Effect of Bi–Sr replacement and oxygen doping on vortex-matter phase transitions in Bi 2+ xSr 2− xCaCu 2O 8+ δ

Abstract

To investigate the effect of oxygen doping level with a wide range on vortex-matter phase transitions, magnetization for H‖ c was measured for Bi 2+ x Sr 2− x CaCu 2O 8+ δ single crystal with x=0.05–0.25 and various doping level δ. Optimum condition against δ for the superconducting transition temperature was found to depend on x, which is reasonably explained from the difference in valences between Bi and Sr. The second-peak of the low temperature magnetization curve was observed in all the samples, ranging from ∼120 Oe for the mostly underdoped sample to ∼1300 Oe for the most overdoped one. For the optimally-doped samples the second-peak was observed at ∼400 Oe almost independent of x. Vortex-melt transition field, H m, was well evaluated for the as-grown samples and for the slightly overdoped ones. The shift of the melting line, H m( T), with x and δ can be explained from the change in anisotropy and penetration depth. Near the superconducting transition temperature vortex transition is attributed to the transition due to the decoupling of pancake vortices, while at lower temperatures to the melting transition with both the electromagnetic- and the Josephson-coupling between vortices.