Electronic Fine Structure in the Electron-Hole Plasma inSrB6

Electron-hole mixing-induced fine structure in alkaline earth hexaborides leads to lower energy (temperature) scales, and thus a stronger tendency toward an excitonic instability than in their doped counterparts (viz. ${\mathrm{Ca}}_{1-x}{\mathrm{La}}_x{\mathrm{B}}_6$, $x\approx 0.005\backslash )$, which are high-Curie-temperature, small-moment ferromagnets. Comparison of Fermi surfaces and spectral distributions with de Haas–van Alphen, optical, transport, and tunneling data indicates that ${\mathrm{SrB}}_6$ remains a fermionic semimetal down to (at least) 5 K, rather than forming an excitonic condensate. For the doped system the Curie temperature is higher than the degeneracy temperature.