Linear spin waves in a trapped Bose gas

TL;DR

This paper develops a theoretical model for spin-wave collective modes in a trapped ultra-cold Bose gas, deriving analytic expressions for frequencies and damping rates, and comparing them with numerical simulations.

Contribution

It introduces a moment method-based theory for spin-wave modes in trapped Bose gases, providing analytic formulas and insights into mode behavior across different regimes.

Findings

Frequency depends on temperature-independent function of peak density

Mode frequencies decrease as 1/n with increasing density

Radial and axial excitations are approximately decoupled

Abstract

An ultra-cold Bose gas of two-level atoms can be thought of as a spin-1/2 Bose gas. It supports spin-wave collective modes due to the exchange mean field. Such collective spin oscillations have been observed in recent experiments at JILA with ${}^{87}$Rb atoms confined in a harmonic trap. We present a theory of the spin-wave collective modes based on the moment method for trapped gases. In the collisionless and hydrodynamic limits, we derive analytic expressions for the frequencies and damping rates of modes with dipole and quadrupole symmetry. We find that the frequency for a given mode is given by a temperature independent function of the peak density $n$, and falls off as $1/n$. We also find that, to a very good approximation, excitations in the radial and axial directions are decoupled. We compare our model to the numerical integration of a one dimensional version of the kinetic equation and find very good qualitative agreement. The damping rates, however, show the largest deviation for intermediate densities, where one expects Landau damping -- which is unaccounted for in our moment approach -- to play a significant role.