Continuous 3D freezing transition in layered superconductors

We use Ginzburg-Landau theory to study the H-c2 transition in layered superconductors with field parallel to the layers, finding a continuous 3D freezing transition to a triangular vortex supersolid in the three-dimensional XY universality class. If screening effects are neglected, off-diagonal-long-range order survives only for d > d(lc) = 5/2. The partial breaking of the lowest Landau level degeneracy induced by layering leads to a local selection of a triangular lattice structure, in contrast to the global free energy minimization in, e.g., Abrikosov's calculation. Our results are relevant to artificially layered superconductors and to strongly anisotropic high T-c materials.