Multiple-source optical spectral weight transfer in ferromagnetic EuB6

Abstract

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, $\text{69}$,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${}_c$.

5 pages, 5 figures