We report measurements of dynamic resistance of high quality oxygen deficient SNS edge junctions with ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$ ${(T}_c=60\mathrm{}\hspace{0.5em}\mathrm{K})$ as the superconductor (S) and ${\mathrm{YBa}}_2{\mathrm{Cu}}_{2.55}{\mathrm{Fe}}_{0.45}{\mathrm{O}}_y$ as the barrier (N). Below the gap a series of peaks in the dynamic resistance is identified as due to multiple Andreev reflections in the barrier ${(V}_n=2\mathrm{}\Delta /en),$ while above the gap, the peaks are attributed to McMillan-Rowell oscillations $\left(\Delta {V=hv}_F^{\prime }{/4\mathrm{ed}}_N\right).\mathrm{}$ Analysis of the subgap series in the 60 K ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.6}$ yields a gap value ${\Delta }_{a-b}$ of $16\pm 1.5\hspace{0.5em}\mathrm{meV},$ and a coupling constant $2\Delta {/kT}_c$ of $6.2\pm \mathrm{0.5.}$ The McMillan-Rowell series yield the renormalized Fermi velocity of quasiparticles in the barrier $v_F^{\prime }=(1.5\mathrm{}\pm 0.1)\times {10}^7\hspace{0.5em}\mathrm{c}\mathrm{m}/\mathrm{s}\mathrm{e}\mathrm{c}.$

• Received 25 March 1999

DOI:https://doi.org/10.1103/PhysRevB.60.9287