CaB6: A New Semiconducting Material for Spin Electronics

Ferromagnetism was recently observed at unexpectedly high temperatures in La-doped ${\mathrm{CaB}}_6$. The starting point of all theoretical proposals to explain this observation is a semimetallic electronic structure calculated for ${\mathrm{CaB}}_6$ within the local density approximation. Here we report the results of parameter-free quasiparticle calculations of the single-particle excitation spectrum which show that ${\mathrm{CaB}}_6$ is not a semimetal but a semiconductor with a band gap of $0.8\pm 0.1\mathrm{eV}$. Magnetism in ${\mathrm{La}}_x{\mathrm{Ca}}_{1-x}{\mathrm{B}}_6$ occurs just on the metallic side of a Mott transition in the La-induced impurity band.