Localized Bose–Einstein Condensation of ⁴He Confined in Nanoporous Media

Abstract

⁴He confined in nanoporous media is an excellent model boson system in a confinement potential. We have studied the superfluid and thermodynamic properties of ⁴He confined in a porous glass that has nanopores of 2.5 nm diameter. It is shown that the confinement of ⁴He in nanopores produces a new quantum state, the localized Bose–Einstein condensation (LBEC) state, and markedly alters the pressure–temperature (P–T) phase diagram. In the LBEC state the global phase coherence is destroyed by strong atomic correlation or by the random potential of a substrate. The formation of LBEC leads to an anomalous reduction of the superfluid transition temperature T_c, determined by the torsional oscillator technique, and the behavior of a quantum phase transition near 0 K. The contributions of roton and phonon excitations to the heat capacity provide definitive evidence of the LBEC state.