MgB2: an old material, a new superconductor. An extensive scanning tunneling spectroscopy study

We have performed an extensive scanning tunneling spectroscopy study on different MgB2 samples. In the standard N–I–S geometry on powders, we observed spectra characterized by pronounced double gap structures that can be modeled by supposing two independent tunneling channels, with gap magnitudes Δσ=7.0±0.5 meV and Δπ=3.0±0.4 meV, i.e., well above and well below the classical BCS limit. In the conductance characteristics of oriented thin films, the presence of the small gap Δπ was predominant and only in few cases a weaker structure appeared around Δσ=7.0 meV. Recent results on as-grown c-axis oriented single crystals yielded only small gap confirming the 3D π-band nature of this feature, that survives in magnetic field higher than 5 T.