Hybridization-mediated quasiparticle and phonon dynamics in single crystal cerium films

We report the ultrafast optical pump-probe spectroscopy measurements on the single-crystal cerium films. Our experimental results of temperature-dependent quasiparticle dynamics reveal development of the hybridization between localized f moments and conduction electrons, i.e., evolving from fluctuating hybridization to collective hybridization. Exotic phonon renormalization is discovered to appear at the emerging temperature (T^†) of fluctuating hybridization apart from its known presence at the coherent temperature (T*), and can hardly be explained by the mean-field theory. The quasiparticle relaxation at high temperatures indicates coexistence of nonthermal electron-electron scattering and inelastic Kondo scattering, while its behavior at low temperatures suggests onset of α phase in the γ-phase dominated film. We also extract the indirect gap below T*, representing the appearance of collective hybridization. Our findings provide novel information about the hybridization and phase evolution in the heavy fermion systems.