Jun 25 - Jun 29, Bin Wang
Jun 18 - Jun 22, Johannes Schachenmayer
Jun 11 - Jun 15, Saubhik Sarkar
Jun 4 - Jun 8, Xiaopeng Li
Jun 29
1. arXiv:1206.6740 [pdf, ps, other]
Higher optical lattice bands in real-time dynamics: effective Hamiltonian approach
Mateusz Lacki, Dominique Delande, Jakub Zakrzewski
The extended effective Bose-Hubbard Hamiltonian which takes into account higher lattice bands is discussed for systems with several particles per side. It is shown that the renormalization of Hamiltonian parameters depends on the dimension of the problem studied. Therefore, mean field phase diagrams do not scale with the coordination number of the lattice. The effect of Hamiltonian parameters renormalization on the dynamics in reduced one-dimensional optical lattice potential is analyzed both for the quasi adiabatic quench through superfluid-Mott insulator transition and for absorption in periodically modulated lattice.
2. arXiv:1206.6797 (cross-list from cond-mat.str-el) [pdf, other]
Correlated Phases of Population Imbalanced Fermi-Fermi Mixtures on an Optical Lattice
Chen-Yen Lai, Chuntai Shi, Shan-Wen Tsai
We study a two species fermion mixture with different populations on a square lattice modeled by a Hubbard Hamiltonian with on-site inter-species repulsive interaction. Such a model can be realized in a cold atom system with fermionic atoms in two different hyperfine states loaded on an optical lattice and with tunable inter-species interaction strength via external fields. For a two-dimensional square lattice, when at least one of the fermion species is close to half-filling, the system is highly affected by lattice effects. With the majority species near half-filling and varying densities for the minority species, we find that several correlated phases emerge as the ground state, including a spin density wave state, a charge density wave state with stripe structure, and various p-wave BCS pairing states for both species. We study this system using a functional renormalization group method, determine its phase diagram at weak coupling, discuss the origin and characteristics of each phase, and provide estimates for the critical temperatures.
Jun 28
1. arXiv:1206.6155 [pdf, ps, other]
Phase Diagram of Rydberg atoms in a nonequilibrium optical lattice
Jing Qian, Guangjiong Dong, Lu Zhou, Weiping Zhang
We study the quantum nonequilibrium dynamics of ultracold three-level atoms trapped in an optical lattice, which are excited to their Rydberg states via a two-photon excitation with nonnegligible spontaneous emission. Rich quantum phases including uniform phase, antiferromagnetic phase and oscillatory phase are identified. We map out the phase diagram and find these phases can be controlled by adjusting the ratio of intensity of the pump light to the control light, and that of two-photon detuning to the Rydberg interaction strength. When the two-photon detuning is blue-shifted and the latter ratio is less than 1, bistability exists among the phases. Actually, this ratio controls the Rydberg-blockade and antiblockade effect, thus the phase transition in this system can be considered as a possible approach to study both effects.
Jun 27
1. arXiv:1206.6042 [pdf, ps, other]
Super Tonks-Girardeau state in an attractive one-dimensional dipolar gas
M. D. Girardeau, G. E. Astrakharchik
The ground state of a one-dimensional (1D) quantum gas of dipoles oriented perpendicular to the longitudinal axix, with a strong $1/x^3$ repulsive potential, is studied at low densities $n$. Near contact the dependence of the many-body wave function on the separation $x_{j\ell}$ of two particles reduces to a two-body wave function $Psi_\text{rel}(x_{j\ell})$. Immediately after a sudden rotation of the dipoles so that they are parallel to the longitudinal axis, this wave function will still be that of the repulsive potential, but since the potential is now that of the attractive potential, it will not be stationary. It is shown that as $nd^2\to 0$ the rate of change of this wave function approaches zero. It follos that for small values of $nd^2$, this state is metastable and is an analog of the super Tonks-Girardeau (STG) state of bosons with a strong zero-range attraction. The dipolar system is equivalent to a spinor Fermi gas with spin $z$-components $\sigma_{\uparrow}=\perp$ (perpendicular to longitudinal axis) and $\sigma_{\downarrow}=\parallel$ (parallel to longitudinal axis). A Fermi-Fermi mapping from spinor Fermi to spinless Fermi followed by the standard 1960 Fermi-Bose mapping reduces the Fermi system to a Bose gas. Potential experiments realizing the sudden spin rotation with ultracold dipolar gases are discussed, and a few salient properties of these states are accurately evaluated by a Monte Carlo method.
2. arXiv:1206.5918 [pdf, other]
Enhancement of condensate depletion due to spin-orbit coupling
Xiaoling Cui, Qi Zhou
We show that spin-orbit coupling(SOC) significantly enhances the depletion of a Bose-Einstein condensate. With decreasing anisotropy of SOC, both quantum and thermal depletion increase. Moreover, when SOC and interaction are isotropic, thermal depletion becomes divergent at any finite temperature in three dimension. We also point out that different types of SOC give rise to qualitatively different dependence of condensate depletion on microscopic variables including scattering length, strength of SOC and temperature, a novel feature that can be directly observed once these types of SOC are realized in experiments.
