Infrared and magneto-optical studies of novel condensed matter systems

We will present the results of our recent infrared and magneto-optical measurements on high temperature supercondutors La$_{2-x}$Sr$_x$CuO$_4$, heavy fermion metal YbFe$_4$Sb$_{12}$ and half-metallic ferromagnet Mn$_5$Ge$_3$. The results demonstrate the power of spectroscopic techniques, which allow one to study materials in extreme experimental conditions, such as temperature as low as 4.2 K and magnetic field as high as 33 Tesla. The results reveal failure of free-electron model for these novel condensed matter systems and importance of electron-electron correlations.