Optical spectroscopy study on the electronic structure of $E{u}_{1-x}C{a}_x{B}_6$

Abstract

The optical conductivity ${\sigma }_1(\omega )$ of Eu${}_{1-x}$Ca${}_x$B${}_6$ has been obtained from reflectivity and ellipsometry measurements for series of compositions, $0\le x\le 1$. 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 $x\ge x_c=0.35$. These results show that the valence band and the conduction band of Eu${}_{1-x}$Ca${}_x$B${}_6$ 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 ($\simeq 0.25$ eV) at the X-point of the Brillouin zone.

5 pages, 4 figures