Phase diagram of the superfluid phases of3Hein 98% aerogel

Abstract

The phase diagram of the superfluid phases of 3 He 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 Tc at a pressure of 33.4 bars. This is in contrast to pure 3 He which has a large stable region of A-phase and a polycritical point. The quadratic coefficient for magnetic field suppression of the AB-transition, ga(β), was obtained. The pressure dependence of ga(β) is markedly different from that for 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 ga(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 3 He, 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.

Specific Heat of Disordered SuperfluidHe3

“…The simplest theoretical description [10,11] considers isotropic quasiparticle scattering spread homogeneously throughout the aerogel volume, characterized by a single parameter, the transport-mean-free path, λ T . This model (HISM) provides a qualitatively consistent account of the superfluid transition temperature suppression [10], the suppression of the A-phase and the polycritical point [19,20], measurements of spin diffusion [21] and the thermal conductivity [11,14]. Here we see that the model gives a quantitatively consistent picture for the heat capacity jump over a wide range of pressure.…”

“…The sample was first cooled by nuclear demagnetization. By opening a cadmium superconducting heat switch we could isolate the sample cell from the nuclear stage, allowing the 3 He to warm up slowly owing to the ambient heat leak, typicallyQ ∼ 0.1 nW, starting in the B-phase [19]. Heat pulses were applied with ∆Q in the range 0.1 ∼ 0.5 µJ for 30 ∼ 60 s, and the temperature increments ∆T were measured to obtain the heat capacity C tot = ∆Q/∆T shown in Fig.…”

“…A calculation [10] with the HISM gives a best fit value of λ T = 180 nm. In this fit we rescale strong coupling contributions [19] to the free energy by the ratio T c,a /T c where T c,a is calculated self-consistently within the HISM. We also find reasonable agreement with the prediction from the HISM for the density of states of gapless excitations.…”

Specific Heat of Disordered 3He

“…The mean free path was estimated to be 150 nm from previous measurements of the phase diagram 1 , consistent with transport measurements of spin diffusion 8 and thermal conductivity 9,10 of liquid 3 He in aerogel. Using this mean free path at zero magnetic field, the B-phase is predicted to be stable 5 and experiments agree 11 . We investigated the predictions of both the HISM and IISM on the specific heat jump for B-phase of superfluid 3 He in aerogel and compared the calculation with our experiment 4 .…”

Acoustic Spectroscopy of Superfluid 3He in Aerogel

“…The acoustic impedance for transverse sound exhibits anomalies at phase transitions that mark the superfluid phase diagram of 3 He in 98% porosity silica aerogel [1]. The scattering of 3 He quasiparticles from the silica aerogel strands suppresses T c and stabilizes the B-phase.…”