Supercooling of the A phase of $^3$He

TL;DR

This paper investigates the supercooling behavior of the superfluid $^3$He A phase, revealing how cooling paths influence metastability and supercooling extent, which is crucial for modeling early universe phase transitions.

Contribution

It provides detailed experimental insights into the path dependence of supercooling in $^3$He A phase, highlighting the role of B phase seed elimination and stability regions.

Findings

Supercooling extent varies with cooling trajectory.

Supercooling can significantly exceed the thermodynamic transition temperature.

Path dependence affects metastability and phase seed formation.

Abstract

Because of the extreme purity, lack of disorder, and complex order parameter, the first-order superfluid $^3$He A-B transition is the leading model system for first order transitions in the early universe. Here we report on the path dependence of the supercooling of the A phase over a wide range of pressures below 29.3 bar at nearly zero magnetic field. The A phase can be cooled significantly below the thermodynamic A-B transition temperature. While the extent of supercooling is highly reproducible, it depends strongly upon the cooling trajectory: The metastability of the A phase is enhanced by transiting through regions where the A phase is more stable. We provide evidence that some of the additional supercooling is due to the elimination of B phase seeds formed upon passage through the superfluid transition. A greater understanding of the physics is essential before the $^3$He can be exploited to model transitions in the early universe.