E. Zaremba

Dynamics of Trapped Bose Gases at Finite Temperatures

Starting from an approximate microscopic model of a trapped Bose-condensed gas at finite temperatures, we derive an equation of motion for the condensate wavefunction and a quantum kinetic equation for the distribution function for the excited atoms. The kinetic equation is a generalization of our earlier work in that collisions between the condensate and non-condensate (C12) are now included, in addition to collisions between the excited atoms as described by the Uehling–Uhlenbeck (C22) collision integral. The continuity equation for the local condensate density contains a source term Γ12which is related to the C12collision term. If we assume that the C22collision rate is sufficiently rapid to ensure that the non-condensate distribution function can be approximated by a local equilibrium Bose distribution, the kinetic equation can be used to derive hydrodynamic equations for the non-condensate. The Γ12source terms appearing in these equations play a key role in describing the equilibration of the local chemical potentials associated with the condensate and non-condensate components. We give a detailed study of these hydrodynamic equations and show how the Landau two-fluid equations emerge in the frequency domain ωτμ ≪ τμis a characteristic relaxation time associated with C12collisions. More generally, the lack of complete local equilibrium between the condensate and non-condensate is shown to give rise to a new relaxational mode which is associated with the exchange of atoms between the two components. This new mode provides an additional source of damping in the hydrodynamic regime. Our equations are consistent with the generalized Kohn theorem for the center of mass motion of the trapped gas even in the presence of collisions. Finally, we formulate a variational solution of the equations which provides a very convenient and physical way of estimating normal mode frequencies. In particular, we use relatively simple trial functions within this approach to work out some of the monopole, dipole and quadrupole oscillations for an isotropic trap.

Finite Temperature Excitations of a Trapped Bose Gas

We present a detailed study of the temperature dependence of the condensate and noncondensate density profiles of a Bose-condensed gas in a parabolic trap. These quantities are calculated self-consistently using the Hartree-Fock-Bogoliubov equations within the Popov approximation. Below the Bose-Einstein transition the excitation frequencies have a relatively weak temperature dependence even though the condensate is strongly depleted. As the condensate density goes to zero through the transition, the excitation frequencies are strongly affected and approach the frequencies of a noninteracting trapped gas in the high temperature limit. {copyright} {ital 1997} {ital The American Physical Society}

Weak Binding Potentials and Wetting Transitions

We present ab initio calculations of the adsorption potentials V(Z) of inert gases and hydrogen on the surfaces of various metals. The ratio of the adsorption well depth to that of the adsorbate pair potential is ∼ 3.5, 2, 1.5, 1, 0.9 and 0.9 for adsorption on Mg, Li, Na, K, Rb, and Cs, respectively, with some variation between gases (always smallest for Ne). When this ratio is small, a wetting transition occurs; we predict the wetting temperature Twusing a model of Cheng et al. Comparison is made with other calculations and with experiments.

Novel low-energy collective excitation at metal surfaces

A novel collective excitation is predicted to exist at metal surfaces where a two-dimensional surface state band coexists with the underlying three-dimensional continuum. This is a low-energy acoustic plasmon with linear dispersion at small wave vectors. Since new modern spectroscopies are especially sensitive to surface dynamics near the Fermi level, the existence of surface-state–induced acoustic plasmons is expected to play a key role in a large variety of new phenomena and to create situations with potentially new physics.

SOUND PROPAGATION IN A CYLINDRICAL BOSE-CONDENSED GAS

We study the normal modes of a cylindrical Bose condensate at

Interaction of rare gas atoms with metal surfaces: A pseudopotential approach

T = 0

The interaction of rare gas atoms with metal surfaces : a scattering theory approach

using the linearized time-dependent Gross-Pitaevskii equation in the Thomas-Fermi limit. These modes are relevant to the recent observation of pulse propagation in long, cigar-shaped traps. We find that pulses generated in a cylindrical condensate propagate with little spread at a speed

The Z13 Correction to the Bethe-Bloch Energy Loss Formula

c = \sqrt{g\bar n /m}

Dynamical Instability of a Condensate Induced by a Rotating Thermal Gas

, where

Theory of acoustic surface plasmons

\bar n