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Valence bond crystal in a pyrochlore antiferromagnet with orbital degeneracy

S. Di Matteo·G. Jackeli·C. Lacroix·N. B. Perkins
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Abstract

We discuss the ground state of a pyrochlore lattice of threefold-orbitally-degenerate $S=1/2$ magnetic ions. We derive an effective spin-orbital Hamiltonian and show that the orbital degrees of freedom can modulate the spin exchange, removing the infinite spin-degeneracy characteristic of pyrochlore structures. The resulting state is a collection of spin-singlet dimers, with a residual degeneracy due to their relative orientation. This latter is lifted by a magneto-elastic interaction, induced in the spin-singlet phase-space, that forces a tetragonal distortion. Such a theory provides an explanation for the helical spin-singlet pattern observed in the B-spinel MgTi$_2$O$_4$.

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S. Di Matteo, G. Jackeli, C. Lacroix, N. B. Perkins. 2004-06-23. Valence bond crystal in a pyrochlore antiferromagnet with orbital degeneracy. https://doi.org/10.1103/physrevlett.93.077208

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