Localization of charge carriers by Pr in theTl2Ba2Ca1−xPrxCu2Oysystem

Abstract

We have measured the resistivity and thermoelectric power of Tl 2 Ba 2 Ca 1 - x Pr x Cu 2 O y samples with x≤0.6 in the temperature range 10-300 K. T c decreases with an increase in the Pr concentration and a crossover from a metallic to insulator state is observed in samples with x≃0.4. In the highly resistive samples (x>0.4) conduction is governed by the in-plane variable range hopping of the carriers while in others, localization of bosons appears to be dominating. A spin gap is observed in the resistivity of these samples. The resistivity and thermoelectric power show the localization of the carriers to increase with an increase in Pr concentration. These data, examined in the light of existing models of conduction by localized carriers, show a preference for the model proposed by Nagaosa and Lee wherein the fermion contribution decreases and that of the bosons increases with the increase in the Pr concentration. The Pr L 3 -edge absorption data show Pr to be present in the mixed (3 + and 4 + ) valent state. Correlating the hole density p determined from T c and thermoelectric power with the valence of Pr estimated from x-ray-absorption edge measurements, we find hole filling along with hybridization to be the dominant mechanism for destablization and eventual suppression of superconductivity in these materials.