Multiple-source optical spectral weight transfer in ferromagnetic EuB6

TL;DR

This study provides the first wide-range optical conductivity measurements of EuB6, revealing multiple spectral weight transfers at ferromagnetic transition, explained by band structure calculations, confirming EuB6 as a semimetal with exchange-driven band splitting.

Contribution

First comprehensive optical conductivity data for EuB6 across a broad energy range, linking spectral weight transfer to band structure and magnetic transition.

Findings

Spectral weight decreases at specific energies upon ferromagnetic transition.

Spectral weight transfers to low-energy Drude response.

EuB6 confirmed as a semimetal with exchange-driven band splitting.

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, $\textbf{69}$,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$_{c}$.