Condensation Of Helium In Aerogel And Athermal Dynamics Of The Random-Field Ising Model

High resolution measurements reveal that condensation isotherms of He-4 in high porosity silica aerogel become discontinuous below a critical temperature. We show that this behavior does not correspond to an equilibrium phase transition modified by the disorder induced by the aerogel structure, but to the disorder-driven critical point predicted for the athermal out-of-equilibrium dynamics of the random-field Ising model. Our results evidence the key role of nonequilibrium effects in the phase transitions of disordered systems.