Vortex lattice of layered superconductors in parallel magnetic fields

In the framework of the Lawrence-Doniach theory, we study numerically the vortex lattice of layered superconductors in parallel magnetic fields for the case of material parameters appropriate to YBa2Cu3O7-x. In the direction perpendicular to the layers, the period of the vortex lattice is commensurate with that of the layered structure. The lattice structure with each period is obtained numerically from the free- energy functional with the relaxation method, and the lattice constants for the equilibrium state and the dependence of the magnetization on the magnetic field H are determined. For (H0/H)1/24, the magnetization shows a periodic variation in the plot of (H0/H)1/2. (H0 is defined in text.) For higher magnetic fields (H0/H)1/2<4, the shape of a unit cell for the equilibrium vortex lattice is enlarged in the direction parallel to the layers, and the magnetization curve deviates from the periodic behavior.