Possible orbital crossover in the ferromagnetic Kondo lattice compound CeAgSb2

Utilizing femtosecond time-resolved pump probe spectroscopy, we investigated the ultrafast quasiparticle dynamics of the ferromagnetic Kondo lattice compound CeAgSb2. The photoinduced reflectivity demonstrates a positive relaxation process connected with simple metal physics at room temperature, while an additional negative decay channel associated with Kondo screening effect can be recognized immediately with temperature decreasing. The temperature-dependent amplitude of the latter exhibits a kink at T* approximate to 45 K, which is close to the energy scale of the first excited crystal field splitting Ce 4f level. We propose that an orbital crossover is responsible for this intriguing feature. The dominating 4f orbitals that participated in the Kodno c-f hybridization varies from the ground state below T* to multiple crystal field splitting levels. In the meantime, the hybridization gap changes from 12.6 to 220.4 K correspondingly, which is also in agreement with the fluence-dependent measurements.