1 arXiv:1305.5487v1 [cond-mat.quant-gas] 1 arXiv:1305.5487v1 [cond-mat.quant-gas]
Optical control of atomic interactions in a quantum gas is a long-sought goal of cold atom research. Previous experiments have been hindered by short lifetimes and parasitic deformation of the trap potential. Here, we develop and implement a generic scheme for optical control of Feshbach resonance in quantum gases, which yields long condensate lifetimes sufficient to study equilibrium and nonequilibrium physics with negligible parasitic dipole force. We show that fast and local control of interactions leads to intriguing quantum dynamics in new regimes, highlighted by the formation of van der Waals molecules and partial collapse of a Bose condensate.
We present experimental evidence showing that an interacting Bose condensate in a shaken optical lattice develops a roton-maxon excitation spectrum, a feature normally associated with superfluid helium. The roton-maxon feature originates from the double-well dispersion in the shaken lattice, and can be controlled by both the atomic interaction and the lattice modulation amplitude. We determine the excitation spectrum using Bragg spectroscopy and measure the critical velocity by dragging a weak speckle potential through the condensate -both techniques are based on a digital micromirror device. Our dispersion measurements are in good agreement with a modified Bogoliubov model.PACS numbers: 03.75. Kk, 05.30.Jp, 37.10.Jk, In his seminal papers in the 1940s [1,2], L. D. Landau formulated the theory of superfluid helium-4 (He II) and showed that the energy-momentum relation (dispersion) of He II supports two types of elementary excitations: acoustic phonons and gapped rotons. This dispersion underpins our understanding of superfluidity in helium, and explains many experiments on heat capacity and superfluid critical velocity. What is now called the "rotonmaxon" dispersion in He II has been precisely measured in neutron scattering experiments [3,4] and is generally considered a hallmark of Bose superfluids in the strong interaction regime.The roton-maxon dispersion carries a number of intriguing features that distinguish excitations in different regimes. The low-lying excitations are acoustic phonons with energy E = pv s , where p is the momentum and v s is the sound speed. At higher momenta, the dispersion exhibits both a local maximum at p = p m with energy E = ∆ m and a minimum at p = p r with energy E = ∆ r . The elementary excitations associated with this maximum and minimum are known as maxons and rotons, respectively. The roton excitations, in particular, are known to reduce the superfluid critical velocity below the sound speed. This is best understood based on the Landau criterion for superfluidity in which the critical velocity set by the roton minimum v c ≈ ∆ r p r is lower than the sound speed v s . The roton minimum also suggests the emergence of density wave order [5] and dynamical instability [6].To explore the properties of these unconventional excitations, many theoretical works have proposed schemes for producing the roton-maxon dispersion outside of the He II system. Many proposals have been devoted to atomic systems with long-range or enhanced interactions, e.g. dipolar gases [6][7][8], Rydberg-excited condensates [9], or resonantly interacting gases [10]. Other candidates are 2D Bose gases [11,12], spinor condensates [13,14], and spin-orbit coupled condensates [15,16]. Experimentally, mode softening resulting from cavity-induced interaction has recently been reported [17], which provides strong evidence for an underlying rotonlike excitation spectrum.In this Letter, we generate and characterize an asymmetric roton-maxon excitation spectrum based on a Bose-Einstein condensate (BEC) in a one dimensio...
The low temperature unitary Bose gas is a fundamental paradigm in few-body and many-body physics, attracting wide theoretical and experimental interest. Here we first present a theoretical model that describes the dynamic competition between two-body evaporation and three-body re-combination in a harmonically trapped unitary atomic gas above the condensation temperature. We identify a universal magic trap depth where, within some parameter range, evaporative cooling is balanced by recombination heating and the gas temperature stays constant. Our model is developed for the usual three-dimensional evaporation regime as well as the 2D evaporation case. Experiments performed with unitary 133 Cs and 7 Li atoms fully support our predictions and enable quantitative measurements of the 3-body recombination rate in the low temperature domain. In particular, we measure for the first time the Efimov inelasticity parameter $\eta$ * = 0.098(7) for the 47.8-G d-wave Feshbach resonance in 133 Cs. Combined 133 Cs and 7 Li experimental data allow investigations of loss dynamics over two orders of magnitude in temperature and four orders of magnitude in three-body loss. We confirm the 1/T 2 temperature universality law up to the constant $\eta$ *
We prepare and study strongly interacting two-dimensional Bose gases in the superfluid, the classical Berezinskii-Kosterlitz-Thouless (BKT) transition, and the vacuum-to-superfluid quantum critical regimes. A wide range of the two-body interaction strength 0.05 < g < 3 is covered by tuning the scattering length and by loading the sample into an optical lattice. Based on the equations of state measurements, we extract the coupling constants as well as critical thermodynamic quantities in different regimes. In the superfluid and the BKT transition regimes, the extracted coupling constants show significant down-shifts from the mean-field and perturbation calculations when g approaches or exceeds one. In the BKT and the quantum critical regimes, all measured thermodynamic quantities show logarithmic dependence on the interaction strength, a tendency confirmed by the extended classical-field and renormalization calculations.PACS numbers: 51.30.+i, 67.25.dj, 64.70.Tg, 37.10.Jk Two-dimensional (2D) Bose gases are an intriguing system to study the interplay between quantum statistics, fluctuations, and interaction. For noninteracting bosons in 2D, fluctuations prevail at finite temperatures and Bose-Einstein condensation occurs only at zero temperature. The presence of interaction can drastically change the picture. With repulsive interactions, fluctuations are reduced and superfluidity emerges at finite temperature via the Berezenskii-Kosterliz-Thouless (BKT) mechanism [1,2]. Interacting Bose gases in two dimensions and BKT physics have been actively investigated in many condensed matter experiments [3][4][5][6][7]. In cold atoms, the BKT transition and the suppression of fluctuations are observed based on 2D gases in the weak interaction regimes [8][9][10][11].Extensive theoretical research on 2D Bose systems addresses the role of interactions in the superfluid phase [12][13][14][15][16][17] and near the BKT critical point [18,19]. In the weak interaction regime, the classical φ 4 field theory [18,19] predicts the logarithmic corrections to the critical chemical potential µ c = k B T (g/π) ln(13.2/g) and the critical density n c = λ −2 dB ln(380/g) for small two-body interaction strength g < 0.2. Here k B T is the thermal energy and λ dB is the thermal de Broglie wavelength. The classical-field predictions are consistent with weakly interacting 2D gas experiments [9][10][11]20]. The upper blue shaded area is the superfluid regime, and the red boundary corresponds to the BKT transition regime. The black dashed lineμ = 0 indicates where we evaluate the density and pressure for a vacuum-to-superfuid quantum critical gas. The inset compares the equations of state of a 2D gas and a 2D lattice gas with an almost identical g ≈ 0.4. µ MF = 2 gn/m logarithmically [13]. Here, m is the mass of the boson, n is the density, and 2π is the Planck constant. Defining the superfluid coupling constant as G = m/( 2 κ), where κ = ∂n/∂µ is the compressibility, we can summarize the perturbation expansion result of G as [12]
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