Energy gap from tunneling and metallic contacts ontoMgB2:Possible evidence for a weakened surface layer

Abstract

Point-contact tunnel junctions using a Au tip on sintered ${\mathrm{MgB}}_2$ pellets reveal a sharp superconducting energy gap that is confirmed by subsequent metallic contacts made on the same sample. The peak in the tunneling conductance and the metallic contact conductance follow the BCS form, but the gap values of $4.3-4.6\hspace{0.5em}\mathrm{meV}$ are less than the weak-coupling BCS value of 5.9 meV for the bulk $T_c$ of 39 K. The low value of $\Delta$ compared to the BCS value for the bulk $T_c$ is possibly due to chemical reactions at the surface.