Ultrafast Strongly Correlated Electron Dynamics Revealed with Time-Resolved X-ray Spectroscopies

The relaxation dynamics of nonequilibrium electrons have been extensively studied via a wide range of pump–probe spectroscopies, but the estimation of how far the excited system is from a thermal state is often indirect, especially on short (<10 −15 s) time scales, while capturing the correlated dynamics between localized and conduction electrons remains an important technical challenge. Here, we show how time-resolved x-ray photoemission and absorption spectroscopies can be employed to measure both the effective energy content and ultrafast, correlated charge dynamics in the electronic subsystem. This scheme takes advantage of a time-dependent correlation between the integrated spectral weight of core-level photoemission/absorption peaks and both thermal and electronic correlation functions, enabling unprecedented insights into the ultrafast, time-averaged dynamics of electron correlations in strongly interacting many-body systems. Such an approach allows for the effective electronic temperature to be determined free of any fitting to a thermal distribution function, following appropriate calibration. As a proof of concept, we experimentally verified this scenario by tracking the ultrafast transfer of spectral weight in the x-ray absorption spectrum of a prototypical, strongly correlated material, CeO 2 , following impulsive photoexcitation. Taken together, our theoretical and experimental findings open the door to quantitatively capture how light-induced changes to the electron–electron correlation function can affect the dynamical evolution of a strongly interacting electron system, including key properties such as the on-site Coulomb repulsion and charge screening.