Impurity substitution in bismuth and thallium cuprates: suppression of T_c and estimation of pseudogap

Abstract

Suppression of T_c in bilayer bismuth and thallium cuprates by substitution of Co impurities at Cu sites were taken for examination. T_c suppression data on differently doped Bi2212 and Tl2212 were analysed within the unitary pair-breaking formalism due to Abrikosov and Gorkov, by fitting data points to a phenomenological relation valid for weak coupling d-wave superconductors. Values of the pseudogap magnitude at each doping were thereby estimated within a “Fermi-level density of states suppression” picture. Pseudogap magnitude from our estimation was observed to have a correspondence with a related characteristic temperature T* obtained by thermoelectric power measurements. Effects of pseudogap on the density of states were studied by calculating the susceptibility, which shows a broad peak at high temperature. This peak feature in susceptibility is indicative of an unusual metallic state, which could be explored further by other systematic measurements.