Site-selective doping and superconductivity in (La1−yPry)(Ba2−xLax)Cu3O7−δ

Abstract

Samples in the quaternary system $\left({\mathrm{La}}_{1-y}{\mathrm{Pr}}_y\right)\left({\mathrm{Ba}}_{2-x}{\mathrm{La}}_x\right){\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ have been prepared and characterized using x-ray and neutron diffraction, thermogravimetric analysis, and transport and magnetic measurements. Pr substitutes on the oxygen-depleted La layers for $y>\mathrm{}0.0\mathrm{}$, while La substitutes on the Ba sites for $x=0.0\mathrm{}$. The effect of doping on each site is inferred to be primarily local, affecting immediately adjacent Cu-O layers. The similar suppression of superconductivity that accompanies doping on each of the two distinct sites apparently correlates with the degree of oxidation of the Cu-O sheets (and not the chains), indicating that the sheets support the high-temperature superconductivity. Comparison of orthorhombic and tetragonal samples with similar Ba:La ratios (and $y=0\mathrm{}$) demonstrates that the orthorhombic phase yields the largest Meissner signals and highest transition temperatures in the $\mathrm{La}\left({\mathrm{Ba}}_{2-x}{\mathrm{La}}_x\right){\mathrm{Cu}}_3{\mathrm{O}}_{7+\delta }$ system. The effect on superconductivity of oxygen-vacancy configuration in the Cu-O chain layers is proposed to derive, indirectly, from their influence on the Cu-O sheets. In addition, optimally superconducting $\mathrm{La}\left({\mathrm{Ba}}_{2-x}{\mathrm{La}}_x\right){\mathrm{Cu}}_3{\mathrm{O}}_{7+\delta }$ samples exhibit interesting normal-state magnetic properties, with a paramagnetic susceptibility that decreases steadily with temperature between 350 K and $T_c$.