Superfluidity of Grain Boundaries in Solid Helium-4

TL;DR

This study uses large-scale quantum Monte Carlo simulations to demonstrate that grain boundaries in solid Helium-4 are generally superfluid at low temperatures, with specific orientations affecting their insulating or superfluid nature.

Contribution

It provides the first comprehensive simulation-based evidence that grain boundaries in solid Helium-4 are typically superfluid, challenging previous assumptions about their insulating behavior.

Findings

Grain boundaries are generally superfluid at low temperatures.

Superfluidity depends on grain boundary orientation.

Superfluid grain boundaries can exist in contact with the liquid at the melting line.

Abstract

By large-scale quantum Monte Carlo simulations we show that grain boundaries in Helium-4 crystals are generically superfluid at low temperature, with a transition temperature of the order of ~0.5K at the melting pressure; insulating grain boundaries are found only for special orientations of the grains. We also find that close vicinity to the melting line is not a necessary condition for superfluid grain boundaries, and a grain boundary in direct contact with the superfluid liquid at the melting curve is found to be mechanically stable and the grain boundary superfluidity observed by Sasaki et al. [Science 313, 1098 (2006)] is not just a crack filled with superfluid.