  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.
