Interstitials, Vacancies, and Supersolid Order in Vortex Crystals

TL;DR

This paper explores how vortex defects in Type II superconductors can lead to a supersolid phase characterized by intertwined crystalline and superfluid properties, with implications for experimental observations.

Contribution

It introduces the concept of a vortex supersolid phase arising from defect proliferation, linking it to quantum crystals and providing numerical estimates for its occurrence.

Findings

Interstitials are the preferred defects at high magnetic fields.

Proliferation of defects can occur below the melting temperature of the vortex crystal.

The supersolid phase affects transport, dislocation structures, and diffraction patterns.

Abstract

Interstitials and vacancies in the Abrikosov phase of clean Type II superconductors are line imperfections, which cannot extend across macroscopic equilibrated samples at low temperatures. We argue that the entropy associated with line wandering nevertheless can cause these defects to proliferate at a sharp transition which will exist if this occurs below the temperature at which the crystal actually melts. Vortices are both entangled and crystalline in the resulting ``supersolid'' phase, which in a dual ``boson'' analog system is closely related to a two-dimensional quantum crystal of He$^4$ with interstitials or vacancies in its ground state. The supersolid {\it must} occur for $B\gg B_\times$, where $B_\times$ is the decoupling field above which vortices begin to behave two-dimensionally. Numerical calculations show that interstitials, rather than vacancies, are the preferred defect for $B\gg \phi_0/\lambda_\perp^2$, and allow us to estimate whether proliferation also occurs for $B\,\lot\,B_\times$.The implications of the supersolid phase for transport measurements, dislocation configurations and neutron diffraction are discussed.