FERMI-LIQUID BEHAVIOR OF THE VISCOSITY OF HE-3-HE-4 MIXTURES

We have investigated He-3-He-4 mixtures at He-3-concentrations 0.98% l ess-than-or-equal-to x less-than-or-equal-to 9.5% by the vibrating wir e technique in the temperature range 1 mK less-than-or-equal-to T less -than-or-equal-to 100 mK and at pressures 0 bar less-than-or-equal-to p less-than-or-equal-to 20 bar. In the degenerate regime of the mixtur es the Landau theory of Fermi liquids predicts a temperature dependenc e of the viscosity eta proportional T-2. We report on the first observ ation of this behaviour at 3 mK less-than-or-equal-to T less-than-or-e qual-to 10 mK for all investigated concentrations and pressures. At te mperatures below about 20 mK slip corrections had to be taken into acc ount due to the increase of the quasiparticle mean free path at very l ow temperatures. The low-temperature cut-off in eta T2 = constant indi cates the transition into the ballistic regime of the mixtures, where the mean free path of the quasiparticles exceeds the radius of the vib rating wire. Our results for the pressure dependence of the viscosity as well as for its magnitude show substantial differences from predict ions based on pseudopotential theory. However, a calculation of eta wi th the quasiparticle interaction potential of recent solubility measur ements in mixtures agrees well with our experimental data, in particul ar the pressure independence of eta.