Mean Free Path Effects in ³He Quasiparticles: Measurement of the Spin Diffusion Coefficient in the Collisionless Regime by a Pulsed Gradient NMR Technique

Abstract

The low-temperature properties of bulk liquid ³He are broadly consistent with Landau quasiparticle theory although they do not, in a precise sense, verify its correctness. The temperature dependence in the limit T→0 is that predicted for any Fermi system, and the magnitude of the coefficients cannot be calculated explicitly. Moreover, accurate comparison of the temperature variations below 50 m°K, where it is most significant, is made doubtful by uncertainties in the temperature scale. We report here the results of an experiment of a different sort, namely the direct observation of mean free path effects in the quasiparticles. The present measurements are of the spin diffusion coefficient of ³He confined to narrow channels at temperatures low enough for the quasiparticles to be in the collisionless regime. Our results are in agreement with modified Landau theory which does not involve the usual temperature-dependent mean free path.¹ In these experiments the bulk and the mean-free-path-limited diffusion coefficients can be measured virtually coextensively and simultaneously, which greatly facilitates their direct comparison. A similar experiment on heat conduction has already been reported.²

This work was supported by the Science Research Council and the U.S. Office of Aerospace Research (European Office, Grant EOOAR 68-0023).