Critical temperature variation with nonstoichiometry in high-temperature superconductors

We present a phenomenological model able to explain the critical temperatur e dependence on nonstoichiometry T-c(x) in high-temperature superconductors . We assume that nonisovalent substitutions in a nonsuperconducting materia l lead to the appearance of superconducting regions with spherical shape, i nside which the buckling of the Cu-O bonds (achieved through electrostatic interactions) is favorable to superconductivity. We obtain T-c taking into account its proportionality to the superconducting volume fraction. which i s calculated as a function of doping content x. Our theoretical results fit very well experimental data for T-c versus doping. (C) 2001 Elsevier Scien ce B.V. All rights reserved.