We report critical-current measurements in all high-${\mathit{T}}_{\mathit{c}}$ superconducting-normal-superconductor junctions using ${\mathrm{Y}}_{0.6}$${\mathrm{Pr}}_{0.4}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ (with ${\mathit{T}}_{\mathit{c}\mathit{N}}$=40 K) as the normal metal. Above ${\mathit{T}}_{\mathit{c}\mathit{N}}$, we find a clear exponential dependence of ${\mathit{I}}_{\mathit{c}}$ on the thickness of the barrier which is characteristic of the proximity effect. The order-parameter decay length is about 120 Å for T>60 K, and it diverges as ${\mathit{T}}_{\mathit{c}\mathit{N}}$ is approached. We estimate that ${\xi }_0$ for this material is 80±25 Å.

• Received 22 July 1991

DOI:https://doi.org/10.1103/PhysRevLett.67.3038