Clean and dirty superconductivity in pure, Al-doped, and neutron irradiated MgB2: A far-infrared study

The effects of Al substitution and neutron irradiation on the conduction regime (clean or dirty) of the pi and sigma band of MgB2 have been investigated by means of far-infrared spectroscopy. The intensity reflected by well characterized polycrystalline samples was measured up to 100 cm-1 in both normal and superconducting state. The analysis of the superconducting to normal reflectivity ratios shows that only the effect of the opening of the small gap in the dirty pi band can be clearly observed in pure MgB2, consistently with previous results. In Al-doped samples the dirty character of the pi band is increased, while no definitive conclusion on the conduction regime of the sigma band can be drawn. On the contrary, results obtained for the irradiated sample show that the irradiation-induced disorder drives the ? band in the dirty regime, making the large gap in sigma band observable in far-infrared measurements.