ORBITAL ORDERING IN TRANSITION-METAL COMPOUNDS: I. THE 120-DEGREE MODEL

Abstract:

We study the classical version of the 120◦ -model. This is an attractive nearest-neighbor system in three dimensions with XY (rotor) spins and interaction such that only a particular projection of the spins gets coupled in each coordinate direction. Although the Hamiltonian has only discrete symmetries, it turns out that every constant field is a ground state. Employing a combination of spin-wave and contour arguments we establish the existence of long-range order at low temperatures. This suggests a mechanism for a type of ordering in certain models of transitionmetal compounds where the very existence of long-range order has heretofore been a matter of some controversy.

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