A. M. Thompson | Searcharxiv

arXiv1997

Flux Line Lattices in Artificially Layered Superconductors

The flux line lattice of superconductors has been investigated when there exists a periodicity in the underlying system, such as can occur in artificially layered structures. For small fields parallel to the layers the flux lines enter the sample in sequential rows, with the possibility of jumps in the magnetization as new rows are created. As the field is increased these discontinuities gradually decrease, but there still exist transitions between states that are aligned differently to the periodic direction. Increasing the magnitude of the periodic potential reduces the competition between differently aligned lattices and tends to lock in one particular alignment. The effect of transitions on the shear modulus is also discussed and related to the experiments of Theunissen et al.

cond-mat↗

arXiv1996

Instabilities in the Flux Line Lattice of Anisotropic Superconductors

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.

cond-mat↗