Boltzmann transport theory of magnon-exciton drag

TL;DR

The paper develops a microscopic Boltzmann framework for coupled exciton-magnon transport in CrSBr bilayers, revealing an orbital exciton-magnon coupling mediated by interlayer tunneling that is enabled by canting of layer magnetizations. It derives a realistic magnon spectrum including short-range exchange and long-range dipole-dipole interactions, predicts a negative group velocity for the low-energy magnon branch, and shows that three two-magnon processes lead to sub-picosecond exciton-magnon scattering rates. The exciton-magnon polaron is found to be weak, while magnon-driven drag can synchronize exciton and magnon flows, producing large, nearly isotropic exciton diffusion that can exceed intrinsic anisotropic diffusion. These results provide a theoretical basis for observed anomalous exciton transport in CrSBr and establish magnon-exciton drag as a robust mechanism to control exciton propagation in layered magnetic semiconductors.

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 intra- and interlayer excitons and thereby modulate 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. The latter produces a negative group velocity of magnons at small wavevectors. 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 exciton, and two-magnon emission are highly efficient. By solving the Boltzmann kinetic equation, we evaluate short exciton-magnon scattering time which is in the sub-ps range and strongly decreases with the increase of magnon population. Hence, exciton-magnon scattering is likely to be dominant over other scattering processes related to the exciton-phonon and exciton-disorder interactions. We demonstrate that 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. Nano. (2025)] and establish magnon-exciton drag as a powerful mechanism for controlling exciton propagation in magnetic systems.

Figures (9)

• Figure 1: Schematic illustration of magnon-exciton drag effect. Magnons (wavy red arrows) propagate out of the excitation spot and drag excitons (blue balls). Inset shows antiferromagnet bilayer under study.
• Figure 2: Magnon dispersion for the full Brillouin zone. On this scale two magnon modes are indistinguishable. Inset shows magnon dispersion near the $\Gamma$-point, left magenta lines correspond to the $\Gamma\to X$ direction, right green lines correspond to $\Gamma \to Y$ direction. Dotted and dashed lines show $E^+_{\bm k}$ and $E^-_{\bm k}$, respectively, Eq. \ref{['energ:gen']}. Thin solid black lines show dispersion without dipole-dipole interaction.
• Figure 3: In equilibrium, the magnetization of the neighboring layers is antiparallel and the states of electrons are orthogonal. In the presence of magnons the magnetization is tilted and the mixing of intralayer and interlayer exciton is allowed.
• Figure 4: Exciton-magnon polaron binding energy as a function of temperature. Solid red line is numerical calculation, Eq. \ref{['eq:Exmp-exact']}, dotted blue and dashed black lines are analytical approximations (Eq. \ref{['eq:num1']} and Eq. \ref{['eq:num2']}, respectively). Inset shows the relative polaron mass correction ($\Delta M^{\alpha}_{XMP} / M^\alpha = M_{XMP}^\alpha / M^\alpha - 1$) as a function of temperature, $\alpha = x$ (magenta) and $\alpha = y$ (green).
• Figure 5: Schematic illustration of the relevant two-magnon processes: (a) scattering, (b) two-magnon absorption and (c) two-magnon emission.