Instabilities in the Flux Line Lattice of Anisotropic Superconductors

TL;DR

This paper investigates the stability of flux line lattices in anisotropic superconductors, revealing field-dependent instabilities influenced by anisotropy and tilt angle, with implications for materials like BSCCO.

Contribution

First comprehensive analysis of flux line lattice instabilities in anisotropic superconductors using London theory, highlighting conditions for stability and instability.

Findings

Lattice stable at high fields, unstable at lower fields.

Instabilities depend on anisotropy and tilt angle.

Kinks in critical field related to flux line instabilities.

Abstract

The stability of the flux line lattice has been investigated within anisotropic London theory. This is the first full-scale investigation of instabilities in the `chain' state. It has been found that the lattice is stable at large fields, but that instabilities occur as the field is reduced. The field at which these instabilities first arise, $b^*(\epsilon,\theta)$, depends on the anisotropy $\epsilon$ and the angle $\theta$ at which the lattice is tilted away from the $c$-axis. These instabilities initially occur at wavevector $k^*(\epsilon,\theta)$, and the component of $k^*$ along the average direction of the flux lines, $k_z$, is always finite. As the instability occurs at finite $k_z$ the dependence of the cutoff on $k_z$ is important, and we have used a cutoff suggested by Sudb\ospace and Brandt. The instabilities only occur for values of the anisotropy $\epsilon$ appropriate to a material like BSCCO, and not for anisotropies more appropriate to YBCO. The lower critical field $H_{c_1}(\phi)$ is calculated as a function of the angle $\phi$ at which the applied field is tilted away from the crystal axis. The presence of kinks in $H_{c_1}(\phi)$ is seen to be related to instabilities in the equilibrium flux line structure.