Optical spectroscopy study of the electronic structure of Eu1-xCaxB6

Abstract

The room temperature optical conductivity sigma(1)(omega) of Eu1-xCaxB6 has been obtained from reflectivity and ellipsometry measurements for a series of compositions, 0 <= x <= 1. The interband part of sigma(1)(omega) shifts continuously to higher frequency as Ca content x increases. Also the intraband spectral weight of sigma(1)(omega) decreases rapidly and essentially vanishes for x >= x(c)=0.35. These results show that the valence band and the conduction band of Eu1-xCaxB6 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 EuB6 to the insulating CaB6 where the two bands are separated to open a finite gap (similar or equal to 0.25 eV) at the X point of the Brillouin zone.