A series of MgB$_2$ thin films systematically disordered by neutron irradiation have been studied by Scanning Tunneling Spectroscopy. The c-axis orientation of the films allowed a reliable determination of local density of state of the $\pi$ band. With increasing disorder, the conductance peak moves towards higher voltages and becomes lower and broader, indicating a monotonic increase of the $\pi$ gap and of the broadening parameter. These results are discussed in the frame of two-band superconductivity.
Categories: cond-mat.supr-con