A−Btransition of superfluidHe3in aerogel and the effect of anisotropic scattering

Abstract

We report the results of high-frequency acoustic shear impedance measurements on superfluid 3 He confined in 98% porosity silica aerogel. Using 8.69 MHz continuous wave excitation, we measured the acoustic shear impedance as a function of temperature for the sample pressures of 28.4 and 33.5 bar. We observed the A-B transition on warming in zero magnetic field. Our observations show that the A and B phases in aerogel coexist in a temperature range of about 100 K in width. We propose that differences in the relative stability of the A and B phases arising from anisotropic scattering can account for our observations.

Impurity Effects of Aerogel in Superfluid3He

“…Additional work at various pressures in zero applied field lead to the same conclusions. 21,68) Based on these experimental results and from the theoretical suggestions of Thuneberg et al, 34) Vicente et al 10) proposed that anisotropic scattering might be the origin for stability of the A-like phase. However, within the context of the free energy expansion, eq.…”

“…The relative stability of axial and isotropic phases can be determined for IISM from theparameter combinations taken from Table I that give ÁC Aa and ÁC Ba , eqs. (9) and (10). The two phases are thermodynamically so close that they will be difficult to distinguish directly (5 {10%) in specific heat measurements.…”

“…Concerning the last question, there is currently substantial interest in a metastable equal-spin-pairing state, an A-like phase, that appears on cooling through the normal-tosuperfluid transition. It was suggested by Vicente et al 10) that the origin of this phase might be associated with anisotropic scattering. Subsequently, some theoretical work 11) and a number of experiments [12][13][14][15][16][17] have investigated 3 He in aerogels having global anisotropy, suggesting new order parameter structures and perhaps new kinds of superfluid phases.…”

Recent Progress and New Challenges in Quantum Fluids and Solids

“…However, the experimental and theoretical studies in the past 10 years revealed that anisotropic disorder has significant influence on the structure of the superfluid phase because of the anisotropic nature of the superfluid itself. Silica aerogels that are homogeneous and isotropic on the scale of the quasiparticle mean-free-path, for example, 150 nm for a 98% porosity aerogel have a dramatic effect destabilizing the A-phase in favor of the B-phase which then fills the entire phase diagram at zero magnetic field [41]. On the other hand anisotropic aerogel samples, stretched during growth by promoting radial shrinkage, were found to stabilize the chiral A-phase at all pressures.…”

Acoustic Spectroscopy of Superfluid 3He in Aerogel

“…More recent acoustic tracking experiments by Vicente et al [4] and NMR by Osheroff et al [5] (99.3% aerogel) reveal that the A-like phase is in fact stable in a small temperature window near T c at high pressure. Vicente et al suggest that this stabilization is due to the local anisotropic scattering from the aerogel strands.…”