Optical spectroscopy study on the electronic structure of Eu1-x Ca xB6

The divalent hexaboride compounds RB$_{6}$ have drawn much attention for the last decade due to their interesting electrical and magnetic properties. In EuB$_{6}$, the optical spectroscopy measurement showed an unusual shift of the plasma frequency across the ferromagentic transition. While Ca~bears no magnetic moment, CaB$_{6}$~exhibits~weak ferromagnetism at high temperature. Although the~band structure is a~prerequisite to the understanding of these unconventional phenomena,~the consensus among the theoretical and experimental works has not been reached. In this work, the optical conductivity of Eu$_{1-x }$Ca$_{x}$B$_{6}$ has been obtained from reflectivity and ellipsometry measurements for series of compositions, 0 $<$ x $<$1. The interband part of $\sigma _{1}(\omega )$ shifts continuously to higher frequency as x increases. Also the intraband spectral weight 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 }$move away from each other such that their band overlap decreases with increasing Ca. As a result, the electronic state evolves from the semimetallic structure of EuB$_{6}$ to the insulating CaB$_{6}$ where the two bands are separated to open a finite gap ($\sim $0.25 eV) at the X-point of the Brillouin zone.