Jun 26
1. arXiv:1206.5614 [pdf, ps, other]
Particle number fractionalization of a one-dimensional atomic Fermi gas with synthetic spin-orbit coupling
Dan-Wei Zhang, L.-B. Shao, Zheng-Yuan Xue, Hui Yan, Z. D. Wang, Shi-Liang Zhu
We propose an experimental scheme to simulate the fractionalization of particle number by using a one-dimensional spin-orbit coupled ultracold fermionic gas. The wanted spin-orbit coupling, a kink-like potential, and a conjugation-symmetry-breaking mass term are properly constructed by laser-atom interactions, leading to an effective low-energy relativistic Dirac Hamiltonian with a topologically nontrivial background field. The designed system supports a localized soliton excitation with a fractional particle number that is generally irrational and experimentally tunable, providing a direct realization of the celebrated generalized-Su-Schrieffer-Heeger model. In addition, we elaborate on how to detect the induced soliton mode with the FPN in the system.
2. arXiv:1206.5586 [pdf, ps, other]
A New Non-Abelian Topological Phase of Cold Fermi Gases in Anisotropic and Spin-Dependent Optical Lattices
Beibing Huang, Xiaosen Yang, Shaolong Wan
To realize non-Abelian s-wave topological superfluid (TS) of cold Fermi gases, generally a Zeeman magnetic field larger than superfluid pairing gap is necessary. In this paper we find that using an anisotropic and spin-dependent optical lattice (ASDOL) to trap gases, a new non-Abelian TS phase appears, in contrast to an isotropic and spin-independent optical lattice. A characteristic of this new non-Abelian TS is that Zeeman magnetic field can be smaller than the superfluid pairing gap. By self-consistently solving pairing gap equation and considering the competition against normal state and phase separation, this new phase is also stable. Thus an ASDOL supplies a convenient route to realize TS. We also investigate edge states and the effects of a harmonic trap potential.
3. arXiv:1206.5470 [pdf, other]
C-Field Methods for Non-Equilibrium Bose Gases
Matthew J. Davis, Tod M. Wright, P. Blair Blakie, Ashton S. Bradley, Rob J. Ballagh, Crispin W. Gardiner
We review c-field methods for simulating the non-equilibrium dynamics of degenerate Bose gases beyond the mean-field Gross-Pitaevskii approximation. We describe three separate approaches that utilise similar numerical methods, but have distinct regimes of validity. Systems at finite temperature can be treated with either the closed-system projected Gross-Pitaevskii equation (PGPE), or the open-system stochastic projected Gross-Pitaevskii equation (SPGPE). These are both applicable in quantum degenerate regimes in which thermal fluctuations are significant. At low or zero temperature, the truncated Wigner projected Gross-Pitaevskii equation (TWPGPE) allows for the simulation of systems in which spontaneous collision processes seeded by quantum fluctuations are important. We describe the regimes of validity of each of these methods, and discuss their relationships to one another, and to other simulation techniques for the dynamics of Bose gases. The utility of the SPGPE formalism in modelling non-equilibrium Bose gases is illustrated by its application to the dynamics of spontaneous vortex formation in the growth of a Bose-Einstein condensate.
4. arXiv:1206.5380 [pdf, ps, other]
A two-dimensional Fermi liquid with attractive interactions
B. Fröhlich, M. Feld, E. Vogt, M. Koschorreck, M. Köhl, C. Berthod, T. Giamarchi
We realize and study an attractively interacting two-dimensional Fermi liquid. Using momentum-resolved photoemission spectroscopy we measure the self-energy, determine the contact parameter of the short-range interaction potential, and find their dependence on the interaction strength. We successfully compare the measurements to a theoretical analysis taking properly into account finite temperature, the harmonic trap, and an inhomogeneous peak-density distribution of the array of two-dimensional gases.
Jun 25
1. arXiv:1206.5188 [pdf, ps, other]
Delocalization of ultracold atoms in a disordered potential due to light scattering
Boris Nowak, Jami J. Kinnunen, Murray J. Holland, Peter Schlagheck
We numerically study the expansion dynamics of ultracold atoms in a one-dimensional disordered potential in the presence of a weak position measurement of the atoms. We specifically consider this position measurement to be realized by a combination of an external laser and a periodic array of optical microcavities along a waveguide. The position information is acquired through the scattering of a near-resonant laser photon into a specific eigenmode of one of the cavities. The time evolution of the atomic density in the presence of this light scattering mechanism is described within a Lindblad master equation approach, which is numerically implemented using the Monte Carlo wave function technique. We find that an arbitrarily weak rate of photon emission leads to a breakdown of Anderson localization of the atoms.
2. arXiv:1206.5103 [pdf, ps, other]
Frustrated Bose-Einstein condensates with non-collinear orbital ordering
Zi Cai, Yu Wang, Congjun Wu
We investigate the unconventional Bose-Einstein condensations (BEC) with the orbital degree of freedom in the 3D cubic optical lattice, which give rise to various exotic features absent in conventional scalar and spinor BECs. Orbital angular momentum moments are formed on lattice sites breaking time-reversal symmetry spontaneously. Furthermore, they exhibit orbital frustrations and develop a chiral ordering selected by the "order-from-disorder" mechanism.
3. arXiv:1206.5086 [pdf, ps, other]
Cooperative excitation and many-body interactions in a cold Rydberg gas
Matthieu Viteau, Paul Huillery, Mark G. Bason, Nicola Malossi, Donatella Ciampini, Oliver Morsch, Ennio Arimondo, Daniel Comparat, Pierre Pillet
The dipole blockade of Rydberg excitations is a hallmark of the strong interactions between atoms in these high-lying quantum states. One of the consequences of the dipole blockade is the suppression of fluctuations in the counting statistics of Rydberg excitations, of which some evidence has been found in previous experiments. Here we present experimental results on the dynamics and the counting statistics of Rydberg excitations of ultra-cold Rubidium atoms both on and off resonance, which exhibit sub- and super-Poissonian counting statistics, respectively. We compare our results with numerical simulations using a novel theoretical model based on Dicke states of Rydberg atoms including dipole-dipole interactions, finding good agreement between experiment and theory.
