Transport and structural properties of ${\mathrm{Pr}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films grown by pulsed-laser deposition

We have studied the transport and structural properties of ${\mathrm{Pr}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ thin films grown by pulsed-laser deposition, focusing on Ca substitution levels x≥0.3 for which the bulk material is metastable. Films with 0.4≤x≤0.5 exhibit a superconducting transition due to divalent cation doping on the rare-earth site. ${\mathrm{Pr}}_{0.5}$ ${\mathrm{Ca}}_{0.5}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ epitaxial thin films exhibit a superconducting onset temperature as high as 47 K with ${\mathit{T}}_{\mathit{c}}$(R=0)=35 K. X-ray-diffraction and electrical-transport data suggest that Ca doping levels greater than x=0.5 are possible, although disorder is introduced as the divalent to trivalent cation ratio becomes large. This work demonstrates that 1:2:3-phase superconductivity can be achieved by substituting Ca for Pr, without the presence in the alloy of Y or any other rare-earth element, R, for which R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is superconducting. This result supports the view that hole localization, due to hybridization of the Pr 4f electronic levels with the O 2p orbitals, contributes substantially to the suppression of superconductivity by Pr in ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$, and demonstrates that this suppression can be partially compensated by appropriate hole doping with Ca.