Dipolar Bose gas in highly anharmonic traps

By means of mean-field theory, we have studied the structure and excitation spectrum of a purely dipolar Bose gas in pancake-shaped trap where the confinement in the x-y plane is provided by a highly anharmonic potential resulting in an almost uniform confinement in the plane. We show that the stable condensates is characterized by marked radially structured density profiles. The stability diagram is calculated by independently varying the strength of the interaction and the trap geometry. By computing the Bogoliubov excitation spectrum near the instability line we show that soft "angular" rotons are responsible for the collapse of the system. The free expansion of the cloud after the trap is released is also studied by means of time-dependent calculations, showing that a prolate, cigar-shaped condensate is dynamically stabilized during the expansion, which would otherwise collapse. Dipolar condensates rotating with sufficiently high angular velocity show the formation of multiply-quantized giant vortices, while the condensates acquire a ring-shaped form.Comment: 9 pages, 10 figures. Submitted to Phys. Rev.

Shell Effects in the First Sound Velocity of an Ultracold Fermi Gas

We investigate the first sound of a normal dilute and ultracold two-component Fermi gas in a harmonic microtube, i.e. a cylinder with harmonic transverse radial confinement in the length-scale of microns. We show that the velocity of the sound that propagates along the axial direction strongly depends on the dimensionality of the system. In particular, we predict that the first-sound velocity display shell effects: by increasing the density, that is by inducing the crossover from one-dimension to three-dimensions, the first-sound velocity shows jumps in correspondence with the filling of harmonic modes. The experimental achievability of these effects is discussed by considering 40K atoms.Comment: 6 pages, 1 figure, submitted for the Proceedings of the International Symposium on Quantum Fluids and Solids 2007, Kaza

Viscosity-entropy ratio of the unitary Fermi gas from zero-temperature elementary excitations

We investigate the low-temperature behavior of the ratio between the shear viscosity \eta and the entropy density s in the unitary Fermi gas by using a model based on the zero-temperature spectra of both bosonic collective modes and fermonic single-particle excitations. Our theoretical curve of \eta/s as a function of the temperature T is in qualitative agreement with the experimental data of trapped ultracold 6Li atomic gases. We find the minimum value \eta/s \simeq 0.44 (in units of \hbar/k_B) at the temperature T/T_F \simeq 0.27, with T_F the Fermi temperature.Comment: 10 pages, 2 figures, prepared for the special issue "Correlations in Quantum Gases" of J. Low Temp, Phy

Supersolid structure and excitation spectrum of soft-core bosons in 3D

By means of a mean-field method, we have studied the zero temperature structure and excitation spectrum of a three-dimensional soft-core bosonic system for a value of the interaction strength that favors a crystal structure made of atomic nano-clusters arranged with FCC ordering. In addition to the longitudinal and transverse phonon branches expected for a normal crystal, the excitation spectrum shows a soft mode related to the breaking of gauge symmetry, which signals a partial superfluid character of the solid. Additional evidence of supersolidity is provided by the calculation of the superfluid fraction, which shows a first-order drop, from 1 to 0.4, at the liquid-supersolid transition and a monotonic decrease as the interaction strength parameter is increased. The conditions for the coexistence of the supersolid with the homogeneous superfluid are discussed, and the surface tension of a representative solid-liquid interface is calculated.Comment: 11 pages, 11 figure

Self-bound droplet of Bose and Fermi atoms in one dimension: Collective properties in mean-field and Tonks-Girardeau regimes

We investigate a dilute mixture of bosons and spin-polarized fermions in one-dimension. With an attractive Bose-Fermi scattering length the ground-state is a self-bound droplet, i.e. a Bose-Fermi bright soliton where the Bose and Fermi clouds are superimposed. We find that the quantum fluctuations stabilize the Bose-Fermi soliton such that the one-dimensional bright soliton exists for any finite attractive Bose-Fermi scattering length. We study density profile and collective excitations of the atomic bright soliton showing that they depend on the bosonic regime involved: mean-field or Tonks-Girardeau.Comment: 9 pages, 5 figures, to be published in Phys. Rev.

Emulation of lossless exciton-polariton condensates by dual-core optical waveguides: Stability, collective modes, and dark solitons

We propose a possibility to simulate the exciton-polariton (EP) system in the lossless limit, which is not currently available in semiconductor microcavities, by means of a simple optical dual-core waveguide, with one core carrying the nonlinearity and operating close to the zero-group-velocity-dispersion (GVD) point, and the other core being linear and dispersive. Both 2D and 1D EP systems may be emulated by means of this optical setting. In the framework of this system, we find that, while the uniform state corresponding to the lower branch of the nonlinear dispersion relation is stable against small perturbations, the upper branch is always subject to the modulational instability (MI). The stability and instability are verified by direct simulations too. We analyze collective excitations on top of the stable lower-branch state, which include a Bogoliubov-like gapless mode and a gapped one. Analytical results are obtained for the corresponding sound velocity and energy gap. The effect of a uniform phase gradient (superflow) on the stability is considered too, with a conclusion that the lower-branch state becomes unstable above a critical wavenumber of the flux. Finally, we demonstrate that the stable 1D state may carry robust dark solitons.Comment: 11 pages, 9 figures, to appear in Phys. Rev.

Acetylene on Si(100) from first principles: adsorption geometries, equilibrium coverages and thermal decomposition

Adsorption of acetylene on Si(100) is studied from first principles. We find that, among a number of possible adsorption configurations, the lowest-energy structure is a ``bridge'' configuration, where the C$_2$H$_2$ molecule is bonded to two Si atoms. Instead, ``pedestal'' configurations, recently proposed as the lowest-energy structures, are found to be much higher in energy and, therefore, can represent only metastable adsorption sites. We have calculated the surface formation energies for two different saturation coverages, namely 0.5 and 1 monolayer, both observed in experiments. We find that although, in general, the full monolayer coverage is favored, a narrow range of temperatures exists in which the 0.5 monolayer coverage is the most stable one, where the acetylene molecules are adsorbed in a $2\times 2$ structure. This result disagrees with the conclusions of a recent study and represents a possible explanation of apparently controversial experimental findings. The crucial role played by the use of a gradient-corrected density functional is discussed. Finally, we study thermal decomposition of acetylene adsorbed on Si(100) by means of finite-temperature Molecular Dynamics, and we observe an unexpected behavior of dehydrogenated acetylene molecules.Comment: 8 pages, 3 figures (submitted to J. Chem. Phy

Dilation-induced phases of gases absorbed within a bundle of carbon nanotubes

A study is presented of the effects of gas (especially H2) absorption within the interstitial channels of a bundle of carbon nanotubes. The ground state of the system is determined by minimizing the total energy, which includes the molecules' interaction with the tubes, the inter-tube interaction, and the molecules' mutual interaction (which is screened by the tubes). The consequences of swelling include a significant increase in the gas uptake and a 3 per cent increase in the tubes' breathing mode frecuency.Comment: 4 page