Effect of Finite Size on Superfluidity; Third Sound

When the quantum liquid, He4, is bounded in size and when one or more of its dimensions becomes comparable with a characteristic healing length, the superfluid state is altered. In particular, the superfluid onset temperature is lowered and the superfluid fraction at a given temperature is reduced. Third sound is a surface wave that propagates in liquid-helium films where only the superfluid component moves. Third-sound measurements have been made in films several atomic layers thick from which reduced onset temperatures and superfluid fractions have been determined. These results will be discussed in the framework of current theories.