Vortex Lines or Vortex-Line Chains at the Lower Critical Field in Anisotropic Superconductors?

The vortex state at the lower critical field, H_{c1}, in clean anisotropic superconductors placed in an external field tilted with respect to the axis of anisotropy (c-axis) is considered assuming two possible arrangements: dilute vortex-lines or dilute vortex-line chains. By minimizing the Gibbs free energies in the London limit for each possibility we obtain the corresponding lower critical fields as a function of the tilt angle. The equilibrium configuration at H_{c1} for a given tilt angle is identified with that for which H_{c1} is the smallest. We report results for parameter values typical of strong and moderate anisotropy. We find that for strong anisotropy vortex-line chains are favored for small tilt angles (< 7.9^o) and that at 7.9^o there is coexistence between this configuration and a vortex-line one. For moderate anisotropy we find that there is little difference between the vortex-line and the vortex-chain lower critical fields.