4. arXiv:1206.5050 [pdf, other]
Pairing in a two-dimensional Fermi gas with population imbalance
M. J. Wolak, B. Grémaud, R. T. Scalettar, G. G. Batrouni
Pairing in a population imbalanced Fermi system in a two-dimensional optical lattice is studied using Determinant Quantum Monte Carlo (DQMC) simulations and mean-field calculations. The approximation-free numerical results show a wide range of stability of the Fulde-Ferrell-Larkin-Ovshinnikov (FFLO) phase. Contrary to claims of fragility with increased dimensionality we find that this phase is stable across wide range of values for the polarization, temperature and interaction strength. Both homogeneous and harmonically trapped systems display pairing with finite center of mass momentum, with clear signatures either in momentum space or real space, which could be observed in cold atomic gases loaded in an optical lattice. We also use the harmonic level basis in the confined system and find that pairs can form between particles occupying different levels which can be seen as the analog of the finite center of mass momentum pairing in the translationally invariant case. Finally, we perform mean field calculations for the uniform and confined systems and show the results to be in good agreement with QMC. This leads to a simple picture of the different pairing mechanisms, depending on the filling and confining potential.
5. arXiv:1206.5023 [pdf, other]
Ultracold Fermionic Feshbach Molecules of $^{23}$Na$^{40}$K
Cheng-Hsun Wu, Jee Woo Park, Peyman Ahmadi, Sebastian Will, Martin W. Zwierlein
We report on the formation of ultracold fermionic Feshbach molecules of $^{23}$Na$^{40}$K, the first fermionic molecule that is chemically stable in its ground state. The lifetime of the nearly degenerate molecular gas exceeds 100 ms in the vicinity of the Feshbach resonance. The measured dependence of the molecular binding energy on the magnetic field demonstrates the open-channel character of the molecules over a wide field range and implies significant singlet admixture. This will enable efficient transfer into the singlet vibrational ground state, resulting in a stable molecular Fermi gas with strong dipolar interactions.
Jun 22
1. arXiv:1206.4988 [pdf, other]
Simulating Quantum Fields with Cavity QED
Sean Barrett, Klemens Hammerer, Sarah Harrison, Tracy E. Northup, Tobias J. OsborneAs the realization of a fully operational quantum computer remains distant, \emph{quantum simulation}, whereby one quantum system is engineered to simulate another, becomes a key goal of great practical importance. Here we report on a variational method exploiting the natural physics of cavity QED architectures to simulate strongly interacting quantum fields. Our scheme is broadly applicable to any architecture involving tunable and strongly nonlinear interactions with light; as an example, we demonstrate that existing cavity devices could simulate models of strongly interacting bosons. The scheme can be extended to simulate systems of entangled multicomponent fields, beyond the reach of existing classical simulation methods.
2. arXiv:1206.4984 [pdf, ps, other]
Ultra-cold fermions in the flatland: evolution from BCS to Bose superfluidity in two-dimensions with spin-orbit and Zeeman fieldsLi Han, C. A. R. Sá de MeloWe discuss the evolution from BCS to Bose superfluidity for ultracold fermions in two-dimensions and in the presence of simultaneous spin-orbit and Zeeman fields. We analyze several thermodynamic properties to characterize different superfluid phases including pressure, compressibility, induced polarization, and spin susceptibility. Furthermore, we compute the momentum distribution and construct topological invariants for each of the superfluid phases.
3. arXiv:1206.4974 [pdf, other]
Dipolar fermions in a two-dimensional lattice at non-zero temperature
Anne-Louise Gadsbolle, G. M. BruunWe examine density ordered and superfluid phases of fermionic dipoles in a two-dimensional square lattice at non-zero temperature. The critical temperature of the density ordered phases is determined and is shown to be proportional to the coupling strength for strong coupling. We calculate the superfluid fraction and demonstrate that the Berezinskii-Kosterlitz-Thouless transition temperature of the superfluid phase is proportional to the hopping matrix element in the strong coupling limit. We finally analyze the effects of an external harmonic trapping potential.
4. arXiv:1206.4962 [pdf, other]
Time dependent impurity in ultracold fermions: orthogonality catastrophe and beyond
Michael Knap, Aditya Shashi, Yusuke Nishida, Adilet Imambekov, Dmitry A. Abanin, Eugene Demler
Recent experimental realization of strongly imbalanced mixtures of ultracold atoms opens new possibilities for studying impurity dynamics in a controlled setting. We discuss how the techniques of atomic physics can be used to explore new regimes and manifestations of Anderson's orthogonality catastrophe (OC), which could not be accessed in solid state systems. We consider a system of impurity atoms localized by a strong optical lattice potential and immersed in a sea of itinerant Fermi atoms. Ramsey interference experiments with impurity atoms probe OC in the time domain, while radio-frequency (RF) spectroscopy probes OC in the frequency domain. The OC in such systems is universal for all times and is determined by the impurity scattering length and Fermi wave vector of itinerant fermions. We calculate the universal Ramsey response and RF absorption spectra. In addition to the standard power-law contribution, which corresponds to the excitation of multiple particle-hole pairs near the Fermi surface, we identify a novel contribution to OC that comes from exciting one extra particle from the bottom of the itinerant band. This gives rise to a non-analytic feature in the RF absorption spectra, which evolves into a true power-law singularity with universal exponent 1/4 at the unitarity. Furthermore, we discuss the manifestations of OC in spin-echo experiments, as well as in the energy counting statistic of the Fermi gas following a sudden quench of the impurity state. Finally, systems in which the itinerant fermions have two or more hyperfine states provide an even richer playground for studying non-equilibrium impurity physics, allowing one to explore non-equilibrium OC and to simulate quantum transport through nano-structures. This provides a useful connection between cold atomic systems and mesoscopic quantum transport.
