Finite Temperature Depinning of a Flux Line from a Nonuniform Columnar Defect

TL;DR

This paper investigates how a flux line in a superconductor depins from a nonuniform columnar defect at finite temperature, revealing a transition driven by defect fluctuations and providing exact critical parameters.

Contribution

It introduces a perturbative diagrammatic approach to analyze the depinning transition, yielding exact results for the depinning temperature and localization length divergence.

Findings

Depinning transition occurs at finite temperature due to defect fluctuations.

Perturbative expansion converges in weak pinning regime and becomes exact at transition.

Provides exact critical temperature and localization length divergence.

Abstract

A flux line in a Type-II superconductor with a single nonuniform columnar defect is studied by a perturbative diagrammatic expansion around an annealed approximation. The system undergoes a finite temperature depinning transition for the (rather unphysical) on-the-average repulsive columnar defect, provided that the fluctuations along the axis are sufficiently large to cause some portions of the column to become attractive. The perturbative expansion is convergent throughout the weak pinning regime and becomes exact as the depinning transition is approached, providing an exact determination of the depinning temperature and the divergence of the localization length.