Multiple-Structure Optical Spectral Weight Transfer in Ferromagnetic EuB6

We present the first wide range (2 meV$-$5.5 eV) optical conductivity of EuB$_6$ from reflectivity and ellipsometry measurements. Upon the ferromagnetic transition at T$_{c}$=15.5 K, interband transition $\sigma_{1}(\omega)$ decreases at three different energy $\omega$=0.5, 1.4, 2.75 eV and the lost spectral weight is transferred to the Drude $\sigma_{1}(\omega)$ at $\omega<$0.33 eV. We succeeded in explaining this unprecedented multiple-energy $\sigma_{1}(\omega)$ change using LDA+U calculated band structure by Kunes and Pickett (Phys. Rev. B, $\textbf{69}$,165111). Our finding supports strongly that (1) EuB$_6$ is a semimetal and (2) the exchange-driven band splitting is the primary source of the drastic $\rho(T)$ and $\omega_{p}^{2}$ changes at T $<$ T$_{c}$.