X-ray absorption and optical spectroscopy studies of (Mg$_{1-x}$Al$_x$)B$_2$

TL;DR

This study investigates how aluminum doping affects the electronic structure and superconductivity in (Mg$_{1-x}$Al$_{x}$)B$_{2}$ using X-ray absorption and optical spectroscopy, revealing a correlation between hole carrier reduction and $T_c$ suppression.

Contribution

It provides a detailed experimental analysis of the evolution of electronic structure with Al doping and links it to superconducting properties, including the role of superstructure effects.

Findings

Hole carrier concentration decreases with Al doping.

Superconducting transition temperature $T_c$ is most suppressed at x=0.4.

Electronic structure evolution is gradual despite significant $T_c$ suppression.

Abstract

X-ray absorption spectroscopy and optical reflectance measurements have been carried out to elucidate the evolution of the electronic structure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$ for $\emph{x}$ = 0.0,0.1, 0.2, 0.3, and 0.4. The important role of B 2$\emph{p}$ $\sigma$ hole states to superconductivity has been identified, and the decrease in the hole carrier number is $\emph{quantitatively}$ determined. The rate of the decrease in the hole concentration agree well with the theoretical calculations. On the other hand,while the evolution of the electronic structure is gradual through the doping range, $T_c$ suppression is most significant at $\emph{x}$ = 0.4. These results suggest that the superstructure in (Mg$_{1-x}$Al$_{x}$)B$_{2}$, in addition to the $\sigma$ holes, can affect the lattice dynamics and contributes to the $T_c$ suppression effect. Other possible explanations like the topological change of the $\sigma$ band Fermi surface are also discussed.