arXiv subjects

H. Smith

Publications and source records attributed to H. Smith.

42 records · Page 3Linked to original sources

arXiv1999

Kinetic Theory of Collective Modes in Atomic Clouds above the Bose-Einstein Transition Temperature

We calculate frequencies and damping rates of the lowest collective modes of a dilute Bose gas confined in an anisotropic trapping potential above the Bose-Einstein transition temperature. From the Boltzmann equation with a simplified collision integral we derive a general dispersion relation that interpolates between the collisionless and hydrodynamic regimes. In the case of axially symmetric traps we obtain explicit expressions for the frequencies and damping rates of the lowest modes in terms of a phenomenological collision time. Our results are compared with microscopic calculations and experiments.

cond-mat↗

arXiv1999

The Surface of a Bose-Einstein Condensed Atomic Cloud

We investigate the structure and collective modes of a planar surface of a trapped Bose-Einstein condensed gas at zero temperature. In the long-wavelength limit we find a mode similar to the gravity wave on the surface of a fluid with the frequency $ω$ and the wavenumber $q$ related by $ω^2=Fq/m$. Here $F$ is the force due to the confining potential at the surface and $m$ is the particle mass. At shorter wavelengths we use a variational approach and find corrections to $ω^2$ of order $q^4 \ln{q}$. We demonstrate the usefulness of the concept of an effective surface tension for describing both static and dynamic properties of condensed atomic clouds.

cond-mat↗

arXiv1998

Vortices in Bose-Einstein-Condensed Atomic Clouds

The properties of vortex states in a Bose-Einstein condensed cloud of atoms are considered at zero temperature. Using both analytical and numerical methods we solve the time-dependent Gross-Pitaevskii equation for the case when a cloud of atoms containing a vortex is released from a trap. In two dimensions we find the simple result that the time dependence of the cloud radius is given by $(1+ω^2t^2)^{1/2}$, where $ω$ is the trap frequency. We calculate and compare the expansion of the vortex core and the cloud radius for different numbers of particles and interaction strengths, in both two and three dimensions, and discuss the circumstances under which vortex states may be observed experimentally.

cond-mat↗

arXiv1998

Relaxation Processes in Clouds of Trapped Bosons above the Bose-Einstein Condensation Temperature

We present a unified account of damping of low-lying collective modes and of relaxation of temperature anisotropies in a trapped Bose gas in the collisionless regime. By means of variational techniques, we show that the relaxation times for the two situations are closely related to the simplest variational estimate of the viscous relaxation time. We derive rather precise theoretical expressions for the characteristic relaxation times, and compare our results with experiment.

cond-mat↗

arXiv1997

Damping of Hydrodynamic Modes in a Trapped Bose Gas above the Bose-Einstein Transition Temperature

We calculate the damping of low-lying collective modes of a trapped Bose gas in the hydrodynamic regime, and show that this comes solely from the shear viscosity, since the contributions from bulk viscosity and thermal conduction vanish. The hydrodynamic expression for the damping diverges due to the failure of hydrodynamics in the outer parts of the cloud, and we take this into account by a physically motivated cutoff procedure. Our analysis of available experimental data indicates that higher densities than have yet been achieved are necessary for investigating hydrodynamic modes above the Bose-Einstein transition temperature.

cond-mat↗

arXiv1994

Bloch oscillations, Zener tunneling and Wannier-Stark ladders in the time-domain

We present a time-domain analysis of carrier dynamics in a semiconductor superlattice with two minibands. Integration of the density-matrix equations of motion reveals a number of new features: (i) for certain values of the applied static electric field strong interband transitions occur; (ii) in static fields the complex time-dependence of the density-matrix displays a sequence of stable plateaus in the low field regime, and (iii) for applied fields with a periodic time-dependence the dynamic response can be understood in terms of the quasienergy spectra.

cond-mat↗