High-Tc ferromagnetism in the SrB6 family:: A case of doped spin-1 Mott insulator in a valence-bond solid phase -: art. no. 212403

Doped divalent hexaborides such as Sr1-xLaxB6 exhibit a high T-c ferromagnetism. We isolate a degenerate pair of 2p orbitals of boron with two valence electrons, invoke electron correlation and Hund coupling, to suggest that the undoped state is better viewed as a spin-1 Mott insulator; it is predicted to be a type of 3d Haldane gap phase with a spin gap similar to0.1 eV, much smaller than the charge gap of >1.0 eV seen in angle-resolved photoemission spectroscopy. The experimentally seen high T-c "ferromagnetism" is argued to be a complex magnetic order in disguise-either a canted six-sublattice antiferromagnetic (approximate to120degrees) order or its quantum melted version, a chiral spin liquid state, arising from a type of double-exchange mechanism.