Critical Field of Al-Doped MgB2 Samples: Correlation with the Suppression of Sigma-Band Gap

TL;DR

This study investigates how aluminum doping affects the upper critical magnetic field in MgB2 superconductors, revealing a direct link between the critical field and the sigma-band gap, with a transition from clean to dirty regimes at higher doping levels.

Contribution

It establishes a clear correlation between Al doping, the suppression of the sigma-band gap, and the critical field behavior in MgB2, providing new insights into the doping effects on superconductivity.

Findings

Bc2 decreases with Al doping due to sigma-gap suppression

Transition from clean to dirty limit occurs at x=0.2 doping

Strong correlation between Bc2 and sigma-band gap

Abstract

In this Letter, the study of the effect of Al substitution on the upper critical field, Bc2, in AlxMg1-xB2 samples is presented. We find a straightforward correlation between Bc2 and the sigma-band gap, Delta_sigma, evaluated by point-contact measurements. Up to x=0.2 Bc2 can be well described within a clean limit model and its decrease with x is directly related to the suppression of Delta_sigma. For larger doping we observed the crossover to the dirty regime driven mostly by the strong decrease of Delta_sigma rather than by the increase of the sigma-band scattering rate