5. arXiv:1206.4778 [pdf, ps, other]
Momentum-resolved radio-frequency spectroscopy of ultracold atomic Fermi gases in a spin-orbit coupled lattice
Xia-Ji Liu
We investigate theoretically momentum-resolved radio-frequency (rf) spectroscopy of a noninteracting atomic Fermi gas in a spin-orbit coupled lattice. This lattice configuration has been recently created at MIT [Cheuk et al., arXiv:1205.3483] for 6Li atoms, by coupling the two hyperfine spin-states with a pair of Raman laser beams and additional rf coupling. Here, we show that momentum-resolved rf spectroscopy can measure single-particle energies and eigenstates and therefore resolve the band structure of the spin-orbit coupled lattice. In our calculations, we take into account the effects of temperatures and harmonic traps. Our predictions are to be confronted with future experiments on spin-orbit coupled Fermi gases of 40K atoms in a lattice potential.
6. arXiv:1206.4756 [pdf, other]
Quantized Adiabatic Transport in Momentum Space
Derek Y. H. Ho, Jiangbin Gong
Though topological aspects of energy bands are known to play a key role in quantum transport in solid-state systems, the implications of Floquet band topology for transport in momentum space (i.e., acceleration) are not explored so far. Using a ratchet accelerator model inspired by existing cold-atom experiments, here we characterize a class of extended Floquet bands of one-dimensional driven quantum systems by Chern numbers, reveal topological phase transitions therein, and theoretically predict the quantization of adiabatic transport in momentum space. Numerical results confirm our theory and indicate the feasibility of experimental studies.
7. arXiv:1206.4772 [pdf, ps, other]
Space-time crystals of trapped ions
Tongcang Li, Zhe-Xuan Gong, Zhang-Qi Yin, H. T. Quan, Xiaobo Yin, Peng Zhang, L.-M. Duan, Xiang Zhang
Great progresses have been made in exploring exciting physics of low dimensional materials in last few decades. Important examples include the discovering and synthesizing of fullerenes (zero dimensional, 0D), carbon nanotubes (1D) and graphene (2D). A fundamental question is whether we can create materials with dimensions higher than that of conventional 3D crystals, for example, a 4D crystal that has periodic structures in both space and time. Here we propose a space-time crystal of trapped ions and a method to realize it experimentally by confining ions in a ring-shaped trapping potential with a static magnetic field. The ions spontaneously form a spatial ring crystal due to Coulomb repulsion. This ion crystal can rotate persistently at the lowest quantum energy state in magnetic fields with fractional fluxes. The persistent rotation of trapped ions produces the temporal order, leading to the formation of a space-time crystal. We show that these space-time crystals are robust for direct experimental observation. The proposed space-time crystals of trapped ions provide a new dimension for exploring many-body physics and emerging properties of matter.
Jun 21
1. arXiv:1206.4507 [pdf, ps, other]
Controlling chemical reactions of a single particle
Lothar Ratschbacher, Christoph Zipkes, Carlo Sias, Michael KöhlThe control of chemical reactions is a recurring theme in physics and chemistry. Traditionally, chemical reactions have been investigated by tuning thermodynamic parameters, such as temperature or pressure. More recently, physical methods such as laser or magnetic field control have emerged to provide completely new experimental possibilities, in particular in the realm of cold collisions. The control of reaction pathways is also a critical component to implement molecular quantum information processing. For these undertakings, single particles provide a clean and well-controlled experimental system. Here, we report on the experimental tuning of the exchange reaction rates of a single trapped ion with ultracold neutral atoms by exerting control over both their quantum states. We observe the influence of the hyperfine interaction on chemical reaction rates and branching ratios, and monitor the kinematics of the reaction products. These investigations advance chemistry with single trapped particles towards achieving quantum-limited control of chemical reactions and indicate limits for buffer gas cooling of single ion clocks.
Jun 20
1. arXiv:1206.4064 [pdf, ps, other]
Superfluidity of Dirac Fermions in a Tunable Honeycomb Lattice: Cooper Pairing, Collective Modes, and Critical Currents
Shunji Tsuchiya, R. Ganesh, Arun ParamekantiMotivated by recent experiments on atomic Dirac fermions in a tunable honeycomb optical lattice, we study the attractive Hubbard model of superfluidity in the anisotropic honeycomb lattice. At weak-coupling, we find that the maximum mean field pairing transition temperature, as a function of density and interaction strength, occurs for the case with isotropic hopping amplitudes. In this isotropic case, we go beyond mean field theory and study collective fluctuations, treating both pairing and density fluctuations for interaction strengths ranging from weak to strong coupling. We find evidence for a sharp sound mode, together with a well-defined Leggett mode over a wide region of the phase diagram. We also calculate the superfluid order parameter and collective modes in the presence of nonzero superfluid flow. We find that the superfluid order parameter and density fluctuations exhibit nontrivial dependence on superflow. The softening of these collective modes leads to dynamical instabilities involving stripe-like density modulations as well as a Leggett-mode instability associated with the natural sublattice symmetry breaking charge-ordered state on the honeycomb lattice. We delineate regimes of the phase diagram where the critical current is limited by depairing or by such collective instabilities, and discuss experimental implications of our results.
