Coherent lock-in measurement of strong correlation strength in thin film FeSe

We use time- and angle-resolved photoemission spectroscopy (trARPES) to achieve a coherent lock-in between the correlation strength and a dynamical structural modulation in MBE-grown FeSe thin films. The trARPES data reveal periodic modulations in the Fe d_yz band, from which we extract the oscillation of the effective mass as a function of pump–probe delay. In parallel, previous trXRD study provides the corresponding oscillation of the lattice structure. By synchronizing the temporal evolution of both electronic and structural degrees of freedom, we directly measure how electronic correlations in FeSe respond to atomic motion in real time. Complementary theoretical calculations correlate specific lattice distortions with changes in effective mass, providing a quantitative bridge between structure and correlation dynamics.