Ultrafast carrier and lattice dynamics in the Dirac semimetal NiTe2

Here, using the ultrafast optical pump-probe spectroscopy, we have studied the quasiparticle dynamics in the type-II Dirac semimetal NiTe2. Anomalous dynamic optical responses are detected around a critical temperature T* (-60 K), which is also evidenced by the transport experiment. In specific, our results reveal a phonon -assisted electron-hole recombination process showing an anomaly at T* <^>' 60 K. We discover an unusual phonon renormalization behavior, manifested by the T-dependent phonon energy and lifetime exhibiting anomalies at T*. We unveil that the observed anomalous T-dependencies can be attributed to the sudden shift of the phonon mode involved in the recombination, which originates from the abrupt change of electronic structure near the Fermi surface. These findings can provide deeper insight into the nonequilibrium carrier and lattice properties in the topological materials.