2. arXiv:1206.4143 [pdf, ps, other]
The effects of disorder in dimerized quantum magnets in mean field approximations
Abdulla Rakhimov, Shuhrat Mardonov, E. Ya. Sherman, Andreas SchillingWe study theoretically the effects of disorder on Bose-Einstein condensates (BEC) of bosonic triplon quasiparticles in doped dimerized quantum magnets. The condensation occurs in a strong enough magnetic field Hc, where the concentration of bosons in the random potential is sufficient to form the condensate. The effect of doping is partly modeled by delta - correlated disorder potential, which (i) leads to the uniform renormalization of the system parameters and (ii) produces disorder in the system with renormalized parameters. These approaches can explain qualitatively the available magnetization data in the Tl_(1-x)K_(x)CuCl_3 compound taken as an example. In addition to the magnetization, we found that the speed of the Bogoliubov mode has a peak as a function of doping parameter, x. No evidence of the pure Bose glass phase has been obtained in the BEC regime.
3. arXiv:1206.4276 [pdf, other]
Analog Hawking radiation from an acoustic black hole in a flowing polariton superfluid
Dario Gerace, Iacopo CarusottoWe theoretically study Hawking radiation processes from an analog acoustic black hole in a flowing superfluid of exciton-polaritons in a one-dimensional semiconductor microcavity. Polaritons are coherently injected into the microcavity by a laser pump with a suitably tailored spot profile. An event horizon with a large analog surface gravity is created by inserting a defect in the polariton flow along the cavity plane. Experimentally observable signatures of the analog Hawking radiation are identified in the scattering of phonon wavepackets off the horizon, as well as in the spatial correlation pattern of quantum fluctuations of the polariton density. The potential of these table-top optical systems as analog models of gravitational physics is quantitatively confirmed by numerical calculations using realistic parameters for state-of-the-art devices.
Jun 19
1. arXiv:1206.3984 [pdf, other]
Quantum Breathing of an Impurity in a One-dimensional Bath of Interacting Bosons
Sebastiano Peotta, Davide Rossini, Marco Polini, Francesco Minardi, Rosario FazioBy means of time-dependent density-matrix renormalization-group (TDMRG) we are able to follow the real-time dynamics of a single impurity embedded in a one-dimensional bath of interacting bosons. We focus on the impurity breathing mode, which is found to be well-described by a single oscillation frequency and a damping rate. If the impurity is very weakly coupled to the bath, a Luttinger-liquid description is valid and the impurity suffers an Abraham-Lorentz radiation-reaction friction. For a large portion of the explored parameter space, the TDMRG results fall well beyond the Luttinger-liquid paradigm.
2. arXiv:1206.3687 [pdf, other]
Matter wave scattering on an amplitude-modulated optical lattice
Pierrick Cheiney, Charlotte Fabre, François Vermersch, Giovanni Luca Gattobigio, Renaud Mathevet, Thierry Lahaye,David Guery-OdelinWe experimentally study the scattering of guided matter waves on an amplitude-modulated optical lattice. We observe different types of frequency-dependent dips in the asymptotic output density distribution. Their positions are compared quantitatively with numerical simulations. A semiclassical model that combines \emph{local} Floquet-Bloch bands analysis and Landau-Zener transitions provides a simple picture of the observed phenomena in terms of elementary \emph{Floquet photon} absorption-emission processes and envelope induced reflections. Finally, we propose and demonstrate the use of this technique with a bichromatic modulation to design a tunable sub-recoil velocity filter.
3. arXiv:1206.3904 [pdf, ps, other]
Liquid and crystal phase of dipolar fermions in two dimensions
N. Matveeva, S. GiorginiThe liquid and crystal phase of a single-component Fermi gas with dipolar interactions are investigated using quantum Monte Carlo methods in two spatial dimensions and at zero temperature. The dipoles are oriented by an external field perpendicular to the plane of motion, resulting in a purely repulsive 1/r^3 interaction. In the liquid phase we calculate the equation of state as a function of the interaction strength and other relevant properties characterizing the Fermi-liquid behavior: effective mass, discontinuity at the Fermi surface and pair correlation function. In the high density regime we calculate the equation of state of the Wigner crystal phase and the critical density of the liquid to solid first order phase transition. Close to the freezing density we also search for the existence of a stripe phase, but such a phase is never found to be energetically favorable.
4. arXiv:1206.3670 [pdf, ps, other]
Ground-State Phase Diagram of the Two-Dimensional Extended Bose-Hubbard Model
Takahiro Ohgoe, Takafumi Suzuki, Naoki KawashimaWe investigate the ground-state phase diagram of the soft-core Bose-Hubbard model with the nearest-neighbor repulsion on a square lattice by using an unbiased quantum Monte Carlo method. In contrast to the previous study[P. Sengupta {\it et. al.}, Phys. Rev. Lett. {\bf 94}, 207202 (2005)], we present the ground-state phase diagrams up to large hopping parameters. As a result, in addition to the known supersolid above half-filling, we find supersolid even below and at half-filling for large hopping parameters. Furthermore, for the strong nearest-neighbor repulsion, we show that the supersolid phase occupies a remarkably broad region in the phase diagram. The results are in qualitative agreement with that obtained by the Gutzwiller mean-field approximation[M. Iskin, Phys. Rev. A {\bf 83}, 051606(R) (2011) and T. Kimura, Phys. Rev. A {\bf 84}, 063630 (2011)].
