Valence-bond crystal in a pyrochlore antiferromagnet with orbital degeneracy.
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 magnetoelastic 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 MgTi2O4.
Laboratori Nazionali di Frascati INFN, via E. Fermi 40, C.P. 13, I-00044 Frascati (Roma) Italy.
S
S
Di Matteo
G
G
Jackeli
C
C
Lacroix
N B
NB
Perkins