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

TL;DR

This study uses far-infrared spectroscopy to explore how aluminum doping and neutron irradiation affect the conduction regimes of MgB2's bands, revealing disorder-induced changes in superconducting gaps.

Contribution

It provides new insights into how doping and irradiation alter the clean or dirty nature of MgB2's bands, especially observing the large gap in the sigma-band after irradiation.

Findings

Al doping increases the dirty character of the π-band.

Neutron irradiation drives the σ-band into the dirty regime.

The large gap in the σ-band becomes observable after irradiation.

Abstract

The effects of Al substitution and neutron irradiation on the conduction regime (clean or dirty) of the $\pi$- and $\sigma$-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 $\pi$-band can be clearly observed in pure MgB$_{2}$, 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 $\sigma$-band in the dirty regime, making the large gap in $\sigma$-band observable for the first time in far-infrared measurements.