A vibrating-wire technique has been used to directly measure the viscosity increase in a dilute 3 He-4 He solution caused by brute-force spin polarization up to 40%. The viscosity of a solution with degeneracy temperature T^ -19.5 mK was measured over the temperature range 6.1-100 mK in magnetic fields of 1.00 and 7.96 T. Spin polarization caused a marked decrease in the mechanical Q of the viscometer at low temperatures. The observed viscosity is compared with calculations of the transport coefficients of dilute quantum gases for all temperatures and spin polarizations. PACS numbers: 67.65,+z, 51.20.+d, 67.60.Fp Spin polarization offers a powerful tool for investigating the quantum state of a fluid. Even when singleparticle scattering is spin independent, spin polarization of the fluid affects scattering rates and transport properties because of quantum statistics [1]. Thus, the viscosity of a Fermi gas is predicted to increase upon spin polarization because Fermi statistics permit s-wave scattering only between unlike spin states [2]. We have made the first measurements of this effect that can be compared directly with theory, using the "gas" of 3 He quasiparticles in a very dilute 3 He-4 He solution as our sample.At the low temperatures of our experiment (T<0A K), the 4 He solvent is a superfluid with a negligible density of excitations (phonons and rotons) [3]. It serves simply as a "mechanical vacuum" that renormalizes the mass of the dissolved 3 He atoms to m = 2.255m3 (m^ is the bare atomic mass) [4], and renormalizes the interaction between 3 He atoms. The renormalized interaction is much weaker than the bare interaction, making this the only neutral Fermi gas that may be cooled through its degeneracy temperature 7> = ft 2 (3/r 2 rt3) 1/2 /2&flm without condensing into a liquid (n?> is the 3 He number density) [3]. This is significant because great progress has recently been made in calculating the transport properties of quantum gases over wide ranges of temperature and spin polarization [5][6][7]. Unlike pure liquid 3 He, which also displays interesting polarization dependence in its transport [8], very dilute 3 He-4 He solutions promise a direct, quantitative test of theory.The predicted effects of spin polarization are quite dissimilar for the various transport coefficients. The magnitude of the spin-diffusion coefficient is not greatly changed [5] unless the polarization P-l or T<^T F , but rotational symmetry in spin space is broken, permitting the propagation of spin waves, as has been observed [9]. Conversely, the thermal conductivity K and the viscosity 77 are both predicted to be strongly polarization dependent for any 77 7>, provided the thermal de Broglie wavelength exceeds the range of the interparticle interaction [2]. Thus, Leduc and co-workers [10] have observed a change in the thermal conductivity of 3 He gas upon spin polarization for r»7>.Evidence for the polarization dependence of K and 77 has been seen in the damping of second-sound waves in a dilute 3 He-4 He solution, originally ...
