wimberger
We report measurements of dynamical tunneling rates of a Bose-Einstein condensate across a barrier in classical phase space. The atoms are initially prepared in quantum states which extend over a classically regular island region. Our experimental data is supported by numerical simulations taking into account imperfections mainly from spontaneous emission. Furthermore we predict experimentally accessible parameter ranges over which direct tunneling could be readily observed if spontaneous emission is further suppressed.
3. arXiv:1305.1800 [pdf, ps, other]
Phase Space Crystals: A new way to create a quasienergy bandstructure
Lingzhen Guo, Michael Marthaler, Gerd Schön
The space periodicity of crystals leads to the energy bandstructure. Systems driven periodically in time show a discrete time translation symmetry, which in the frame of the Floquet theory leads to the concept of quasienergy. Here we propose a novel way to create a bandstructure of the quasienergy spectrum. The system, e.g. an ion trapped in a potential, shows no spatial periodicity, but it is driven by a time-dependent field coupling highly nonlinearly to one of its degrees of freedom (e.g., ~ q^n). The bandstructure in quasienergy arises as a consequence of the n-fold discrete periodicity in phase space induced by this driving field. We propose an explicit model to realize such a phase space crystal and analyze its bandstructure in the frame of a tigh-binding approximation. In spite of an orbital circular symmetry the quasienergy bandstructure shows a peculiar asymmetry. It arises due to the loss of phase reflection symmetry of the quantum system which renders the clockwise and anti-clockwise motion in phase space non-equivalent. The resulting bandstructure of the quasienergy spectrum determines the emission spectrum, which shows resonant peaks grouping together into several clusters due to intra-band and inter-band transitions. The phase space crystal opens new ways to engineer energy bandstructures and emission spectra, with the added advantage that its properties can be changed in situ by tunning the driving field's parameters.
May 8
1. arXiv:1305.1301 [pdf, other]
Spontaneous emissions and thermalization of cold bosons in optical lattices
Johannes Schachenmayer, Lode Pollet, Matthias Troyer, Andrew John Daley
We study the thermalization of excitations generated by spontaneous emission events for cold bosons in an optical lattice. Computing the dynamics described by the many-body master equa- tion, we characterize equilibration timescales in different parameter regimes. For simple observables, we find regimes in which the system relaxes rapidly to values in agreement with a thermal distribu- tion, and others where thermalization does not occur on typical experimental timescales. Because spontaneous emissions lead effectively to a local quantum quench, this behavior is strongly depen- dent on the low-energy spectrum of the Hamiltonian, and undergoes a qualitative change at the Mott Insulator-superfluid transition point. These results have important implications for the un- derstanding of thermalization after localized quenches in isolated quantum gases, as well as the characterization of heating in experiments.
2. arXiv:1305.1423 [pdf, other]
Observation of the universal jump across the Berezinskii-Kosterlitz-Thouless transition in two-dimensional Bose gases
Jiho Noh, Jeongwon Lee, Jongchul Mun
The physics in two-dimensional (2D) systems is very different from what we observe in three-dimensional (3D) systems. Thermal fluctuations in 2D systems are enhanced, and they prevent the conventional Bose-Einstein condensation (BEC) at non-zero temperatures by destroying the long-range order. However, a phase transition to a superfluid phase is still expected to occur in a 2D system along with an emergence of a quasi-long-range order, explained by the Berezinskii-Kosterlitz-Thouless (BKT) mechanism. Within the BKT mechanism, a universal jump of the superfluid density in a 2D Bosonic system was theoretically predicted by Nelson and Kosterlitz, and was first observed in 2D \textsuperscript{4}He films by Bishop and Reppy. Recent experiments in trapped ultracold 2D Bose gas systems have shown signatures of the BKT transition, and its superfluidity. However, the universal jump in the superfluid density was not observed in these systems. Here we report the first observation of the universal jump in the superfluid density using an optically trapped ultracold 2D Bose gas. The measured superfluid phase space density at the BKT transition agrees well with the predicted value within our measurement uncertainty. Additionally, we measure the phase fluctuations in our density profiles to show that the BKT transition occurs first, followed by the BEC transition.
