Effect of surface on the conductance characteristics of Au-Bi₂Sr₂CaCu₂O_8+δ (single crystal) point contact junctions

Abstract

We report the effect of surface treatments on the dynamic conductance curves (G=dI/dV-V) of Au-Bi₂Sr₂CaCu₂O_8+δ (single crystal) point contact junctions of variable junction conductances (100 mS࣡G࣡100 μS). We find that if the crystal surface is cleaved freshly just prior to making contacts, all irreproducible sharp multiple features often observed in tunneling data of Bi(2212) oxide superconductors disappear. If the cleaved crystal surfaces are left under ambient conditions for a few days and the tunneling experiments are repeated, these multiple features reappear. We also find that if the current in the junction is made to pass predominantly through the bulk (and not along the surface), gap features are sharper. The observed conductance curves are fitted to a modified model [G. E. Blonder et al., Phys. Rev. B 25, 4515 (1982)] and estimated gap values are Δ=28 to 30 meV corresponding to the ratio 2Δ/κ_BTc=7.5 with lifetime broadening Γ/Δ=0.2. We conclude that the sharp multiple features observed in Bi(2212) tunneling curves has no intrinsic origin in the bulk and they arise from the surface only.