Impurity Effects on the A_1-A_2 Splitting of Superfluid 3He in Aerogel

TL;DR

This paper investigates how impurity effects, particularly solid-like 3He layers on aerogel, influence the A_1-A_2 splitting in superfluid 3He, proposing that high magnetic fields could reveal this splitting.

Contribution

It introduces a model considering solid-liquid exchange coupling effects on superfluid 3He in aerogel, predicting conditions under which the A_1-A_2 splitting can be observed.

Findings

No A_1-A_2 splitting observed at B<5 kG

Estimated exchange coupling J ≈ 0.1 mK

Higher fields B>20 kG may reveal the splitting

Abstract

When liquid 3He is impregnated into silica aerogel a solid-like layer of 3He atoms coats the silica structure. The surface 3He is in fast exchange with the liquid on NMR timescales. The exchange coupling of liquid 3He quasiparticles with the localized 3He spins modifies the scattering of 3He quasiparticles by the aerogel structure. In a magnetic field the polarization of the solid spins gives rise to a splitting of the scattering cross-section of for `up' vs. `down' spin quasiparticles, relative to the polarization of the solid 3He. We discuss this effect, as well as the effects of non-magnetic scattering, in the context of a possible splitting of the superfluid transition for $\uparrow\uparrow$ vs. $\downarrow\downarrow$ Cooper pairs for superfluid 3He in aerogel, analogous to the A_1-A_2 splitting in bulk 3He. Comparison with the existing measurements of T_c for B< 5 kG, which show no evidence of an A_1-A_2 splitting, suggests a liquid-solid exchange coupling of order J = 0.1 mK. Measurements at higher fields, B > 20 kG, should saturate the polarization of the solid 3He and reveal the A_1-A_2 splitting.