Local stress and superfluid properties of solid Helium-4

TL;DR

This paper investigates the origin of supersolidity in Helium-4, showing that local strain near defects induces superfluidity, explaining experimental observations of supersolid behavior.

Contribution

It demonstrates that local strain at crystalline defects in Helium-4 can induce superfluidity, providing a microscopic mechanism for supersolidity.

Findings

Local strain near defects triggers superfluidity in Helium-4

Vacancy creation gap closes under moderate stress

Defect cores can become superfluid

Abstract

More than half a century ago Penrose asked: are the superfluid and solid state of matter mutually exclusive or do there exist "supersolid" materials where the atoms form a regular lattice and simultaneously flow without friction? Recent experiments provide evidence that supersolid behavior indeed exists in Helium-4 -- the most quantum material known in Nature. In this paper we show that large local strain in the vicinity of crystalline defects is the origin of supersolidity in Helium-4. Although ideal crystals of Helium-4 are not supersolid, the gap for vacancy creation closes when applying a moderate stress. While a homogeneous system simply becomes unstable at this point, the stressed core of crystalline defects (dislocations and grain boundaries) undergoes a radical transformation and can become superfluid.