Optical spectroscopy and electronic band structure of ferromagnetic EuB₆

We present an optical conductivity of EuB₆ over a wide range (2 meV-5.5 eV) from reflectivity and ellipsometry measurements. Upon the ferromagnetic transition at T_c =15.5 K, interband transition σ₁ (ω) decreases below ω=3.3 eV and the lost spectral weight is transferred to the Drude σ₁ (ω) at ω<0.33 eV. Further a high-energy optical peak at around 4 eV exhibits a large splitting below T_c. We explain these high-energy σ₁ (ω) changes, that is, spectral-weight recovery and peak splitting, using local-density approximation+U band-structure calculations. Our finding suggests strongly that: (1) EuB₆ is a semimetal and (2) the exchange-driven band splitting is the primary source of the drastic ω_p² change at T<T_c.