Order produced by quantum disorder in the Heisenberg rhombohedral antiferromagnet

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Abstract
In the classical approximation the Heisenberg rhombohedral antiferromagnet shows infinite degeneracy of the ground-state energy corresponding to infinite inequivalent iso-energetic helices whose wave-vectors Q belong to lines in the reciprocal space, called 'degeneration lines'. In absence of anisotropy the spin wave energy spectrum shows 'soft lines' vanishing for all wave-vectors falling on the degeneration lines. These soft lines destroy long-range order (LRO) at any finite temperature. The authors show that the zero-point motion, which is generally expected to contrast with the onset of LRO, favours in this case a single wave-vector removing the infinite degeneracy of the ground state, so LRO is restored by quantum disorder.