Direct Evidence for Edge-Contaminated Vortex Phase in a Nb Single Crystal using Neutron Diffraction

TL;DR

This study provides direct neutron diffraction evidence of an edge-contaminated, disordered vortex phase in Nb superconductors, revealing its metastability and coexistence with ordered vortex states, and clarifying the peak effect phenomenon.

Contribution

It presents the first direct observation of an edge-contaminated vortex phase in a superconductor using neutron diffraction, confirming its metastability and coexistence with ordered vortex states.

Findings

Edge vortex state is disordered and persists after annealing.

Bulk vortex matter reorders upon thermal annealing.

Edge-contaminated vortex state explains the peak effect in superconductors.

Abstract

We report the first direct observation of a disordered vortex matter phase existing near the edge of a bulk type-II superconductor Nb using a novel position-sensitive neutron diffraction technique. This "edge-contaminated" vortex state was implicated in previous studies using transport techniques and was postulated to have played a significant role in the behavior of vortex dynamics in a wide range of type-II superconductors. It is found that upon thermal annealing, the vortex matter in the bulk undergoes re-ordering, suggesting that the edge-contaminated bulk vortex state is metastable. The edge vortex state remains disordered after repeated thermal annealing, indicating spatial coexistence of a vortex glass with a Bragg glass. This observation resolves many outstanding issues concerning the peak effect in type-II superconductors.