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Journal of Experimental and Theoretical Physics

Spin waves in cold gases with nondegenerate levels

T. L. Andreeva, P. L. Rubin

Published Nov 1, 2008• 1 citation
Publisher

Abstract

The existence of spin waves of a new type is predicted for ultracold paramagnetic gases with nondegenerate energy levels. The waves are generated by creating a nonequilibrium population in the absence of external polarization and are damped not only by diffusion, but also by a previously ignored mechanism due to level splitting. Theoretical calculations are in fair agreement with experimental observations of spin waves in magnetically trapped clouds of rubidium atoms.

DOI
10.1134/s1063776108110113

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The existence of spin waves of a new type is predicted for ultracold paramagnetic gases with nondegenerate energy levels. The waves are generated by creating a nonequilibrium population in the absence of external polarization and are damped not only by diffusion, but also by a previously ignored mechanism due to level splitting. Theoretical calculations are in fair agreement with experimental observations of spin waves in magnetically trapped clouds of rubidium atoms.
We derive the linearized spin dynamics for paramagnetic quantum liquids. In these systems, the existence of quantum-mechanical exchange in the presence of (an externally induced) broken symmetry in spin space gives rise to a coherent precession of the spins damped by diffusion. Such collective oscillations occur as a result of the precession of the spin current about a macroscopic local exchange field generated either by an external polarizing magnetic field (as in dilute nondegenerate spin-polarized gases) or by the long-lived nonequilibrium polarization (as in degenerate Fermi liquids). We use a unified approach based on the Kadanoff-Baym formulation of the kinetic equations to describe these phenomena. The connection between strongly and weakly interacting paramagnetic systems is made obvious by considering two special cases: (1) a weakly interacting spin-polarized quantum gas and (2) a parametrization of strongly correlated degenerate Fermi liquids in terms a weakly interacting gas of ``quasiparticles.'' Transport coefficients are calculated with a novel method which may be generalizable to systems with no translational invariance.
A variety of solitary waves, such as solitons, vortex rings, solitonic vortices, and more complex entities, have recently been predicted to exist. They can move in superfluid ultracold gases along elongated traps. The theoretical description of this motion requires knowledge of the inertial soliton mass and the effective number of particles in it as functions of the soliton energy. While these functions can be calculated by a microscopic theory, it is also possible to express them directly in terms of observable quantities, such as the order parameter phase jump and the particle number depletion in the soliton. In this article, the corresponding equations are derived in a simple and physically clear way and applied to the recently predicted `magnetic soliton' in mixtures of Bose gases in various spin states.