Oxygen Doping Effects on the Re-entrant Point of Vortex Melting Line in Bi2Sr2CaCu2O8+ƒÔ and YBa2Cu3O7-ƒÔ Superconducting System

Abstract
The complexity of vortex morphology of high Tc superconductor is marked among others by the existence of ¡§re-entrant¡¨ phenomena of vortex melting lines in the B-T diagram. This peculiar behavior has its origin in the large anisotropy, disorder and thermal fluctuation effects. Materials having those characterictics are Bi2Sr2CaCu2O8+ƒÔ (BSCCO-2212) and YBa2Cu3O7-ƒÔ (YBCO-123). The re-entrant point (B*,T*) can be regarded as the intersection point between the low field melting curve and the high field melting curve. The values of B* and T* generally depend on the critical temperature (Tc), anisotropy factor (£`) and penetration depth (£f). The results of analysis based on the data of Tc, £`, and £f from the samples of BSCCO-2212 and YBCO-123 single crystals show that B*>Bc1 and T*