Photoinduced coherent acoustic phonon dynamics inside Mott insulator Sr2IrO4 films observed by femtosecond X-ray pulses

摘要

We investigate the transient photoexcited lattice dynamics in a layeredperovskite Mott insulator Sr2IrO4 by femtosecond X-ray diffraction using alaser plasma-based X-ray source. Ultrafast structural dynamics of Sr2IrO4 thinfilms are determined by observing the shift and broadening of the (0012) Braggdiffraction after excitation by 1.5 eV and 3.0 eV pump photons for films withdifferent thicknesses. The observed transient lattice response can be wellinterpreted as a distinct three-step dynamics due to the propagation ofcoherent acoustic phonons generated by the photoinduced quasiparticles (QP).Employing a normalized phonon propagation model, we found that the photoinducedangular shifts of the Bragg peak collapse into a universal curve afterintroducing a normalized coordinates to account for different thicknesses andpump photon energies, pinpointing the origin of the lattice distortion and itsearly evolution. In addition, a transient photocurrent measurement indicatesthat the photoinduced QPs are charge neutral excitons. Mapping the phononpropagation and correlating its dynamics with the QP by ultrafast X-raydiffraction (UXRD) establish a powerful way to study electron-phonon couplingand uncover the exotic physics in strongly correlated systems undernonequilibrium conditions.