Jun 15
1. arXiv:1206.3181 [pdf, other]
On a new twist in the dynamics of Bose-Einstein condensation
Boris Nowak, Thomas GasenzerThe dynamical process of Bose-Einstein (BE) condensation of a dilute ultracold gas is found to occur in two distinctly different forms. If the particle flux into the low-energy modes is sufficiently strong, the Bose gas behaves predominantly like an incompressible fluid. It exhibits chaotic (Vinen) superfluid turbulence which marks the presence of a \emph{non-thermal fixed point} of the evolution. The approach of this state can be identified by the build-up of a characteristic power-law single-particle spectrum $n(k)\sim k^{-5}$. Alternatively, for a weak flux in energy space, during the condensation process phase defects reminiscent of superfluid turbulence appear but a separation of the incompressible and compressible components, as well as the $k^{-5}$ scaling of the low-energy modes are absent.
Jun 14
1. arXiv:1206.2770 [pdf, other]
Roton spectroscopy in a harmonically trapped dipolar Bose-Einstein condensate
P. B. Blakie, D. Baillie, R. N. BissetWe study a harmonically trapped Bose-Einstein condensate with dipole-dipole interactions in a regime where a roton spectrum emerges. We show that the roton spectrum is clearly revealed in the static and dynamic structure factors which can be measured using Bragg spectroscopy. We develop and validate a theory based on the local density approximation for the dynamic structure factor.
Jun 13
1. arXiv:1206.2410 [pdf, ps, other]
Quench dynamics of the interacting Bose gas in one dimension
Deepak Iyer, Natan AndreiWe obtain an exact expression for the time evolution of the interacting Bose gas following a quench from a generic initial state using the Yudson representation for integrable systems. We study the time evolution of the density and noise correlation for a small number of bosons and their asymptotic behavior for any number. We show that for any value of the coupling, as long as it is repulsive, the system asymptotes towards a strongly repulsive gas, while for any value of an attractive coupling the long time behavior is dominated by the maximal bound state. This occurs independently of the initial state and can be viewed as an emerging "dynamic universality".
2. arXiv:1206.2368 (cross-list from cond-mat.stat-mech) [pdf, ps, other]
Quantum interface unbinding transitions
Pawel JakubczykWe consider interfacial phenomena accompanying bulk quantum phase transitions in presence of surface fields. On general grounds we argue that the surface contribution to the system free energy involves a line of singularities characteristic of an interfacial phase transition, occurring below the bulk transition temperature T_c down to T=0. This implies the occurrence of an interfacial quantum critical regime extending into finite temperatures and located within the portion of the phase diagram where the bulk is ordered. Even in situations, where the bulk order sets in discontinuously at T=0, the system's behavior at the boundary may be controlled by a divergent length scale if the tricritical temperature is sufficiently low. Relying on an effective interfacial model we compute the surface phase diagram in bulk spatial dimensionality $d\geq 2$ and extract the values of the exponents describing the interfacial singularities in $d\geq 3$.
Jun 12
1. arXiv:1206.2274 [pdf, ps, other]
Efimov Physics and the Three-Body Parameter within a Two-Channel Framework
P. K. Sørensen, D. V. Fedorov, A. S. Jensen, N. T. ZinnerWe calculate shallow three-body bound states in the universal regime, defined by Efimov, with inclusion of both scattering length and effective range parameters. The universal spectrum is recovered for the least bound states, whereas for larger binding energies we find corrections to the universal scaling laws. We recover known results for broad Feshbach resonances with small effective range, whereas in the case of narrow resonances we find a distinct non-monotonic behavior of the threshold at which the lowest Efimov trimer merges with the three-body continuum. To address the issue of the physical origin of the three-body parameter we provide a physically clear model for the relation between three-body physics and typical two-body atom-atom interactions. Our results demonstrate that experimental information from narrow Feshbach resonances and/or mixed systems are of vital importance to pin down the relation of two- and three-body physics in atomic systems.
2. arXiv:1206.2202 [pdf, ps, other]
Condensate wave function and elementary excitations of bosonic polar molecules: beyond the first Born approximation
Chao-Chun Huang, Daw-Wei Wang, W. C. WuWe investigate the condensate wave function and elementary excitations of strongly interacting bosonic polar molecules in a harmonic trap, treating the scattering amplitude beyond the standard first Born approximation (FBA). By using an appropriate trial wave function in the variational method, effects of the leading order correction beyond the FBA have been investigated and shown to be significantly enhanced when the system is close to the phase boundary of collapse. How such leading order effect of going beyond the FBA can be observed in a realistic experiment is also discussed.
3. arXiv:1206.2198 (cross-list from cond-mat.other) [pdf, ps, other]
Vortices in a rotating Bose-Einstein condensate under extreme elongation in a harmonic plus quartic trap
C. C. Huang, C. H. Liu, W. C. WuThe behaviors of a rapid rotating Bose-Einstein condensate under extreme elongation in a 2D anisotropic harmonic plus quartic trap are investigated. Due to the quartic trap, the system remains stable at high rotating velocity, $\Omega\geq \omega_\perp$ ($\omega_\perp$ is the radial harmonic trap frequency), and vortex lattices form even in the absence of the repulsive s-wave interaction ($g$). When $g$ is present, the interplay between $g$ and the quartic trap potential can lead to rich vortex lattice transition states as a function of $\Omega$, to which vortex lattices vanish eventually at some higher $\Omega$.
