Hysteretic solidification of surfaceHe4measured by the modification of the specularity of

Abstract

We have measured the superfluid fraction of 3 He in pores of sintered silver. The superfluid fraction, which is suppressed by confinement, increases with pressure due to the decreasing coherence length. Coating the surfaces with a thin film of 4 He partially restores the suppressed superfluidity, but above 17 bars this effect diminishes as the 4 He film solidifies. This solidification occurs over a range of about 9 bars and is hysteretic. With a slightly thicker 4 He film, solidification occurs at pressures greater than that required for bulk (25 bars).PACS numbers: 67.50.Fi, 67.40.Hf This Letter describes a series of experiments on 4 He and 3 He confined within the pores of sintered silver. The 3 He functions as both a hydrostatic medium to generate pressure as well as a detector of specular scattering of 3 He quasiparticles at the surfaces. The 4 He, which is preferentially plated onto the surfaces, serves as the agent to induce specularity as well as the material whose solidification is studied in this experiment.The breaking of superfluid pairs of 3 He by a diffusely scattering surface produces a reduction in superfluid density and transition temperature which were predicted [1,2] and observed in a number of experiments [3][4][5][6][7][8]. Freeman et ai [7] observed that the transition temperature and superfluid fraction were restored to bulk values by the addition of 4 He, as is expected for a specularly scattering surface. Kim et ai [8] also saw an increase in the superfluid fraction of 3 He in a porous medium formed from //m-sized particles following the addition of 4 He. In the normal fluid, Tholen and Parpia [9] induced slip, which is associated with specular scattering, by the addition of similar coverages of 4 He to the surfaces.One of the outstanding questions is whether the conversion of the 4 He fluid layer to a solid reduces the specularity. We find that solidification does destroy the specularity of the interface, and this destruction of specularity is used in this experiment to indicate the phase of the surface 4 He. The process of solidification occurs over a range of pressures starting below 25 bars, and is strongly dependent on the 4 He film thickness.In this experiment, the 3 He was confined in a region with pore sizes comparable to the coherence length in the superfluid. The expected diffuse scattering of 3 He should break pairs and suppress the total superfluid fraction below the bulk value. Specular scattering induced by a surface layer of 4 He is expected to reduce or eliminate this suppression. By measuring the superfluid fraction following a change in pressure, we can determine the dependence of the specularity on the phase of the 4 He surface layer.The experiments were carried out in a torsion oscillator cell which contained a 0.75-cm-diam 0.75-cm-tall cylindrical plug of silver powder (nominal diameter of 700 A) sintered to 68% of solid density, with a 0.75-mm-diam hole drilled down the length of the sinter to promote thermal equilibrium. The cell was located in close proximity to an ...

Effect of Magnetic Boundary Conditions on SuperfluidHe3in Nematic Aerogel

“…However, experiments with silica aerogels do not show a clear evidence of the magnetic channel. In particular, the observed A-like and B-like phases correspond to A and B phases of bulk 3 He, regardless of presence or absence of 4 He [8][9][10][11][12][13][14]. Moreover, the superfluid transition temperature of 3 He in aerogel (T ca ) is independent on the coverage at high pressures [8,12] but slightly higher in presence of the coverage at lower pressures [15,16] probably due to the change of the scattering specularity.…”

“…We found that ∆T ca in pure He in nafen-243 we are not able to distinguish whether the observed superfluid phase is the A phase or the DA phase because a rather large suppression of Tca does not allow us to use the rescaled bulk value of ΩA in Eq. 2 for determination of K. more strongly depends on nafen porosity than in presence of 4 He. It is especially clear for nafen-910: although its porosity is extremely low, we observed the superfluid transition for 2.5-coverage (Fig.…”

“…At temperatures T ∼ 1 mK, where 3 He is superfluid, the scattering occurs only on aerogel strands and boundary conditions may be varied by a small amount of 4 He which covers the strands by a few atomic layers. In pure 3 He the strands are covered by ∼ 2 atomic layers of paramagnetic solid 3 He [2] and the scattering is diffusive but in presence of more than ≈ 2.5 layers of 4 He it is nearly specular at low pressures and becomes purely diffusive above ≈ 25 bar [3][4][5][6][7]. The 4 He coverage also removes the solid 3 He, and spin is conserved during the scattering.…”

“…The necessary temperatures were obtained by a nuclear demagnetization cryostat and determined by a quartz tuning fork. To obtain the desirable coverage we add a fixed amount of 4 He into the empty chamber at T < 100 mK. We assign 2.5-coverage to the minimal amount of 4 He which at 29.3 bar results in absence of the paramagnetic NMR signal from solid 3 He.…”

“…We note that aerogel strands in Refs. [28][29][30] were covered by ≈ 2.5 atomic layers of 4 He to remove the paramagnetic 3 He. In this work we describe experiments with 3 He in N-aerogels in pure 3 He and with strands covered by 2.2 and 2.5 atomic layers of 4 He (2.2-coverage and 2.5-coverage).…”