Bose-Einstein condensation of bound magnon pairs (Vol. 42, No. 1)

Strong quantum fluctuations can destroy conventional dipole-type magnetic ordering. A quantum magnet remains, then, in a disordered spin-liquid state down to zero temperature. The enhanced fluctuations may also stabilize magnetic analogues of liquid crystals, states with partially broken rotational symmetries characterized by tensor order parameters. The prime candidates for such exotic spin-nematic states are frustrated magnetic systems with competing interactions.

Here, we investigate theoretically a microscopic mechanism for the spin-nematic ground state based on the competition of ferro- and antiferromagnetic interactions. In a strong magnetic field, local magnetic moments become completely polarized. Elementary excitations are single spin-flips or magnons. In the majority of quantum antiferromagnets spin-flips repel each other. Upon decreasing external field this leads to single-particle condensation, which can be regarded as an analogue of the Bose-Einstein condensation discovered for cold atomic gases in optical traps. Various exotic quantum states of bosonic particles find their analog in conventional magnetic structures.

The above conventional scenario changes if some of the exchange bonds are ferromagnetic. In this case, spin flips gain the interaction energy by occupying two adjacent sites. This may lead to formation of bound magnon pairs. Because of their lower energy, the bound pairs start to condense prior to the onset of single particle condensation (Fig.). We have developed the microscopic description of the Bose condensation of bound magnon pairs, which form a quantum state analogous to the condensate of electron pairs in superconductors. Our theory predicts the presence of such a spin-nematic phase in the frustrated chain material LiCuVO₄. The pulsed-field measurements on LiCuVO₄ and other related compounds should lead to the first observation of this exotic off-diagonal order in solid-state systems.