Phase diagram of the superfluid phases of ³He in 98% aerogel

The phase diagram of the superfluid phases of ³He in 98% aerogel was determined in the range of pressure from 15 to 33 bars and for fields up to 3 kG using high-frequency sound. The superfluid transition in aerogel at 33.4 bars is field independent from 0 to 5 kG and shows no evidence of an A₁-A₂ splitting. The first-order transition between the A and B phases is suppressed by a magnetic field, and exhibits strong supercooling at high pressures. We show that the equilibrium phase in zero applied field is the B phase with at most a region of A phase ≲20 μK just below T_c at a pressure of 33.4 bars. This is in contrast to pure ³He which has a large stable region of A phase and a polycritical point. The quadratic coefficient for magnetic-field suppression of the AB transition, g_a(β), was obtained. The pressure dependence of g_a(β) is markedly different from that to the pure superfluid, g₀(β), which diverges at a polycritical pressure of 21 bars. We compare our results with calculations from the homogeneous scattering model for g_a(β), defined in a Ginzburg-Landau theory in terms of strong-coupling parameters β. We find qualitatively good agreement with the experiment if the strong-coupling corrections are rescaled from known values of the β’s for pure ³He, reduced by the suppression of the superfluid transition temperature. The calculations indicate that the polycritical pressure in the aerogel system is displaced well above the melting pressure and out of experimental reach. We cannot account for the puzzling supercooling of the aerogel AB transition in zero applied field within the framework of known nucleation scenarios.