Superfluid Phase Stability of $^3$He in Axially Anisotropic Aerogel

TL;DR

This study investigates how axial anisotropy in compressed aerogel affects the stability of superfluid phases of helium-3, finding that uniform anisotropy does not stabilize the A-like phase but influences phase nucleation.

Contribution

The paper experimentally examines the effect of axial anisotropy on superfluid helium-3 phases in aerogel, revealing that uniform anisotropy does not stabilize the A-like phase as predicted.

Findings

Uniform axial anisotropy does not stabilize the A-like phase.

An increase in supercooling of the A-like phase at lower pressure.

Modification to B-like phase nucleation in anisotropic aerogels.

Abstract

Measurements of superfluid $^3$He in 98% aerogel demonstrate the existence of a metastable \emph{A}-like phase and a stable \emph{B}-like phase. It has been suggested that the relative stability of these two phases is controlled by anisotropic quasiparticle scattering in the aerogel. Anisotropic scattering produced by axial compression of the aerogel has been predicted to stabilize the axial state of superfluid $^3$He. To explore this possiblity, we used transverse acoustic impedance to map out the phase diagram of superfluid $^3$He in a $\sim 98$% porous silica aerogel subjected to 17% axial compression. We have previously shown that axial anisotropy in aerogel leads to optical birefringence and that optical cross-polarization studies can be used to characterize such anisotropy. Consequently, we have performed optical cross-polarization experiments to verify the presence and uniformity of the axial anisotropy in our aerogel sample. We find that uniform axial anisotropy introduced by 17% compression does not stabilize the \emph{A}-like phase. We also find an increase in the supercooling of the \emph{A}-like phase at lower pressure, indicating a modification to \emph{B}-like phase nucleation in \emph{globally} anisotropic aerogels.