Control of Hole Distribution and Superconducting Properties in Bi_2Sr_2(Ca_(1-x)Y_x)Cu_2O_(8+δ)

摘要

The dependence of H_(irr) on the concentration and distribution of holes in the Bi_2Sr_2(Ca_(1-x)Y_x)Cu_2O_(8+δ) (Bi-2212) system was investigated. Coulometric Cu(I)/(II) and Fe(II)/(III) titrations were applied to determine oxygen contents and distinct Cu and Bi valences and to thereby estimate the hole distribution among the SrO-Bi_2O_(2+δ)SrO and (Ca_(1-x)Y_x)Cu_2O_4 blocks. With increasing Y-substitution level, x, continuous increase in the oxygen content and decrease in the thickness of the SrO-Bi_2O_(2+δ)SrO "blocking" block were observed. The extra oxygen taken in the structure only partially counteracts the trivalent-Y to divalent-Ca substitution, and consequently the hole concentration in both blocks was found to decrease. With increasing x and δ, H_(irr) was not enhanced but rather depressed. This suggests that the thickness of the blocking block is not the primary parameter in determining the H_(irr).