Spin Dynamics in a Photoexcited Mott Insulator on a Square Lattice

The recent development of time-resolved resonant inelastic x-ray scattering (trRIXS) opens a new avenue for probing collective two-particle excitations, 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 single-magnetic excitation if one uses incident x rays tuned for L edge in transition metals. Therefore, trRIXS is a good tool to investigate single-magnon excitation in photoexcited Mott insulators. On the other hand, time-resolved two-magnon Raman scattering (trTMRS) is suitable for investigating two-magnon excitation in photoexcited Mott insulators. In order to give theoretical predictions to trRIXS, we investigate momentum dependent single-magnon excitation that evolves after pumping within a femtosecond timescale in an antiferromagnetic Mott insulator on a square lattice [1]. Using a numerically exact-diagonalization technique based on the time-dependent Lanczos method for a half-filled Hubbard model on a square lattice, we find a characteristic temporal oscillation for the intensity of the dynamical spin correlation function describing single-magnon excitation. This oscillation shows an antiphase behavior for two orthogonal directions that 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. The photoexcitation of the Mott insulator on a square lattice weakens the intensity of two- magnon excitation as observed in trTMRS [2]. However, the spectral changes in the low-energy region below the two-magnon excitation have not yet been clearly understood. After turning off a pump pulse tuned for an absorption edge, we find that a new magnetic signal clearly emerges well below the two-magnon energy [3]. We find that low-energy excitation is predominantly created via an excitonic state at the absorption edge. This exciton-assisted magnetic excitation may provide a possible explanation for the low-energy spectral weight in the recent trTMRS experiment [2].<br/>[1] K. Tsutsui, K. Shinjo, and T. Tohyama, Phys. Rev. Lett. <b>126</b>, 127404 (2021).<br/>[2] J.-A. Yang, N. Pellatz, T. Wolf, R. Nandkishore, and D. Reznik, Nat. Commun. <b>11</b>, 2548 (2020).<br/>[3] K. Tsutsui, K. Shinjo, S. Sota, and T. Tohyama, Commun. Phys. <b>6</b>, 41 (2023).