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Multiple-source 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 $σ_{1}(ω)$ decreases at three different energy $ω$=0.5, 1.4, 2.75 eV and the lost spectral weight is transferred to the Drude $σ_{1}(ω)$ at $ω<$0.33 eV. We succeeded in explaining this unprecedented multiple-energy $σ_{1}(ω)$ 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 $ρ(T)$ and $ω_{p}^{2}$ changes at T $<$ T$_{c}$.