Self-energy of half-metallic ferromagnet Mn$_5$Ge$_3$ calculated from infrared spectroscopy data

We will report the results of our infrared and optical spectroscopy study of a half-metallic ferromagnet Mn$_5$Ge$_3$. This compound is currently being investigated as potential injector of spin polarized currents into germanium. Infrared measurements have been performed over a broad frequency (30 - 50000~cm$^{-1}$) and temperature (10 - 300 K) range. From the complex optical conductivity $\sigma(\omega)$ we extract the electron self-energy $\Sigma(\omega)$. The calculation of $\Sigma (\omega)$ is based on novel numerical technique which uses Levenberg--Marquardt algorithm for solution of systems of non- linear equations. Obtained self-energy provides a new insight into electron correlations in Mn$_5$Ge$_3$.