Effect of praseodymium on the normal-state and superconducting properties ofRBa2Cu3Oy: A comparative study of the role of the Pr ion onRand Ba sites

Abstract

We present results of a comparative study of the normal-state and superconducting properties in the $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ system with substitutions by Pr for the R and Ba sites. The temperature dependences of the resistivity and thermopower were measured for the ${\mathrm{SmBa}}_{2\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ $(x=0.0--0.35)$ samples and analyzed in comparison with data for the ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ system. It was observed that the $\mathrm{P}\stackrel{\ensuremath{\rightarrow}}{r}\mathrm{Ba}$ substitution affects all the properties of the $R{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ system, both the normal state and superconducting, much stronger compared to the $\mathrm{P}\stackrel{\ensuremath{\rightarrow}}{r}\mathrm{Y}$ one. This manifests itself in an increasing oxygen content, an orthorhombic-to-tetragonal transition at $x=0.3--0.35,$ a stronger rise of the thermopower value, and a faster ${T}_{c}$ drop. The results obtained are analyzed within a phenomenological narrow-band model in order to reveal both common peculiarities and specific features of the Pr influence on the band spectrum parameters in the normal state when substituting for different lattice positions. We have observed that the Pr doping leads to a significant modification of the band spectrum, resulting in a strong band broadening and states localization. At the same time, changes of the total effective width of the conduction band ${W}_{D}$ and the ${T}_{c}$ value in two systems with the different kind of Pr doping are shown to correspond to each other so that the ${T}_{c}{(W}_{D})$ dependence follows the universal correlation for the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ system with different deviations from stoichiometry. Based on our analysis and in agreement with earlier suggestions, we conclude that the superconductivity suppression in both Pr-doped systems could be mainly attributed to the hybridization between $4f$ states of the Pr ion and the conduction-band states. In addition, Pr in ${\mathrm{SmBa}}_{2\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}$ on the Ba site acts like other trivalent rare-earth impurities inducing the extra hole filling effect that leads to a stronger ${T}_{c}$ suppression compared to ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}.$