Narrow bridge structures were precisely fabricated (using novel ion-beam-milling techniques) on pulsed-laser-deposited ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ films. The bridges have a length scale down to L≃500 Å. Supercurrents were observed to flow across the bridge structures with critical current densities up to ${\mathit{J}}_{\mathit{c}}$≃1.3×${10}^9$ A/${\mathrm{cm}}^2$, the highest critical current densities yet reported. The current-density magnitudes are consistent with the Onsager-Feynman vortex-ring-creation mechanism (for producing voltages across narrow constrictions) in the length scale region of ξ≪L≤Λ, with ξ the coherence length and Λ the London penetration depth. The highest ${\mathit{J}}_{\mathit{c}}$ is in good agreement with the critical depairing mechanism.

• Received 11 February 1991

DOI:https://doi.org/10.1103/PhysRevLett.66.1785