Jun 11
1. arXiv:1206.1648 [pdf, other]
Signatures of the superfluid to Mott insulator transition in equilibrium and in dynamical ramps
D. Pekker, B. Wunsch, T. Kitagawa, E. Manousakis, A. S. Sørensen, E. DemlerWe investigate the equilibrium and dynamical properties of the Bose-Hubbard model and the related particle-hole symmetric spin-1 model in the vicinity of the superfluid to Mott insulator quantum phase transition. We employ the following methods: exact-diagonalization, mean field (Gutzwiller), cluster mean-field, and mean-field plus Gaussian fluctuations. In the first part of the paper we benchmark the four methods by analyzing the equilibrium problem and give numerical estimates for observables such as the density of double occupancies and their correlation function. In the second part, we study parametric ramps from the superfluid to the Mott insulator and map out the crossover from the regime of fast ramps, which is dominated by local physics, to the regime of slow ramps with a characteristic universal power law scaling, which is dominated by long wavelength excitations. We calculate values of several relevant physical observables, characteristic time scales, and an optimal protocol needed for observing universal scaling.
2.arXiv:1206.1604 (cross-list from cond-mat.str-el) [pdf, ps, other]
Integer quantum Hall effect for bosons: A physical realization
T. Senthil, Michael LevinA simple physical realization of an integer quantum Hall state of interacting two dimensional bosons is provided. This is an example of a "symmetry-protected topological" (SPT) phase which is a generalization of the concept of topological insulators to systems of interacting bosons or fermions. Universal physical properties of the boson integer quantum Hall state are described and shown to correspond to those expected from general classifications of SPT phases.
Jun 8
1. arXiv:1206.1569 [pdf, other]
Atom-dimer p-wave resonance for fermionic mixtures with different masses
R. Combescot, X. Leyronas
We show that, near a Feshbach resonance, a strong p-wave resonance is present at low energy in atom-dimer scattering for $^6$Li-$^{40}$K fermionic mixtures. This resonance is due to a virtual bound state, in the atom-dimer system, which is present at this low energy. When the mass ratio between the two fermionic elements is increased, this virtual bound state goes to a known real bound state which appears when the mass ratio reaches 8.17. This resonance should affect a number of physical properties. These include the equation of state of unbalanced mixtures at very low temperature but also the equation of state of balanced mixtures at moderate or high temperature. The frequency and the damping of collective modes should also provide a convenient way to evidence this resonance. Finally it should be possible to modify the effective mass of one the fermionic species by making use of an optical lattice. This would allow to study the strong dependence of the resonance as a function of the mass ratio of the two fermionic elements.
2. arXiv:1206.1547 [pdf, ps, other]
Broken Z2 symmetries and fluctuations in statistical mechanics
Pierre Gaspard
An analogy is developed between the breaking of space-inversion symmetry in equilibrium statistical mechanics and the breaking of time-reversal symmetry in nonequilibrium statistical mechanics. In this way, similar relationships characterizing fluctuations are obtained in both contexts.
3. arXiv:1206.1407 [pdf, ps, other]
Axion topological field theory of topological superconductors
Xiao-Liang Qi, Edward Witten, Shou-Cheng Zhang
Topological superconductors are gapped superconductors with gapless and topologically robust quasiparticles propagating on the boundary. In this paper, we present a topological field theory description of three-dimensional time-reversal invariant topological superconductors. In our theory the topological superconductor is characterized by a topological coupling between the electromagnetic field and the superconducting phase fluctuation, which has the same form as the coupling of "axions" with an Abelian gauge field. As a physical consequence of our theory, we predict the level crossing induced by the crossing of special "chiral" vortex lines, which can be realized by considering s-wave superconductors in proximity with the topological superconductor. Our theory can also be generalized to the coupling with a gravitational field.
4. arXiv:1206.1332 [pdf, ps, other]
Quantum Criticality in Topological Insulators and Superconductors: Emergence of Strongly Coupled Majoranas and Supersymmetry
Tarun Grover, Ashvin Vishwanath
We study symmetry breaking quantum phase transitions in topological insulators and superconductors where the single electron gap remains open in the bulk. Specifically, we consider spontaneous breaking of the symmetry that protects the gapless boundary modes, so that in the ordered phase these modes are gapped. Here we determine the fate of the topological boundary modes right at the transition where they are coupled to the strongly fluctuating order parameter field. Using a combination of exact solutions and renormalization group techniques, we find that the surface fermionic modes either decouple from the bulk fluctuations, or flow to a strongly coupled fixed point which remains gapless. In addition, we study transitions where the critical fluctuations are confined only to the surface and find that in several cases the critical point is naturally supersymmetric. This allows a determination of critical exponents and points to an underlying connection between band topology and supersymmetry. Finally, we study the fate of gapless Majorana modes localized on point and line defects in topological superconductors at bulk criticality, which is analogous to a quantum impurity problem. Again, an interplay of topology and strong correlations causes these modes to remain gapless but in a strongly coupled state. Experimental candidates for realizing these phenomena are discussed.
Jun 7
1. arXiv:1206.1276 [pdf, other]
Zero-bias peaks in spin-orbit coupled superconducting wires with and without Majorana end-states
Jie Liu, Andrew C. Potter, K.T. Law, Patrick A. Lee
One of the simplest proposed experimental probes of a Majorana bound-state is a quantized (2e^2/h) value of zero-bias tunneling conductance. When temperature is somewhat larger than the intrinsic width of the Majorana peak, conductance is no longer quantized, but a zero-bias peak can remain. Such a non-quantized zero-bias peak has been recently reported for semiconducting nanowires with proximity induced superconductivity. In this paper we analyze the relation of the zero-bias peak to the presence of Majorana end-states, by simulating the tunneling conductance for multi-band wires with realistic amounts of disorder. We show that this system generically exhibits a (non-quantized) zero-bias peak even when the wire is topologically trivial and does not possess Majorana end-states. We make comparisons to recent experiments, and discuss the necessary requirements for confirming the existence of a Majorana state.
