Thermoelectric power of charge-neutral Nd1-2xCaxMxBa2Cu3O7-d M=Th and Pr: Evidence for different types of localization

Abstract

The superconducting Tc of Nd1-2xCaxMxBa2Cu3O7-d (M=Th or Pr!is strongly depressed with increasing<br><br>doping concentration in spite of the fact that these dopings are nominally charge neutral for small x. We have inquired into the reasons for this behavior by studies of the thermoelectric power ,S, in sintered samples with x up to 0.1 and for temperatures from Tc up to room temperature. S(x,T) was analyzed in terms of two different semiempirical models, which assume the existence of a narrow electron band. In both models and for both dopings the bands broaden with increasing x and the tendency for localization increases. For Ca-Th doping this can be explained by weak electronic disorder. For Ca-Pr doping, the increase of electronic disorder is weaker, and the results indicate that charge localization in addition is important.