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

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.

Figures (21)

• Figure 1: Schematic illustration of time-resolved diffuse scattering. (A) Time-resolved non-resonant diffuse scattering to probe phonons. (B) Time-resolved resonant diffuse scattering to probe magnons. (Top) Periodic arrangement of atoms or spins produces corresponding Bragg reflections in reciprocal space and low-energy collective excitations. (Bottom) Photoexcitation shakes the atomic positions or randomizes spin orientations, represented by the creation of low-energy quasiparticles along the dispersion curve, i.e., phonons and magnons, respectively. The enhanced density of low-energy quasiparticles results in diffuse scattering around Bragg reflections as streaks, enabling momentum-resolved tracking of collective excitation dynamics in non-equilibrium states.
• Figure 2: Resonant diffuse scattering from non-equilibrium magnons.
• Figure 3: Momentum dependence of time-resolved resonant diffuse scattering.
• Figure 4: Energy transfer from magnons to phonons. (A-C) Possible magnon-phonon coupling mechanisms: (A) magnon-phonon interconversion, (B) magnon number-conserved scattering, and (C) anomalous scattering, where two magnons convert into a phonon. Insets show corresponding Feynman diagrams, where q and k represent the momentum of quasiparticles. (D) Long timescale comparison of the $(1/2\ 0\ {-1/2})$ RXD intensity (green), RDS intensity at $(0.47, 0, -0.53)$ (orange) (bottom), and NRDS intensities integrated around the $(0\ 0\ {-2})$ Bragg peak within 0.1 r.l.u. (Supplementary Text) in the collinear AFM (blue) and PM (red) phases (top), revealing a correlation between magnon annihilation and phonon creation during magnetic recovery. The used pump laser fluence was 3 mJ/cm$^{2}$. Error bars represent standard error. (E, F) Simulated two-dimensional distribution patterns of the scattering rate for the lowest-energy optical phonons (E) and the acoustic magnons (F) via the anomalous scattering. Here, we use the simplified orthorhombic unit cell, as mentioned in the text.
• Figure S1: Schematic of experimental setup for tr-RXD and tr-RDS measurements.