Competition between thermal and quantum fluctuations in a Heisenberg rhombohedral antiferromagnet

Abstract

The Heisenberg rhombohedral antiferromagnet (RAF) in the classical approximation is characterised by infinite degeneracy of the ground state corresponding to infinite inequivalent helices whose wavevectors Q belong to lines in the reciprocal space called degeneration lines LQ. A rich unusual scenario that called a degenerate helix (DH) is set up: this entails the presence of 'soft lines' in the magnon energy spectrum, the absence of LRO in 3D at any finite temperature and possible algebraic decay of the correlation function. Zero-point-motion fluctuations destroy this phenomenology, picking out a certain helix of the manifold LQ. It may be conjectured that the classical DH scenario could reappear at intermediate temperature when thermal fluctuations overcome quantum fluctuations. Indeed, the authors have found that this is the case-and conclude that the DH is not a mere artefact of the classical approximation but a genuine possibility for helimagnetic systems.