Longitudinal disordering of vortex lattices in anisotropic superconductors

Vortex disordering in superconducting crystals is shown to be markedly sensitive to penetration-depth anisotropy. At low temperature and high magnetic field, the muon-spin-rotation spectra for the highly anisotropic ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ material are found to be anomalously narrow and symmetric about the applied field, in a manner consistent with a layered vortex sublattice structure with pinning-induced misalignment between layers. In contrast, spectra for the less-anisotropic ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ compounds taken at comparable fields are broader and asymmetric, showing that the vortex lattices are aligned parallel to the applied-field direction.