Kim and Chan have recently observed Non-Classical Rotational Inertia (NCRI) for solid $^4$He in Vycor glass, gold film, and bulk. Their low $T$ value of the superfluid fraction, $\rho_{ s}/\rho\approx0.015$, is consistent with what is known of the atomic delocalization in this quantum solid. By including a lattice mass density $\rho_{ L}$ distinct from the normal fluid density $\rho_{ n}$, we argue that $\rho_{ s}(T)\approx\rho_{ s}(0)-\rho_{ n}(T)$, and we develop a model for the normal fluid density $\rho_{ n}$ with contributions from longitudinal phonons and ``defectons'' (which dominate). The Bose-Einstein Condensation (BEC) and macroscopic phase inferred from NCRI implies quantum vortex lines and quantum vortex rings, which may explain the unusually low critical velocity and certain hysteretic phenomena.
Categories: cond-mat.other cond-mat.supr-con