MgB2: directional tunnelling and two-band superconductivity

We have studied the anisotropic superconductor MgB2 using a combination of scanning electron microscopy and scanning tunnelling spectroscopy. Tunnelling spectroscopy performed on thin films and pellets reveals two distinct energy gaps at Δ1 = 2.3 meV and Δ2 = 7.1 meV. On different crystallites within the polycrystalline sample different spectral weights of the partial densities of states (PDOS) were observed. They reflect different tunnelling directions with respect to the crystallographic orientation of the grain in a multiband system. Indeed when tunnelling in the c-axis films only one superconducting gap is observed, which is associated with the 3D band in this system. Temperature evolution of the tunnelling spectra reveals that both gaps close simultaneously near the bulk critical temperature. Our experimental findings are consistent with the two-band superconductivity scenario in the presence of strong pair interaction between the two bands [1]