Clean and Dirty Superconductivity in Pure, Al doped, and Neutron Irradiated MgB2: a Far-Infrared Study

Abstract

The effects of Al substitution and neutron irradiation on the conduction regime (clean or dirty) of the $π$- and $σ$-band of MgB$_{2}$ 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 $π$-band can be clearly observed in pure MgB$_{2}$, consistently with previous results. In Al-doped samples the dirty character of the $π$-band is increased, while no definitive conclusion on the conduction regime of the $σ$-band can be drawn. On the contrary, results obtained for the irradiated sample show that the irradiation-induced disorder drives the $σ$-band in the dirty regime, making the large gap in $σ$-band observable for the first time in far-infrared measurements.