arXiv subjects

T. Nikuni

Publications and source records attributed to T. Nikuni.

25 records · Page 2Linked to original sources

arXiv1999

Two-fluid hydrodynamics of a Bose gas including damping from normal fluid transport coefficients

We extend our recent work on the two-fluid hydrodynamics of the condensate and non-condensate in a trapped Bose gas by including the dissipation associated with viscosity and thermal conduction. For purposes of illustration, we consider the hydrodynamic modes in the case of a uniform Bose gas. A finite thermal conductivity and shear viscosity give rise to a damping of the first and second sound modes in addition to that found previously due to the lack of diffusive equilibrium between the condensate and non-condensate. The relaxational mode associated with this equilibration process is strongly coupled to thermal fluctuations and reduces to the usual thermal diffusion mode above the Bose-Einstein transition. In contrast to the standard Landau two-fluid hydrodynamics, we predict a damped mode centered at zero frequency, in addition to the usual second sound doublet.

cond-mat.stat-mech↗

arXiv1999

Realization of Bose-Einstein condensation of dilute magnons in TlCuCl$_3$

The recent observation [Oosawa et al. J. Phys. : Condens. Matter {\bf 11}, 265 (1999)] of the field-induced Néel ordering in a spin-gap magnetic compound TlCuCl$_3$ is interpreted as a Bose-Einstein Condensation of magnons. A mean-field calculation based on this picture is shown to describe well the temperature dependence of the magnetization. The present system opens a new area for studying Bose-Einstein condensation of thermodynamically large number of particles in a grand-canonical ensemble.

cond-mat.stat-mech↗

arXiv1999

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 includes collisions between the condensate and non-condensate atoms ($C_{12}$), in addition to collisions between the excited atoms as described by the Uehling-Uhlenbeck ($C_{22}$) collision integral. Assuming that the $C_{22}$ collision rate is sufficiently rapid to produce a local equilibrium Bose distribution, the kinetic equation can be used to derive hydrodynamic equations for the non-condensate. These equations include a description of the equilibration of the local chemical potentials of the condensate and non-condensate components which gives rise to a new relaxational mode associated with the exchange of atoms between the two components. We show how the Landau two-fluid equations emerge in the frequency domain $ωτ_μ\ll 1$, where $τ_μ$ is a characteristic relaxation time of the equilibration process. This process provides an additional source of damping of the collective modes (first and second sound in the case of a uniform system). Our equations are consistent with the generalized Kohn theorem. Finally, a variational solution of the equations is developed which is used to determine some of the monopole, dipole and quadrupole normal modes of a trapped Bose gas in an isotropic trap.

cond-mat.stat-mech↗

arXiv1999

Two-fluid dynamics for a Bose-Einstein condensate out of local equilibrium with the non-condensate

We extend our recent work on the two-fluid hydrodynamics of a Bose-condensed gas by including collisions involving both condensate and non-condensate atoms. These collisions are essential for establishing a state of local thermodynamic equilibrium between the condensate and non-condensate. Our theory is more general than the usual Landau two-fluid theory, to which it reduces in the appropriate limit, in that it allows one to describe situations in which a state of complete local equilibrium between the two components has not been reached. The exchange of atoms between the condensate and non-condensate is associated with a new relaxational mode of the gas.

cond-mat↗

arXiv1998

Hydrodynamic modes and pulse propagation in a cylindrical Bose gas above the Bose-Einstein transition

We study hydrodynamic oscillations of a cylindrical Bose gas above the Bose-Einstein transition temperature using the hydrodynamic equations derived by Griffin, Wu and Stringari. This extends recent studies of a cylindrical Bose-condensed gas at T=0. Explicit normal mode solutions are obtained for non-propagating solutions. In the classical limit, the sound velocity is shown to be the same as a uniform classical gas. We use a variational formulation of the hydrodynamic equations to discuss the propagating modes in the degenerate Bose-gas limit and show there is little difference from the classical results.

cond-mat↗

arXiv1998

Hydrodynamic damping in trapped Bose gases

Griffin, Wu and Stringari have derived the hydrodynamic equations of a trapped dilute Bose gas above the Bose-Einstein transition temperature. We give the extension which includes hydrodynamic damping, following the classic work of Uehling and Uhlenbeck based on the Chapman-Enskog procedure. Our final result is a closed equation for the velocity fluctuations $δv$ which includes the hydrodynamic damping due to the shear viscosity $η$ and the thermal conductivity $κ$. Following Kavoulakis, Pethick and Smith, we introduce a spatial cutoff in our linearized equations when the density is so low that the hydrodynamic description breaks down. Explicit expressions are given for $η$ and $κ$, which are position-dependent through dependence on the local fugacity when one includes the effect of quantum degeneracy of the trapped gas. We also discuss a trapped Bose-condensed gas, generalizing the work of Zaremba, Griffin and Nikuni to include hydrodynamic damping due to the (non-condensate) normal fluid.

cond-mat↗

arXiv1997

Two-Fluid Hydrodynamics for a Trapped Weakly-Interacting Bose Gas

We derive the coupled equations of motion for the condensate (superfluid) and non-condensate (normal fluid) degrees of freedom in a trapped Bose gas at finite temperatures. Our results are based on the Hartree-Fock-Popov approximation for the time-dependent condensate wavefunction and an assumption of local equilibrium for the non-condensate atoms. In the case of a uniform weakly-interacting gas, our formalism gives a microscopic derivation of the well-known two-fluid equations of Landau. The collective modes in a parabolically trapped Bose gas include the analogue of the out-of-phase second sound mode in uniform systems.

cond-mat↗