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

TL;DR

This study investigates how substituting calcium for europium in EuB$_6$ alters its electronic structure, revealing a transition from semimetallic to insulating behavior with increasing Ca content through optical spectroscopy.

Contribution

It provides detailed optical spectroscopy data showing the evolution of electronic bands and the metal-insulator transition in Eu$_{1-x}$Ca$_x$B$_6$ with Ca substitution.

Findings

Interband spectral features shift to higher frequencies with Ca content.

Intraband spectral weight diminishes and vanishes for x ≥ 0.35.

A finite energy gap of approximately 0.25 eV opens at the X-point.

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, $0\leq x\leq 1$. 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 $x\geq x_{c}=0.35$. 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.