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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. 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 LiCuVO4 at high fields.