Structure and superconducting properties of Tl_1-yPb_yY_1-xCa_xSr₂Cu₂O₇: dependence of T_c on hole concentration

Abstract

Structure and superconducting properties of Tl_1-yPb_yY_1-xCa_xSr₂Cu₂O₇ (y=0.0, 0.1, 0.25 and 0.5) , derived from the parent insulator TlYSr₂Cu₂O₇, have been investigated for different values of x. XANES studies show Pb to be in the 4+ state while Tl is in the 3+ state, suggesting thereby that in this series, (x-y) approximately corresponds to the hole concentration. The in-plane Cu---O distance decreases with increase in x for all values of y. The apical Cu---O distance as well as the Tl(Pb)---O(2) distance show anomalies at x=0.5 in the series with y=0.25; at this composition, T_c also reaches a maximum. In general, the composition (value of x) at which the T_c of Tl_1-yPb_yY_1-xCa_xSr₂Cu₂O₇ reaches a maximum depends on the Pb content and the maximum T_c itself increases with increase in y, reaching a value of 105 K at y=0.5. More interestingly, a maximum T_c occurs at an (x-y) value of ~0.25, which is close to the hole concentration at maximum T_c in other cuprate superconductors containing two CuO₂ layers.