Boltzmann transport theory of magnon-exciton drag
Phys. Rev. B 113, 245424 – Published 23 June, 2026
DOI: https://doi.org/10.1103/s8bd-vb42
Abstract
We develop a microscopic theory of magnon-exciton drag effect in a bilayer van der Waals antiferromagnetic semiconductor CrSBr. Effective exciton-magnon coupling arises from an orbital mechanism: Magnons tilt the layer magnetizations, enabling charge carrier tunneling that mixes intralayer and interlayer excitons and thereby modulates the exciton energy. We derive the effective Hamiltonian of exciton-magnon coupling, based on our calculation of the magnon spectrum taking into account short-range exchange interaction between Cr-ion spins, single-ion anisotropy, and long-range dipole-dipole interactions. We show that despite rather small renormalization of exciton's energy and effective mass by the exciton-magnon interaction, the three key two-magnon processes, exciton-magnon scattering, two-magnon absorption by an exciton, and two-magnon emission, are highly efficient. By solving the Boltzmann kinetic equation, we evaluate a short exciton-magnon scattering time which is in the sub-ps range and strongly decreases with the increase in magnon population. Hence, exciton-magnon scattering is likely to be dominant over other scattering processes related to exciton-phonon and exciton-disorder interactions. We demonstrate that nonequilibrium magnons can efficiently drag excitons, resulting in a large and nearly isotropic exciton propagation that can significantly exceed the intrinsic anisotropic diffusion. Our results provide a theoretical basis for recent observations of anomalous exciton transport in CrSBr [F. Dirnberger et al., Nat. Nanotechnol. 21, 65 (2026)] and establish magnon-exciton drag as a powerful mechanism for controlling exciton propagation in magnetic systems.
