BOUNDARY EFFECTS AND SPIN-WAVES IN SPIN-POLARIZED QUANTUM GASES

Boundary conditions are derived for spin dynamics of spin-polarized qu antum gases near nonmagnetic walls. We are interested mostly in bounda ry-induced line shifts and attenuation of spin waves, and in the possi bility of having a macroscopic boundary condition for systems close to a Knudsen ballistic regime. We consider the effects caused by roughne ss of the wall and by surface adsorption. By a proper coordinate trans formation, we reduce the problem of particle collisions with an inhomo geneous nonmagnetic wall to an equivalent problem with a specular homo geneous wall but with stochastic bulk imperfections. As a result, the boundary effects are described by some additional bulklike transverse spin-diffusion coefficient inversely proportional to the angular harmo nics of the correlation function of surface inhomogeneities. This lead s to an effective macroscopiclike boundary condition For transverse sp in dynamics responsible for the boundary effects in spin-wave resonanc es. The situation changes drastically at low temperatures because of a n appearance of an adsorbed boundary layer which renormalizes the mole cular field near the wall, and leads to additional effective spin-exch ange processes. The experimental implications for helium and hydrogen systems are discussed.