Vortex phase diagram and electromagnetic anisotropy of cation composition controlled Bi2212 single crystals

In the present study, vortex phase diagram and pinning properties of Bi2212 single crystals under $H$ // $c$ have been systematically studied as functions of cation and oxygen compositions, while these matters were well understood for slightly cation-nonstoichiometric samples almost ten years ago. We confirmed that cation stoichiometry largely affects vortex state and pinning strength. A particular crystal with a cation composition of nearly 2:2:1:2 ``Bi2212'' exhibited strong bulk pinning behaviors even in the high temperature region, resulting in disappearance of magnetic reversible region below the first-order-transition (FOT) temperature of vortex state. In addition, entropy change of the vortex at FOT of ``Bi2212'' crystals was found to be apparently larger than that of conventional ones. On the contrary, Bi and Ca-rich and Sr-poor single crystals showed poor pinning behaviors with low irreversibility fields and critical current density. Systematic enhancement of in-plane anisotropy in resistivity, decreases in \textit{$\rho$}$_{c}$ and increases of penetration depth with approaching the cation stoichiometric composition suggested that disordered crystal lattice due to partial substitutions of Bi and Ca for Sr-site, which is common for conventional Bi2212 single crystals, degraded superconducting properties of Bi2212.