Time Scales of Excited State Relaxation in α-RuCl₃ Observed by Time-Resolved Two Photon Photoemission Spectroscopy

The existence of long-lived hot carriers in correlated materials is significant in the search for new quantum phasescreated by external driving and is also needed to quantify the strength and nature of complex electronic interactions. We have studied the photoexcitation dynamics in a spin-orbit assisted Mott insulator α-RuCl₃ using time resolved two photon photoemission spectroscopy and differential reflection spectroscopy. We find that photoexcited carriers (doublons) in the upper Hubbard band (UHB) rapidly relax to Mott-Hubbard excitons (MHE) on a time scale of less than 200 fs. Subsequently, further fast relaxation of excitons occurs with a time constant of ~ 400 fs. After this process, the population of Mott-Hubbard excitons persists for very long times of at least several hundred picoseconds in agreement with several theoretical predictions about long-lived excited states in large gap Mott insulators.