Anisotropic Strong Coupling Effects on Superfluid 3He in Aerogels - Conventional Spin-Fluctuation Approach -

TL;DR

This paper extends the spin-fluctuation theory to analyze how anisotropic scattering in aerogels influences the strong coupling effects on superfluid helium-3, explaining experimental phase behaviors in anisotropic media.

Contribution

It introduces a modified Ginzburg-Landau free energy framework incorporating anisotropic scattering effects on strong coupling contributions for superfluid helium-3 in aerogels.

Findings

Anisotropic scattering broadens the stability of the A-phase.

Strong coupling corrections to quadratic terms can change sign depending on cutoff.

The theory explains disappearance of polar phase and absence of certain phases in experiments.

Abstract

Motivated by recent experiments on liquid $^3$He reporting emergence of novel superfluid phases in globally anisotropic aerogels, our previous theory on superfluid $^3$He in globally anisotropic aerogels is extended to incorporate effects of an anisotropy of the quasiparticle scattering cross section on the strong coupling (SC) contributions to the Ginzburg-Landau (GL) free energy on the basis of the spin-fluctuation (paramagnon) approach to the SC contributions developed by Brinkman et al. [Phys. Rev. A {\bf 10}, 2386 (1974)]. In the globally isotropic case, impurity effects on the SC correction destabilize the A-phase even at higher pressures of about 30 (bar) and make the B-phase the only state in equilibrium, while SC contributions accompanied by a global stretched anisotropy to the GL quartic terms generally tend to broaden the stability region of the A-phase compared with that of the B-phase. In particular, in contrast to the cases in bulk and in the isotropic aerogel, the SC corrections to the GL {\it quadratic} terms are not negligible in the globally anisotropic case but may change the sign of the apparent anisotropy depending on the magnitude of the frequency cutoff of the normal paramagnon propagator. Based on this sign change of the apparent anisotropy, we discuss different strange observations on superfluid $^3$He in porous media such as disappearance of the polar superfluid phase at higher pressures seen in nematically-ordered aerogels and absence of the B-phase and of the A-phase with planar ${\hat {\bf l}}$-vector in a stretched aerogel.