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

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.