High-speed scanning tunnelling spectroscopy was used at low temperature to study the quasiparticle excitation spectrum on the surface of c-axis-oriented superconducting thin films of MgB2. The tunnelling spectra measured on as-grown films were compared with those acquired on chemically etched samples. In most cases the scanning tunnelling spectroscopy revealed only one small superconducting gap to be present in the tunnelling spectra, consistent with c-axis tunnelling and the particular electronic band structure of MgB2. It was found that the etching led to the enhancement of the gap energy by 25% from 2.2 to 2.8meV, and to the modification of the temperature dependence of the superconducting gap which, in both cases, clearly had a non-BCS shape. It was argued that the modification of the electronic structure at the surface of the material due to the etching was responsible for these changes.

Quasiparticle State Density on the Surface of Superconducting Thin Films of MgB2. F.Bobba, D.Roditchev, R.Lamy, E.M.Choi, H.J.Kim, W.N.Kang, V.Ferrando, C.Ferdeghini, F.Giubileo, W.Sacks, S.I.Lee, J.Klein, A.M.Cucolo: Superconductor Science and Technology, 2003, 16, 167-70