Hierarchical quasiparticle dynamics in antiferromagnets revealed by time- and momentum-resolved X-ray scattering

TL;DR

This study uncovers the hierarchical energy transfer pathways in photoexcited antiferromagnetic CuO using advanced X-ray scattering and simulations, revealing ultrafast spin dynamics and magnon interactions across momentum space.

Contribution

It introduces a momentum-resolved approach to observe low-energy magnons and elucidates the hierarchical energy flow in antiferromagnets, advancing understanding beyond phenomenological models.

Findings

Femtosecond generation of non-thermal magnons after excitation

Real-time momentum-resolved magnon thermalization within picoseconds

Recovery bottlenecks caused by quasiparticle dispersion mismatch

Abstract

Energy flows among coupled subsystems are essential for ultrafast dynamics and high-speed technologies. In magnetic materials, spin fluctuations -- magnons -- mediate these flows in ultrafast magnetism. Yet momentum-resolved access to low-energy magnons governing the microscopic dynamics has been lacking. Using time-resolved resonant diffuse scattering alongside complementary time-resolved X-ray techniques and quantum-kinetic simulations, we unveil the hierarchical energy pathways among correlated systems in the photoexcited antiferromagnet CuO. Above-bandgap excitation triggers near-instantaneous spin disorder, generating non-thermal magnons throughout reciprocal space within femtoseconds. Real-time momentum-resolved tracking reveals picosecond magnon quasi-thermalization, followed by nanosecond recovery via momentum-selective magnon-phonon scattering. The quasiparticle dispersion mismatch creates recovery bottlenecks that control non-equilibrium lifetimes. This microscopic framework transcends phenomenological models and generalizes across materials, establishing design principles for ultrafast control of material properties.