Pump-probe Nano-spectroscopy of Mott Insulating Ca₂RuO₄

Ca2RuO4 is a Mott Insulator that exhibits a temperature-induced insulator to metal transition occurring slightly above room temperature. Further, Ca2RuO4 exhibits a current-driven IMT at room temperature and current-induced diamagnetism below 50K. These discoveries, particularly the latter, imply that the application of current can expose a previously hidden state in Ca2RuO4. Motivated by the current induced transition, we conducted nanoscale pump-probe measurements to explore the phase diagram further. In our experiments, we study both the static and photo-induced response with nano-FTIR measurements to interrogate the dynamics of the spectral response throughout the mid-IR. The static spectra reveal a large phonon resonance of the in-plane stretching mode of the Ru-O bonds in the insulating state, while the metallic state demonstrates a flat, Drude like response. The photo-induced spectra reveal an initial suppression of the phonon mode and a noticeable increase in the Drude response at later times. We interpret the response as an initial injection of charge carriers that screen the phonon mode while their thermalization at later times leads to the Drude-like response.