  Dirac semimetal PdTe2 single-crystal temperature-dependent ultrafast carrier
and phonon dynamics were studied using ultrafast optical pump-probe
spectroscopy. Two distinct carrier and coherent phonons relaxation processes
were identified in the 5 K - 300 K range. Quantitative analysis revealed a fast
relaxation process ({\tau}_f) occurring on a subpicosecond time scale which
originated from electron-phonon thermalization. This was followed by a slower
relaxation process ({\tau}_s) with a time scale of ~ 7-9.5 ps which originated
from phonon-assisted electron-hole recombination. Two significant vibrational
modes resolved at all measured temperatures and corresponded to Te atoms
in-plane (E_g), and out-of-plane (A_1g), motion. As temperature increased both
phonon modes softened markedly. A_1g mode frequency monotonically decreased as
temperature increased. Its damping rate remained virtually unchanged. As
expected, E_g decreased uniformly as temperatures rose. At temperatures above
80 K, there was insignificant change. Test results suggested that pure
dephasing played an important role in the relaxation processes. PdTe2 phonon is
thought responsible for its superconductive properties. Examining phonons
behavior should improve the understanding of its complex superconductivity.
