Path-Dependent Supercooling of the $^3$He Superfluid A-B transition

TL;DR

This study investigates the path-dependent nature of the superfluid $^3$He A-B transition near the polycritical point, revealing non-standard nucleation behavior and implications for simulating early universe phase transitions.

Contribution

It demonstrates that the superfluid $^3$He A-B transition exhibits path-dependent behavior not explained by existing nucleation theories, providing new insights into first-order phase transitions.

Findings

Transition line depends on the path taken in pressure-temperature space.

Depressurizing at constant temperature can cross the transition line after $T_c$.

Standard nucleation mechanisms do not account for observed path dependence.

Abstract

We examine the discontinuous first-order superfluid $^3$He A to B transition in the vicinity of the polycritical point (2.232 mK and 21.22 bar). We find path-dependent transitions: cooling at fixed pressure yields a well defined transition line in the temperature-pressure plane, but this line can be reliably crossed by depressurizing at nearly constant temperature after transiting $T_{\rm c}$ at a higher pressure. This path dependence is not consistent with any of the standard B-phase nucleation mechanisms in the literature. This symmetry breaking transition is a potential simulator for first order transitions in the early universe.