We have studied NMR spin echoes in 3 He liquid in a magnetic field #o = 8 T for temperatures T>4.5 mK. The data are analyzed in terms of the Leggett-Rice effect to obtain the transverse spindiffusion coefficient D± (T). For T<20 mK we find that D± is less than measured in earlier experiments at lower Bo. This phenomenon has been predicted for degenerate Fermi systems, due to the phase space created for quasiparticle scattering by spin polarization. The effect we have measured is more pronounced than suggested by scaling a dilute-gas theory result by a Fermi-liquid factor.PACS numbers: 67.65. +Z, 67.55.-s In degenerate Fermi liquids in which interparticle collisions dominate scattering ( 3 He and certain metallic systems), the transport coefficients diverge as the temperature T-* 0. This fundamental property is due to the restriction of scattering phase space to a shell of width kg about the Fermi surface. Recently, it was independently predicted by Meyerovich [1] and by Jeon and Mullin [2] that the divergence in the transverse spin-diffusion coefficient D± oc T~2 is removed by arbitrarily small spin polarization P of the system. Below a temperature T a which depends upon P, spin diffusion becomes anisotropic, with D±6 mK found no significant deviation from either the expected low-field behavior or the measured behavior [5] of D\\ at BQ=9.2 T. These earlier experiments used very dilute 3 He-4 He mixtures in which the 3 He quasiparticles form a weakly interacting gas. Thus, if polarization does cause D± < D\u this must occur at lower temperatures or higher fields.Reason to expect a lower value for T a has come from the kinetic-equation calculations of Jeon and Mullin [2]. For a dilute, weakly polarized Fermi gas with s-wave interactions, they obtained Ta^hyBjlTtkB.Unfortunately this suggests spin-diffusion anisotropy should be difficult to observe in available laboratory fields unless nuclear demagnetization were used to achieve much lower temperatures.In an attempt to observe the effect using conventional dilution refrigeration, we have studied spin diffusion in pure 3 He liquid at BQ^S T. Pure 3 He is a strongly in-teracting system, but for T<£T F it is successfully described by Fermi-liquid theory as a weakly interacting quasiparticle gas with strong mean-field effects. At zero pressure 7> = 1.8 K and mean-field effects enhance the effective magnetic field by a factor of (1 -FFg) _1 = 3.3, wh...

We report measurements of the transverse-spin-diffusion coefficient D& and the spin-rotation parameter Q~~for two very dilute He-He mixtures (x3=1.82X10 and 6.26X10 ) spin polarized by an 8-T magnetic field. Brute-force spin polarization up to 40% was achieved at the lowest temperature, 6 mK.We find that Q~~increases monotonically as the temperature is reduced through the Fermi temperature TF, in disagreement with the only previous experiment but in good agreement with recent theory. Unlike the earlier experiment, which measured spin echoes, the present experiments employed a spin-wave technique that avoids nonlinear excitation of the spin field. We compare our results with the recent calculations of Jeon and Mullin for spin transport in dilute gases with arbitrary polarization and degeneracy. The best fit to the data is obtained by scaling the quasiparticle interaction V(q) proposed by Ebner 0 by a modest factor, 1.07. The corresponding s-wave scattering length is a = -1.21 A. Cxood agreement is found for Q~~(T) at both concentrations and all temperatures, and for D&/Q~~(T) apart from the lower concentration at T & 20 mK. The discrepancy in Dj /Q~j at the lowest temperatures and x3 could be explained by an unanticipated polarization dependence or by modification of the spin-wave boundary condition by processes occurring at the interface between the mixture and the silica cavity wall.

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