Superconducting energy gap in $\mathrm{B}{\mathrm{a}}_{1-\mathrm{x}}{\mathrm{K}}_{\mathrm{x}}\mathrm{Bi}{\mathrm{O}}_3$: Temperature dependence

Abstract

The superconducting energy gap of $\mathrm{B}{\mathrm{a}}_{1-\mathrm{x}}{\mathrm{K}}_{\mathrm{x}}\mathrm{Bi}{\mathrm{O}}_3$ has been measured by tunneling. Despite the fact that the sample was macroscopically single phase with very sharp superconducting transition $T_c$ at 32~$K$, some of the measured tunnel junctions made by point contacts between silver tip and single crystal of $\mathrm{B}{\mathrm{a}}_{1-\mathrm{x}}{\mathrm{K}}_{\mathrm{x}}\mathrm{Bi}{\mathrm{O}}_3$ had lower transition at 20~$K$. Local variation of the potassium concentration as well as oxygen deficiency in $\mathrm{B}{\mathrm{a}}_{1-\mathrm{x}}{\mathrm{K}}_{\mathrm{x}}\mathrm{Bi}{\mathrm{O}}_3$ at the place where the point contact is made can account for the change of $T_c$. The conductance curves of the tunnel junctions reveal the BCS behavior with a small broadening of the superconducting-gap structure. A value of the energy gap scales with $T_c$. The reduced gap amounts to $2\hat{I}”/k{T}_c=4\tilde{A}\cdot 4.3$ indicating a medium coupling strength. Temperature dependence of the energy gap follows the BCS prediction.

2 pages, 3 figures