On superflow in solid ⁴He

Kim and Chan have recently observed nonclassical rotational inertia (NCRI) for solid ⁴He in Vycor glass, porous gold, and bulk. Their low T value of the superfluid fraction, ρ_s∕ρ₀≈0.015, is consistent with what is known of atomic delocalization in this quantum solid. Upon including a lattice mass density ρ_L distinct from the normal fluid density ρ_n (which is zero at T=0), the superfluid hydrodynamics becomes consistent with Galilean transformations. From its microscopic definition, ρ_L is temperature dependent. As a consequence, determination of ρ_n∕ρ₀ from ρ_s∕ρ₀ is not trivial, although a characteristic excitation energy of about 0.35 K can be extracted from the data. The macroscopic phase inferred from the observation of NCRI suggests quantum vortices, whose properties we discuss. Searches for quantum vortex lines and rings are proposed.