Point-like and line-like melting of the vortex lattice in the universal phase diagram of layered superconductors

The phase diagram of layered superconductors in the vortex state is studied by Monte Carlo simulations of the three-dimensional uniformly frustrated $\mathrm{XY}$ model with different anisotropy parameters. In the London regime the phase diagram of layered superconductors is shown to be universal if plotted in scaled temperature and field with the field scale being the two-dimensional (2D)-three-dimensional (3D) crossover field $B_{\mathrm{cr}}.$ We find a very broad crossover region between quasi-two-dimensional and line-like melting regimes ranging from $\sim B_{\mathrm{cr}}$ to $\sim 10B_{\mathrm{cr}}.$ The region is characterized by several distinct features: (i) the melting of the lattice occurs when the Josephson energy is suppressed to 64% of its bare value; (ii) the latent heat at the transition does not change much with the anisotropy parameter; (iii) the jump of the Josephson energy at the transition is equal to the jump of the in-plane energy. The entropy jump reaches a maximum value of $0.45k_B$/vortex/layer at a field $\sim 10B_{\mathrm{cr}}$ and decreases with decreasing field due to an increase in the transition temperature. This behavior is found to be in a good agreement with experimental observations after the renormalization due to the temperature dependence of superconducting parameters is taken into account. The pancake alignment above the transition increases with increasing of the Josephson coupling. At high fields the melting is accompanied by a significant drop in the coupling energy and the destruction of vortex lines, while at small fields the vortex lines preserve at the transition. In the studied region of parameters we find that the line liquid does not have superconductivity along the direction of magnetic field in the thermodynamic limit.