Energy gaps in Bi-Sr-Ca-Cu-O and Bi-Sr-Cu-O systems by electron tunneling

Abstract

We report the results of electron tunneling studies of the Bi-Sr-Ca-Cu-O and Bi-Sr-Cu-O systems by using point-contact tunneling. In these studies, we observed two distinctive energy gaps at each Bi-Sr-Ca-Cu-O system, having extremely large values. The larger values of the ratios 2Δ/${\mathrm{kT}}_c$ are 10–12 for ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathrm{y}}$ and 9–11 for ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{Oi}}_{\mathit{y}}$ and the smaller values are 5.5–6.1 for ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$. These differences can be attributed to the gap anisotropies and the ratios of gaps are 1.7–1.8 for ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{\mathrm{y}}$ and 1.5–1.8 for ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_2$ ${\mathrm{C}}_3$ ${\mathrm{O}}_{\mathit{y}}$. The ratio 2Δ/${\mathrm{kT}}_c$ in the Ca-absent material ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_{\mathrm{y}}$ was obtained as 5.4–7.4. The gap values are quite reproducible for the bulk, single crystal, and thin films.