EXCITATIONS IN CONFINED LIQUID 4He

The spectacular properties of liquid helium at low temperature are generally accepted as the signature of the bosonic nature of this system. Particularly the superfluid phase is identified with a Bose–Einstein condensed fluid. However, the relationship between the superfluidity and the Bose–Einstein condensation is still largely unknown. Studying a perturbed liquid 4 He system would provide information on the relationship between the two phenomena. Liquid 4 He confined in porous media provides an excellent example of a boson system submitted to disorder and finite-size effects. Much care should be paid to the sample preparation, particularly the confining condition should be defined quantitatively. To achieve homogeneous confinement conditions, firstly a suitable porous sample should be selected, the experiments should then be conducted at a lower pressure than the saturated vapor pressure of bulk helium. Several interesting effects have been shown in confined 4 He samples prepared as described above. Particularly we report the observation of the separation of the superfluid-normal fluid transition temperature, T c , from the temperature at which the Bose–Einstein condensation is believed to start, T BEC , the existence of metastable densities for the confined liquid accessible to the bulk system as a short-lived metastable state only and strong clues for a finite lifetime of the elementary excitations at temperatures as low as 0.4 K .