Johan H. Mentink
Altermagnetism deals with space-time symmetries of magnetic media in the exchange approximation. Hence, it will be conceptually very appealing to explore consequences of altermagnetism for magnons at the edge of the Brillouin zone, since their energy is determined by the exchange interaction. However, understanding experimentally observed magnon spectra at such short wavelengths has been challenging even for simple high-symmetry models such as the antiferromagnetic Heisenberg model on a square lattice.
To gain insight into this problem, we review recent theoretical studies [1-4] focused on the space-time dynamics of pairs of counterpropagating antiferromagnetic magnons at the edge of the Brillouin zone. We highlight that the dynamics of such magnon pairs is most naturally explained by considering the spin correlation function as main observable [3,4], making it distinct from the conventional macrospin approximation for magnons, which considers the ferro- or antiferromagnetic order parameters as main observables [5]. Furthermore, we show that the propagation of spin correlations features a d-wave symmetry in real space-time and can occur at velocities that are higher than the highest magnon velocity. This latter can thus be termed supermagnonic propagation and is shown to be caused by the discrete nature of the spins themselves [2]. Finally, we derive a minimal model for the selection rules spin correlations [5], which we find to be consistent with experimental results on time-resolved detection of magnon pairs. Although derived for high-symmetry cubic systems, we emphasize that our model can be easily extended to low-symmetry antiferromagnets, holding the promise for the discovery of altermagnetic magnon spin currents at the space and time scale of the exchange interaction.
[1] G. Fabiani & J.H. Mentink, SciPost Phys 7, 004(2019) [2] G. Fabiani, M.D. Bouman and J.H. Mentink, Phys. Rev. Lett. 127, 097202 (2021) [3] G. Fabiani & J.H. Mentink, PRB 105, 094438 (2022) [4] A. Fedianin, A. Kalashnikova and J.H. Mentink, arXiv:2212.14698, accepted PRB (2023) [5] F. Formisano et al., preprint at arXiv:2303.06996 (2023)