BOUNDARY EFFECTS IN TRANSVERSE SPIN DYNAMICS OF SPIN-POLARIZED QUANTUM GASES

Spin dynamics of spin-polarized quantum gases near non-magnetic walls is discussed. We consider boundary-induced line shifts and attenuation of spin-waves, and a possible "macroscopic" boundary condition for systems close to a Knudsen ballistic regime. By a coordinate transformation, we reduce the problem of collisions with a rough wall to an equivalent problem with a specular wall but with stochastic bulk perturbations. The boundary effects are described by a bulk-like transverse spin-diffusion coefficient inversely proportional to the correlation function of surface inhomogeneities. This leads to an effective macroscopic boundary condition applied for the spin-wave resonances. The situation is changed at low temperatures by an appearance of absorbed boundary layers which lead to additional exchange processes and renormalize the molecular field near the walls. The experimental implications for helium and hydrogen systems are discussed, including the signs of surface spin modes.