Stability of chiral magnon condensates in collinear antiferromagnetic insulators

Quasiequilibrium magnon Bose-Einstein condensates in ferromagnetic insulators have been a field of great interest, while condensation in antiferromagnetic systems has not yet been explored in detail. We analyze the stability of condensed chiral magnons in two antiferromagnetic insulators: a uniaxial easy-axis system and a biaxial system. We show that two-component magnon condensation and intermagnon interactions are essential to create metastable magnon condensation. The uniaxial system with a Rashba-type Dzyaloshinskii-Moriya interaction supports two degenerate condensate populations at finite wave vectors. We find that the condensation state in this model is (meta)stable only when the distribution of condensed magnons between the two populations is symmetric. In addition, we demonstrate the emergence of a zero-sound-like Goldstone mode in antiferromagnetic systems that support two-component magnon condensation. On the other hand, in the biaxial system without Dzyaloshinskii-Moriya interaction, we predict that the magnon condensate cannot stabilize due to the breaking of the magnon degeneracy. Our results suggest that this instability is a general characteristic of single-component quasiequilibrium quasiparticle condensates.