Effect of lead substitution in bismuth-based cuprate systems containing both low-T_c and high-T_c phases

Abstract

We report the results of resistivity, ac-susceptibility, and specific-heat measurements carried out on samples of Bi_2-xPb_xSr₂Ca₂Cu₃O_10+x (0≤× ≤0.7) that were characterized by x-ray diffraction and high-resolution scanning tunneling microscopy. The zero resistance temperature, T_c (R=0), shows a peak at x=0.3 and the susceptibility data indicate that although a high-T_c 2:2:2:3 phase is present at 110 K, there is a second transition at 90 K that seems to indicate the existence of an additional low-T_c 2:1:2:2 phase. The specific-heat data unambiguously indicate a clear and distinct anomaly around T_c for samples with x=0.3–0.4 and smearing of the anomaly for x>0.5. Based on the results obtained from these combinations of experiments, the role of lead substitution in Bi samples, particularly in the stabilization of the 2:2:2:3 phase has been discussed.