Coherent energy scale revealed by ultrafast dynamics of UX ₃ (X = Al, Sn, Ga) single crystals

The temperature dependence of relaxation dynamics of UX ₃ (X = Al, Ga, Sn) compounds is studied using the time-resolved pump-probe technique in reflectance geometry. For UGa ₃, our data are consistent with the formation of a spin density wave gap as evidenced from the quasidivergence of the relaxation time τ near the Néel temperature T _N. For UAl ₃ and USn ₃, the relaxation dynamics shows a change from single-exponential to two-exponential behavior below a particular temperature, suggestive of coherence formation of the 5f electrons with the conduction band electrons. This particular temperature can be attributed to the spin fluctuation temperature T _sf, a measure of the strength of Kondo coherence. Our T _sf is consistent with other data such as resistivity and susceptibility measurements. The temperature dependence of the relaxation amplitude and time of UAl ₃ and USn ₃ were also fitted by the Rothwarf-Taylor model. Our results show that ultrafast optical spectroscopy is sensitive to c-f Kondo hybridization in the f-electron systems.