3. arXiv:1305.1378 [pdf, ps, other]
A simple and efficient numerical method for computing the dynamics of rotating Bose-Einstein condensates via a rotating Lagrangian coordinate
Weizhu Bao, Daniel Marahrens, Qinglin Tang, Yanzhi Zhang
We propose a simple, efficient and accurate numerical method for simulating the dynamics of rotating Bose-Einstein condensates (BECs) in a rotational frame with/without a long-range dipole-dipole interaction. We begin with the three-dimensional (3D) Gross-Pitaevskii equation (GPE) with an angular momentum rotation term and/or long-range dipole-dipole interaction, state the two-dimensional (2D) GPE obtained from the 3D GPE via dimension reduction under anisotropic external potential and review some dynamical laws related to the 2D and 3D GPE. By introducing a rotating Lagrangian coordinate system, the original GPEs are re-formulated to GPEs without the angular momentum rotation which is replaced by a time-dependent potential in the new coordinate system. We then cast the conserved quantities and dynamical laws in the new rotating Lagrangian coordinates. Based on the new formulation of the GPE for rotating BECs in the rotating Lagrangian coordinates, a time-splitting spectral method is presented for computing the dynamics of rotating BECs. The new numerical method is explicit, simple to implement, unconditionally stable and very efficient in computation. It is spectral order accurate in space and second-order accurate in time, and conserves the mass in the discrete level. Extensive numerical results are reported to demonstrate the efficiency and accuracy of the new numerical method. Finally, the numerical method is applied to test the dynamical laws of rotating BECs such as the dynamics of condensate width, angular momentum expectation and center-of-mass, and to investigate numerically the dynamics and interaction of quantized vortex lattices in rotating BECs without/with the long-range dipole-dipole interaction.
May 7
1. arXiv:1305.1177 [pdf, other]
Interaction-induced chiral p_x \pm i p_y superfluid order of bosons in an optical lattice
M. Ölschläger, T. Kock, G. Wirth, A. Ewerbeck, C. Morais Smith, A. Hemmerich
The study of superconductivity with unconventional order is complicated in condensed matter systems by their immense complexity. Optical lattices with their exceptional precision and control allow one to emulate superfluidity avoiding many of the complexities of condensed matter. A promising approach to realize unconventional superfluid order is to employ orbital degrees of freedom in higher Bloch bands. In recent work, indications were found that bosons condensed in the second band of an optical chequerboard lattice might exhibit p_x \pm i p_y order. Here we present experiments, which provide strong evidence for the emergence of p_x \pm i p_y order driven by the interaction in the local $p$-orbitals. We compare our observations with a multi-band Hubbard model and find excellent quantitative agreement.
2. arXiv:1305.1097 [pdf, other]
Dissipative Effects on the Superfluid to Insulator Transition in Mixed-dimensional Optical Lattices
E. Malatsetxebarria, Zi Cai, U. Schollwoeck, M. A. Cazalilla
We study the superfluid to Mott insulator transition of a mixture of heavy bosons and light fermions loaded in an optical lattice. We focus on the effect of the light fermions on the dynamics of the heavy bosons. It is shown that, when the lattice potential is sufficiently deep to confine the bosons to one dimension but allowing the fermions to freely move in three dimensions (i.e. a mixed-dimensionality lattice), the fermions act as an ohmic bath for bosons leading to screening and dissipation effects on the bosons. Using a perturbative renormalization-group analysis, it is shown that the fermion-induced dissipative effects have no appreciable impact on the transition from the superfluid to the Mott-insulator state at integer filling. On the other hand, dissipative effects are found to be very important in the half-filled case near the critical point. In this case, in the presence of a finite incommensurability that destabilizes the Mott phase, the bosons can still be localized by virtue of dissipative effects.
May 6
1. arXiv:1305.0645 [pdf, ps, other]
Supercurrent and dynamical instability of spin-orbit coupled ultracold Bose gases
Tomoki Ozawa, Lev P. Pitaevskii, Sandro Stringari
We investigate the stability of supercurrents in a Bose-Einstein condensate with 1D spin-orbit and Raman couplings. The consequence of the lack of Galilean invariance is explicitly discussed. We show that in the plane wave phase, characterized by uniform density, the supercurrent state can become dynamically unstable, the instability being associated with the occurrence of a complex sound velocity, in a region where the effective mass is negative. We also discuss the emergence of energetic instability in these supercurrent states. We argue that both the dynamical and energetic instabilities in these systems can be generated experimentally through the excitation of the collective dipole oscillation.
