Two-gap state density in MgB2: A true bulk property or a proximity effect?

We report on the temperature dependence of the quasiparticle density of states in the simple binary compound MgB2 directly measured using scanning tunneling microscope (STM). To achieve high quality tunneling conditions. a small crystal of MgB2 is used as a tip in the STM experiment. The "sample" is chosen to be a 2H-NbSe2 single crystal presenting an atomically flat surface. At low temperature the tunneling conductance spectra show a gap at the Fermi energy followed by two well-pronounced conductance peaks on each side. They appear at voltages V-S similar or equal to 3.8 mV and V-L similar or equal to +/- 7.8 mV. With rising temperature both peaks disappear at the T-C of the bulk MgB2, a behavior consistent with the model of two-gap superconductivity. The possibility of a particular proximity effect is also discussed.