A Study of the Electronic Structure and the Effects of Oxygen on the Superconducting Properties of MgB$_2$ by Electron Energy Loss Spectroscopy

The transport properties of MgB$_2$ have a strong dependence on the incorporation of impurities such as oxygen. Different phases of oxygen precipitates have been characterized and it has been found that the presence of oxygen in MgB$_2$ systematically changes the electronic structure of the boron atoms. Among the different phases of oxygen precipitates found in MgB$_2$, those forming coherent superlattice structures of MgB$_2$- MgB$_x$O$_y$ were studied in detail by first principles calculations. This kind of precipitate has been reported to increase the upper critical fields and critical current density without decreasing the critical temperature of MgB$_2$. This effect is reflected by the low critical temperatures calculated for coherent oxygen precipitates with different concentrations of oxygen using density functional theory. These low critical temperatures explain the behavior of the oxygen precipitates as pinning centers and highlight the importance of oxygen on the superconducting properties of MgB$_2$. Additionally, due to the presence of two carrier species given by the B and Mg states in MgB$_2$, a low energy plasmon mode was theoretically proposed. This work presents the first experimental evidence of this plasmon mode, which has a quadratic dispersion, $\omega_p\propto q^2$.