Delocalization of Flux Lines from Extended Defects by Bulk Randomness

TL;DR

This paper investigates how bulk randomness affects the binding of a flux line to extended defects like columnar pins or twin planes, revealing a transition from bound to unbound states with specific critical behavior.

Contribution

It provides the first detailed analysis of the unpinning transition of flux lines from extended defects in three dimensions, supported by transfer matrix simulations and critical exponent estimates.

Findings

Flux lines are always bound to planar defects in 3D.

Unpinning transition occurs from columnar pins.

Localization length divergence characterized by a critical exponent   1.3 1.3 6.

Abstract

We study the delocalization by bulk randomness of a single flux line (FL) from an extended defect, such as a columnar pin or twin plane. In three dimensions, the FL is always bound to a planar defect, while there is an unpinning transition from a columnar pin. Transfer matrix simulations confirm this picture, and indicate that the divergence of the localization length from the columnar defect is governed by a liberation exponent $\nu_\perp =1.3 \pm 0.6$, for which a ``mean-field'' estimate gives $\nu_\perp \approx 0.78$. The results, and their extensions, are compared to other theories. The effects may be observable in thin samples close to $H_{c1}$.