Optical spectroscopy study of the electronic structure of Eu _1-xCa _xB ₆

The room temperature optical conductivity σ ₁(ω) of Eu _1-xCa _xB ₆ has been obtained from reflectivity and ellipsometry measurements for a series of compositions, 0 ≤x≤1. The interband part of σ ₁(ω) shifts continuously to higher frequency as Ca content x increases. Also the intraband spectral weight of σ ₁(ω) decreases rapidly and essentially vanishes for x≥x _c=0.35. These results show that the valence band and the conduction band of Eu _1-xCa _xB ₆ move away from each other such that their band overlap decreases with increasing Ca substitution. As a result, the electronic state evolves from the semimetallic structure of EuB ₆ to the insulating CaB ₆ where the two bands are separated to open a finite gap (≃0.25 eV) at the X point of the Brillouin zone.