We develop a three-dimensional flux line lattice model in the layered superconductors, taking into account both long-range magnetic interactions between pancake vortices located in different layers for a single flux line and those between vortices located in the same layer for different flux lines. Using Langevin dynamic simulations, we study the disorder and thermally driven melting transitions from a disentangled Bragg glass ͑BG͒ with quasi-long-range order to an entangled amorphous vortex glass ͑VG͒ or a vortex liquid ͑VL͒ in the disorder strength-temperature phase diagram. Owing to nonmonotonous temperature dependence of the interactions between vortices, the BG-VG melting line exhibit unusual temperature behavior, reproducing the inverse melting phenomenon observed recently on BSCCO crystals.

Using molecular dynamic simulations for the melting transition of a flux line lattice(FLL) with point disordered pinnings, thermal fluctuations and magnetic interactions between pancake vortices, we study the disorder-driven melting transition from a disentangled and ordered Bragg glass (BG) to an entangled amorphous vortex glass (VG) or a vortex liquid (VL) in the pinning strength-temperature phase diagram. A portion of the BG region is found to be sandwiched in between the VG phase at lower temperatures and VL phase at higher temperatures, exhibiting inverse melting behavior observed recently on BSCCO crystals.

Gd is successfully embedded into YBa 2 Cu 3 O 7-δ by the solid state reaction. Rietveld refinements show that Gd replaces Cu in the unit cell. By magnetization and resistivity measurements, the effects of Gd -doping in YBCO on the critical current density J c and the critical temperature T c in YBa 2 Cu 3 O 7-δ( YBCO ) have been investigated. J c varies with the applied field and the content of Gd and has a maximum at x = 0.03. The characteristic behavior of J c with varying the Gd content can be explained in terms of the competition between two different effects caused by Gd -doping in YBCO: the inhomogeneity of grains distribution in nano-scale and the variation of superconductivity.

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