Superflow in Solid 4He

TL;DR

This paper models superfluidity in solid helium-4, explaining experimental NCRI observations through a quantum vortex framework and a detailed normal fluid density that includes phonons and defectons.

Contribution

It introduces a model distinguishing lattice and normal fluid densities, incorporating defectons and phonons, to explain NCRI and superfluid behavior in solid helium-4.

Findings

NCRI observed in various solid helium-4 samples with low superfluid fraction.

A model linking superfluid density to normal fluid contributions from phonons and defectons.

Quantum vortices potentially explain low critical velocities and hysteresis phenomena.

Abstract

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.