Size effects in superfluid helium II - I. Experiments in porous systems

Abstract

The properties of the superfluid phase of liquid helium are modified by size effects. We discuss here experiments on helium contained in very fine compressed powders (∼ 100 to 104 Å) : lowering of the onset temperature for superfluidity, T0, and superfluid properties for T < To : critical flow rate, specific heat, superfluid fraction. These last data are obtained from the measurement of the hydrodynamic sound velocity that propagates in such porous systems : 4th sound. We present in some detail the techniques of the measurements and the physical properties of 4th sound. A discussion of the results is presented in analogy with the study of the proximity effect in superconductors