Critical behavior of the liquid gas transition of 4 He confined in a silica aerogel

TL;DR

This study investigates how quenched disorder in silica aerogel affects the critical behavior of liquid-gas transition in 4He, revealing a shifted and altered phase transition near the critical point.

Contribution

It provides experimental evidence on the impact of quenched disorder on the liquid-gas critical transition, relating findings to the RFIM framework.

Findings

Hysteresis disappears below a certain temperature T ch.

Slow relaxations indicate energy barriers near T ch.

Critical behavior appears shifted and not directly observable due to disorder.

Abstract

We have studied 4 He confined in a 95% porosity silica aerogel in the vicinity of the bulk liquid gas critical point. Both thermodynamic measurements and light scattering experiments were performed to probe the effect of a quenched disorder on the liquid gas transition, in relation with the Random Field Ising Model (RFIM). We find that the hysteresis between condensation and evaporation present at lower temperatures disappears at a temperature T ch between 25 and 30 mK below the critical point. Slow relaxations are observed for temperatures slightly below T ch , indicating that some energy barriers, but not all, can be overcome. Above T ch , no density step is observed along the (reversible) isotherms, showing that the critical behavior of the equilibrium phase transition in presence of disorder, if it exists, is shifted to smaller temperatures, where it cannot be observed due to the impossibility to reach equilibrium. Above T ch , light scattering exhibits a weak maximum close to the pressure where the isotherm slope is maximal. This behavior can be accounted for by a simple model incorporating the compression of 4 He close to the silica strands.