Magnon pairing in quantum spin nematic

Competing ferro-and antiferromagnetic exchange interactions may lead to the formation of bound magnon pairs in the high-field phase of a frustrated quantum magnet. With decreasing field, magnon pairs undergo a Bose-condensation prior to the onset of a conventional one-magnon instability. We develop an analytical approach to study the zero-temperature properties of the magnon-pair condensate, which is a bosonic analog of the BCS superconductors. The representation of the condensate wave function in terms of the coherent bosonic states reveals the spin-nematic symmetry of the ground state and allows one to calculate various static properties. Sharp quasiparticle excitations are found in the nematic state with a small finite gap. We also predict the existence of a long-range-ordered spin-nematic phase in the frustrated chain material LiCuVO(4) at high fields. Copyright (C) EPLA, 2010