Optical spectral weight changes accompanying ferromagnetic transition in EuB6

EuB6 have drawn much attention for the last decade due to their interesting electrical and magnetic properties. As a prerequisite to the understanding of these unconventional phenomena, the band structure of EuB6 was extensively studied both theoretically and experimentally. However, there is no consensus of opinion on the electronic ground state. We present temperature dependent optical reflectivity and spectroscopic ellipsometry measurements on ferromagnetic EuB$_{6}$ taken over a wide spectral range 0.002-5.5 eV. In addition to a large blue shift of the plasma frequency below T$c$=15.5 K, two prominent changes in the interband region are observed: a systematic decrease of the interband spectral weight below 3.3 eV and the splitting of 4 eV peak with a decrease in T. We can explain the observed optical spectral weight changes within the semi-metal picture of EuB6.