1 99 9 Excitonic ferromagnetism in the hexaborides

In a recent letter to Nature Young et al . reported ferromagnetism in Ca1−xLaxB6 — a material with no partially filled dor f -orbitals. Three features are noteworthy, the narrow concentration range of doping (0 ∼ x < ∼ 0.01), the small magnetic moment (∼ 0.07μB/La), and the high Curie temperature (TC ∼ 600 K). The parent compound CaB6 is a poor conductor which shows a small overlap EG (< 0) between a boron-derived valence band and a calcium-derived conduction band at the X-point in the Brillouin zone. The valence and conduction bands belong to different irreducible representations (X ′ 3 and X3, respectively) such that the interband dipole matrix element vanishes at the X-point. As a result the dielectric constant (κ) and the effective masses (mh and me) remain finite as EG → 0, which, as pointed by Halperin and Rice, favors formation of an excitonic condensate.