  Point-contact tunnel junctions using a Au tip on sintered MgB2 pellets reveal
a sharp superconducting energy gap that is confirmed by subsequent metallic
Sharvin contacts made on the same sample. The peak in the tunneling conductance
and the Sharvin contact conductance follow the BCS form, but the gap values of
4.3 meV are less than the weak-coupling BCS value of 5.9 meV for the bulk Tc of
39 K. The low value of Delta compared to the BCS value for the bulk Tc is
possibly due to chemical reactions at the surface.
