Electron Tunneling into Superconducting Bismuth-Copper Oxide Systems

Abstract

The energy gaps of the polycrystal1ine Bi-Sr-Ca-Cu-O and Bi-Sr-Cu-O superconductors have been measured by means of the point-contact tunneling technique. We find that the energy gaps at T=OK are 2Δ=76-81meV in Bi₂Sr₂Ca₁Cu₂O_y(Tc=76K), 91–100meV in Bi₂Sr₂Ca₂Cu₃O_y(Tc=105K) and 3.0–3.5meV in Bi₂Sr₂Cu₁O_y (Tc=6.5K). Temperature variations of these energy gaps are also extracted. The ratios 2Δ/kTc are 11.6–12.4 for Bi₂Sr₂Ca₁Cu₂O_y, 10.0–11.0 for Bi₂Sr₂Ca₂Cu₃O_y, extremely larger than BCS value. 2Δ/kTc of Bi₂Sr₂Cu₁O_y is 5.4–6.2, larger than those of the usual strong coupling superconductors. These results give the information about the difference of the pairing nature on the superconducting state between these materials which have multiple Cu0₂ layers or a single CuO₂ layer between the SrO layers.