Critical Behavior of the Liquid Gas Transition of 4He Confined in a Silica Aerogel

We have studied 4He 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 Tch between 25 and 30 mK below the critical point. Slow relaxations are observed for temperatures slightly below Tch, indicating that some energy barriers, but not all, can be overcome. Above Tch, 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 Tch, 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 4He close to the silica strands.