ISOTOPIC MASS AND PRESSURE-DEPENDENT CHANGES FOR EXTENDED S-WAVE SUPERCONDUCTORS

We consider a tight-binding model comprising a hopping and an attracti ve interaction term to obtain superconductivity. Exact expressions for the isotope-shift exponent (alpha) and the pressure coefficient of tr ansition temperature (gamma(P)) are derived invoking the isotopic mass and pressure dependence of both the hopping and attractive interactio n terms. The variations of alpha and gamma(P) are studied as a functio n of the chemical potential and the transition temperature. With prope r choice of parameters we obtain large positive values of alpha for lo w-T-c samples and a minimum in alpha near optimum doping. It is found that yp is large and positive for low-T-c underdoped systems and small near optimum doping. These results qualitatively agree with the exper imental results of hole-doped high-T-c oxides. The behaviour of gamma( P) for electron-doped oxide systems can also be explained from the mod el.