ar X iv : c on d - m at / 9 60 11 54 v 1 3 1 Ja n 19 96 Instabilities in the Flux Line Lattice of Anisotropic Superconductors

The stability of the flux line lattice has been investigated with anisotropic London theory. By calculating the normal modes of the elasticity matrix, it has been found 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 * (ǫ, θ), depends on the anisotropy ǫ and the angle θ at which the lattice is tilted away from the c-axis. These instabilities initially occur at wavevector k * (ǫ, θ). The dependence of k * on ǫ and θ is not understood, but the component of k * along the average direction of the flux lines, k z , is always finite. For rigid straight flux lines, the cutoff necessary for London theory has been 'derived' from Landau-Ginzburg theory, where the shape of the vortex core is known. However, for investigating instability at finite k z it is necessary to know the dependence of the cutoff on k z , and we have used a cutoff suggested by Brandt. This is the first investigation of instabilities at finite k z with such a cutoff and the first investigation of instabilities at a general wavevector k = (k x , k y , k z). With this cutoff, the lattice is most unstable to two distinct instabilities, a 'transverse' instability and a 'staircase' instability. The 'transverse' instability is similar in nature to the k = (k x , k y , 0) instability observed by Sudbo and Brandt, but the 'staircase' instability has not been observed previously. 1 It only occurs for values of the anisotropy ǫ appropriate to a material like BSCCO, and not for anisotropies more appropriate to YBCO