Optical spectroscopy study of the electronic structure of Eu 1 − x Ca x B 6

The room temperature optical conductivity {sigma}{sub 1}({omega}) of Eu{sub 1-x}Ca{sub x}B{sub 6} has been obtained from reflectivity and ellipsometry measurements for a series of compositions, 0{approx} =xc = 0.35. These results show that the valence band and the conduction band of Eu{sub 1-x}Ca{sub x}B{sub 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{sub 6} to the insulating CaB{sub 6} where the two bands are separated to open a finite gap ({approx}0.25 eV) at the X point of the Brillouin zone.