Theory of High \tc Ferromagnetism in $Sr{B}_6$ family: A case of Doped Spin-1 Mott insulator in a Valence Bond Solid Phase

Abstract

Doped divalent hexaborides such as $S{r}_{1-x}L{a}_x{B}_6$ exhibit high \tc 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 $\sim 0.1eV$, much smaller than the charge gap of $>1.0eV$ seen in ARPES. The experimentally seen high \tc `ferromagnetism' is argued to be a complex magnetic order in disguise - either a canted 6-sublattice AFM ($\approx 120^0$) order or its quantum melted version, a chiral spin liquid state, arising from a type of double exchange mechanism.

4 pages, 2 figures; minor corrections, references added

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