Optical spectroscopy study on the electronic structure of $Eu_{1-x} Ca_x B_6$

摘要

The optical conductivity $\sigma_{1}(\omega)$ of Eu$_{1-x}$Ca$_{x}$B$_{6}$has been obtained from reflectivity and ellipsometry measurements for series ofcompositions, $0\leq x\leq 1$. The interband part of $\sigma_{1}(\omega) $shifts continuously to higher frequency as Ca-content $x$ increases. Also theintraband spectral weight of $\sigma_{1}(\omega)$ decreases rapidly andessentially vanishes for $x\geq x_{c}=0.35$. These results show that thevalence band and the conduction band of Eu$_{1-x}$Ca$_{x}$B$_{6}$ move awayfrom each other such that their band overlap decreases with increasingCa-substitution. As a result, the electronic state evolves from thesemimetallic structure of EuB$_{6}$ to the insulating CaB$_{6}$ where the twobands are separated to open a finite gap ($\simeq 0.25$ eV) at the X-point ofthe Brillouin zone.