Disorder And Thermally Driven Melting Of The Flux-Line Lattice In Anisotropic Layered Superconductors

we develop 3D flux line lattice (FLL)model in the layered high_Tc superconducto(rHTSC), this model is more practically than the 2D one. Based on the GL phenomenological theory and Langevin flux flow model, we use molecular dynamic simulations and compute the correlation function along the z_axis (C_z) to define the phase transition.We compute the 3D-2D phase transition, and find that at low temperatures and low magnetic fields C_z>0.45, vortex lattices are aligned into elastically distorted without dislocations, we define the flux line is under 3D state; with the increase of temperature or magnetic field, C z<0.45, vortex lattices melt into 2D state. Adjusting parameters to consist with that of Bi2Sr2CaCu2 O8, we get the similar phase diagram to experiment. It is necessary to say that the BG-VG transition line is parallel to the temperature axis. But in earlier theoretical work, the BG-VG transition line decreases with the increasing temperature. This is our work's distinction from others'. We say that it is because the interaction between different layers is so great that weakens other influence. The simulation results suggest our model's rationality and advantage.In the second part, we focus our attention on the temperature dependence of the BG-VG transition line. Considering that the penetration depth of superconductor increases with decreasing temperature, we put up a view that the temperature-related interactions between vortices is nonmonotonic. When 1.0>C_z>0.65, the pancake are aligned into 3D lines and the vortex system is regarded as a BG phase ; while for C_z<0.6 and quickly down to zero ,we define the VL phase; and when C_z<0.65 and keeps between 0.65-0.6 , we define the VG phase. we study the disorder and thermally driven melting transition from a disentangled with quasi-long-range ordered Bragg glass (BG) to an entangled amorphous vortex glass (VG) or a vortex liquid (VL) in the disordered strength-temperature phase diagram by the reduction of layered correlation function along c axis, and get the disorder-temperature phase diagram. From the phase diagram, we found, with the increase of temperature, in the low temperature region, the BG-VG transition line is parallel to temperature axis, but in the intermediate region, it shows an upturn. Such order-disorder transitions resemble the inverse melting in vortex lattices observed recently by Avraham et al. [Nature 411, 451 (2001)]. We attribute the unusual inverse melting behavior to temperaturedependence of the 3D vortex-vortex interactions. And so far there is no related report about the inverse melting behavior.