Instabilities in the Flux Line Lattice of Anisotropic Superconductors

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, $b^*(ε,θ)$, depends on the anisotropy $ε$ and the angle $θ$ at which the lattice is tilted away from the $c$-axis. These instabilities initially occur at wavevector $k^*(ε,θ)$, 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 $ε$ appropriate to a material like BSCCO, and not for anisotropies more appropriate to YBCO. The lower critical field $H_{c_1}(ϕ)$ is calculated as a function of the angle $ϕ$ at which the applied field is tilted away from the crystal axis. The presence of kinks in $H_{c_1}(ϕ)$ is seen to be related to instabilities in the equilibrium flux line structure.