165 research outputs found
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 CH 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 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
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