Liquid-solid transition and phase diagram of ⁴He confined in nanoporous glass

We have studied the liquid-solid (L-S) phase transition of ⁴He confined in nanoporous Gelsil glass that has interconnected nanopores of 2.5-nm diameter. The L-S boundary is determined by a series of measurements of pressure versus temperature along isochores and the rate of temperature change during slow cooling and warming in a semi-adiabatic condition. Below 1K, the freezing pressure is elevated by 1.2 MPa from the bulk freezing pressure, and appears to be independent of temperature. The T-independent freezing pressure strongly suggests the existence of the localized Bose-Einstein condensation state.