Antiphase oscillations in the time-resolved spin structure factor of photoexcited two-dimensional Mott Insulator

The recent development of time-resolved resonant inelastic x-ray scattering (TRRIXS) opens a new avenue for probing collective two-particle excitation, from which one can investigate novel photoinduced nonequilibrium phenomena in the wide range of momentum and energy spaces. RIXS can probe not only charge excitation but also magnetic excitation if one uses incident x rays tuned for L edge in transition metals. If the lifetime of an intermediate state in the L-edge RIXS process is short enough, the dominant contribution to the RIXS spectrum comes from the dynamical charge and spin structure factors. It is also numerically shown that even for a realistic lifetime scale of an intermediate state in cuprate materials, the magnetic excitation in RIXS gives information on the dynamical spin structure factor [1]. Therefore, TRRIXS is an ideal tool for characterizing transient spin dynamics.
We theoretically investigate momentum dependent spin excitation that evolves after pumping within a femtosecond timescale in the antiferromagnetic Mott insulator on a square lattice [2]. Using a numerically exact-diagonalization technique based on the time-dependent Lanczos method for a half-filled Hubbard model with a 4×4-site lattice, we find novel momentum-dependent transient spin dynamics. In particular, we demonstrate characteristic temporal oscillations for the intensity of the dynamical spin structure factor, showing an antiphase behavior for two orthogonal directions in the momentum space, which are parallel and perpendicular to the electric field of a pump pulse. The same behavior is also seen in the static spin structure factor. Their oscillation period in time is determined by two-magnon excitation in the Mott insulator. This theoretical prediction will be confirmed for Mott insulating cuprates and iridates once TRRIXS is ready for a femtosecond timescale.
[1] T. Tohyama and K. Tsutsui, Int. J. Mod. Phys. B 32, 1840017 (2018).
[2] K. Tsutsui, K. Shinjo, and T. Tohyama, Phys. Rev. Lett. 126, 127404 (2021).