Electron energy-loss spectroscopy study of electron-doping in MgB$_2$

TL;DR

This study investigates how electron doping affects the electronic structure of MgB$_2$ using EELS and first-principles calculations, revealing band filling effects that influence superconductivity.

Contribution

It provides detailed experimental and theoretical analysis of electron doping effects on MgB$_2$'s electronic structure, highlighting the role of band filling and interband scattering.

Findings

Both $\sigma$ and $\pi$ bands are filled with doping.

No significant electronic structure difference between Mg and B site doping.

Band filling alone cannot explain all superconducting property variations.

Abstract

The electronic structure of electron-doped polycrystalline Mg$_{1-x}$Al$_{x}$(B$_{1-y}$C$_{y}$)$_2$ was examined by electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) and first-principle electronic structure calculations. We found significant changes in the boron $K$ edge fine structure, suggesting the two bands of the B $K$ edge, the $\sigma$ and the $\pi$ band are being simultaneously filled as the electron doping concentration of Mg$_{1-x}$Al$_{x}$(B$_{1-y}$C$_{y}$)$_2$ was increased. Our density-functional theory calculations confirm the filling of the $\sigma$ band states close to the Fermi level, which is believed to cause the loss of superconductivity in highly doped MgB$_2$, since the electron-phonon coupling of these states is thought to be responsible for the high superconducting transition temperature. Our results do not show significant differences in the electronic structure for electron doping on either the Mg or the B site, although many superconducting properties, such as $T_c$ or H$_{c_2}$ differ considerably for C- and Al- doped \MB. This behavior can not be satisfactorily explained by band filling alone, and effects such as interband scattering are considered.