Quantum disorder versus order-out-of-disorder in the Kugel - Khomskii model

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Abstract
The Kugel - Khomskii model, the simplest model for orbital degenerate magnetic insulators, exhibits a zero temperature degeneracy in the classical limit which could cause genuine quantum disorder. Khaliullin and Oudovenko (1997 Phys. Rev. B 56 R14243) suggested recently that instead a particular classical state could be stabilized by quantum fluctuations. Here we compare their approach with standard random phase approximation and show that it strongly underestimates the strength of the quantum fluctuations, shedding doubt on the survival of any classical state.
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