2.arXiv:1206.1072 [pdf, other]
Wannier Permanents and Featureless Bosonic Mott Insulators on the 1/3 filled Kagome Lattice
S. A. Parameswaran, Itamar Kimchi, Ari M. Turner, D. M. Stamper-Kurn, Ashvin Vishwanath
We study Bose-Hubbard models on tight-binding, non-Bravais lattices, with a filling of one boson per unit cell -- and thus fractional site filling. At integer filling of a unit cell neither symmetry breaking nor topological order is required, and in principle a trivial and featureless (i.e., symmetry-unbroken) insulator is allowed. We demonstrate by explicit construction of a family of wavefunctions that such a featureless Mott insulating state exists at 1/3 filling on the kagome lattice, and construct Hamiltonians for which these wavefunctions are exact ground states. We briefly comment on the experimental relevance of our results to cold atoms in optical lattices. Such wavefunctions also yield 1/3 magnetization plateau states for spin models in an applied field. The featureless Mott states we discuss can be generalized to any lattice for which symmetric exponentially localized Wannier orbitals can be found at the requisite filling, and their wavefunction is given by the permanent over all Wannier orbitals.
3. arXiv:1206.1060 [pdf, ps, other]
Spin-Orbit Coupling in LaAlO$_3$/SrTiO$_3$ interfaces: Magnetism and Orbital Ordering
Mark H Fischer, Srinivas Raghu, Eun-Ah Kim
The combination of Rashba spin-orbit coupling and electron correlations can induce unusual phenomena in the metallic interface between SrTiO$_3$ and LaAlO$_3$. We consider effects of Rashba spin-orbit coupling at this interface in the context of the recent observation of anisotropic magnetism. Firstly, we show how Rashba spin-orbit coupling in a system near a band-edge can account for the observed magnetic anisotropy. Secondly, we investigate the coupling between in-plane magnetic-moment anisotropy and nematicity in the form of an orbital imbalance between d$_{xz}$ / d$_{yz}$ orbitals. We estimate this coupling to be substantial in the low electron density regime. Such an orbital ordering can affect magneto transport.
Jun 6
1. arXiv:1206.0957 [pdf, other]
Quantum dynamics of local phase differences between reservoirs of driven interacting bosons separated by simple aperture arrays
T. J. Volkoff, K. Birgitta Whaley
We present a microscopic derivation of the effective action for the relative phase of coupled, driven reservoirs of interacting condensed bosons, where the coupling is given by tunneling through nanometer sized apertures. The presence of an external driving velocity field and local chemical potential field require imposition of a local U(1) gauge symmetry on the microscopic model. We use the resulting locally gauge invariant effective action to derive the central equations of superfluid hydrodynamics for flow through a single nanoaperture junction from the stationary-phase approximation to the coherent state path-integral at first and second orders of perturbation theory.
For a single junction, the resulting current-phase equation is shown to be consistent with dynamics in a tilted washboard potential. We compute the renormalization group (RG) beta function of the periodic potential component of the effective action for small tunneling amplitudes and use this to analyze the temperature dependence of the low-energy current-phase relation, with application to the transition from linear to sinusoidal current-phase behavior observed in experiments by Hoskinson et al. for liquid $^{4}$He driven through nanoaperture arrays. Extension of the microscopic theory to a two-aperture array shows that interference between the microscopic tunneling contributions for individual apertures leads to a effective coupling between apertures. The resulting multi-aperture current-phase equations are equivalent to a set of equations of coupled pendula, with microscopically derived couplings.
Jun 5
1.arXiv:1206.0564 [pdf, other]
Emergent quantum confinement at topological insulator surfaces
M. S. Bahramy, P. D. C. King, A. de la Torre, J. Chang, M. Shi, L. Patthey, G. Balakrishnan, Ph. Hofmann, R. Arita, N. Nagaosa, F. Baumberger
Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial $\mathbb{Z}_2$ topology. They are therefore widely regarded ideal templates to realize the predicted exotic phenomena and applications of this topological surface state. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure whose origin and properties have proved controversial. Here, we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states. In combination with circular dichroism in angle-resolved photoemission (ARPES) experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study reveals how the full surface-bulk connectivity in topological insulators is modified by quantum confinement.
2.arXiv:1206.0310 [pdf, ps, other]Current and entanglement in a Bose-Hubbard lattice
L. Morales-Molina, S. A. Reyes, M. OrszagWe study the generation of entanglement for interacting cold atoms in an optical lattice. The entanglement is generated by managing the interaction between two distinct atomic species. It is found that the current of one of the species can be used as a good indicator of entanglement generation. The thermalization process between the species is also shown to be closely related to the evolution of the current.
3. arXiv:1206.0614 (cross-list from quant-ph) [pdf, other]
Optomechanical sideband cooling of a thin membrane within a cavity
M. Karuza, C. Molinelli, M. Galassi, C. Biancofiore, R. Natali, P. Tombesi, G. Di Giuseppe, D. Vitali
We present an experimental study of dynamical back-action cooling of the fundamental vibrational mode of a thin semitransparent membrane placed within a high-finesse optical cavity. We study how the radiation pressure interaction modifies the mechanical response of the vibrational mode, and the experimental results are in agreement with a Langevin equation description of the coupled dynamics. The experiments are carried out in the resolved sideband regime, and we have observed cooling by a factor 350 We have also observed the mechanical frequency shift associated with the quadratic term in the expansion of the cavity mode frequency versus the effective membrane position, which is typically negligible in other cavity optomechanical devices.
