Geometric Origin of Chiral Magnons in Antiferromagnets

Chiral magnons are the fundamental carriers of information in magnonic computers, where data is processed and transmitted through spin waves instead of electric currents. Unconventional antiferromagnets (or altermagnets) have emerged as a new, promising platform for ultrafast and low-dissipation magnonics. They combine many sought-after functional properties of conventional ferro- and antiferromagnets, while overcoming a central limitation of the latter -- namely, the lack of chiral magnons. We analytically demonstrate that chiral magnons in antiferromagnets arise from the quantum geometry of the normal-state wave functions. This follows from an analysis of quasiparticle-pole multiplicity in the Dyson equation within the random-phase approximation. We validate the here-derived relations by computing the quantum geometry and excitation spectrum of MnF₂, an altermagnetic insulator in which the existence of chiral magnons has been the subject of recent debate. We show that the nontrivial quantum metric of the material’s ground state makes altermagnetic magnon band splitting inevitable. We connect our findings to experiment by computing the circular dichroism and dynamical structure factor, which probe the system’s quantum metric and spin-wave spectrum, respectively.