Concentration range for superconductivity in high-quality ${\mathrm{Pr}}_{2\mathrm{-}\mathrm{x}}$${\mathrm{Ce}}_{\mathrm{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ thin films

We report ac susceptibility, electrical resistivity, and structural measurements on crystalline ${\mathrm{Pr}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Ce}}_{\mathrm{x}}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{y}}$ (PCCO) thin films prepared by pulsed laser deposition. We find superconductivity occurs within a narrow Ce concentration range (0.13<x<0.20) similar to that found for ceramic samples of this ``electron-doped'' material. Our results are quite different than those recently reported [Phys. Rev. Lett. 74, 4927 (1995)] on PCCO single crystals where superconductivity near 20 K was found for Ce concentration as low as x=0.04. Possible reasons for this difference will be discussed.