Vortex lines or vortex-line chains at the lower critical field in anisotropic superconductors?

Abstract 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 (