Dimer phases in quantum antiferromagnets with orbital degeneracy

Georgi Jackeli (Max Planck Institute, Stuttgart)

We study and solve the ground-state problem of a microscopic model for a family
of orbitally degenerate quantum magnets. The orbital degrees of freedom are
assumed to have directional character and are represented by static Potts-like
variables. In the limit of vanishing Hund's coupling, the ground-state manifold
of such a model is spanned by the hard-core dimer (spin singlet) coverings of
the lattice. The extensive degeneracy of dimer coverings is lifted at a finite
Hund's coupling through an order-out-of-disorder mechanism by virtual triplet
excitations. The relevance of our results to several experimentally studied
systems is discussed.