EFFECT OF BI-SR REPLACEMENT AND OXYGEN DOPING ON VORTEX-MATTER PHASE-TRANSITIONS IN BI2-XCACU2O8+DELTA(XSR2)

To investigate the effect of oxygen doping level with a wide range on vortex-matter phase transitions, magnetization for H parallel to c was measured for Bi2+xSr2-xCaCu2O8+delta single crystal with x = 0.05-0.2 5 and various doping level delta. Optimum condition against delta for the superconducting transition temperature was found to depend on x, w hich is reasonably explained from the difference in valences between B i and Sr. The second-peak of the low temperature magnetization curve w as observed in all the samples, ranging from similar to 120 Oe for the mostly underdoped sample to similar to 1300 Oe for the most overdoped one. For the optimally-doped samples the second-peak was observed at similar to 400 Oe almost independent of x. Vortex-melt transition fiel d, H-m, was well evaluated for the as-grown samples and for the slight ly overdoped ones. The shift of the melting line, H-m(T), with x and d elta can be explained from the change in anisotropy and penetration de pth. Near the superconducting transition temperature vortex transition is attributed to the transition due to the decoupling of pancake vort ices, while at lower temperatures to the melting transition with both the electromagnetic- and the Josephson-coupling between vortices. (C) 1998 Elsevier Science B.V. All rights reserved.