  As a newly emergent type-II Dirac semimetal, Platinum Telluride (PtTe2)
stands out from other 2D noble-transition-metal dichalcogenides for the unique
structure and novel physical properties, such as high carrier mobility, strong
electron-phonon coupling and tunable bandgap, which make the PtTe2 a good
candidate for applications in optoelectronics, valleytronics and far infrared
detectors. Although the transport properties of PtTe2 have been studied
extensively, the dynamics of the nonequilibrium carriers remain nearly
uninvestigated. Herein we employ optical pump-terahertz (THz) probe
spectroscopy (OPTP) to systematically study the photocarrier dynamics of PtTe2
thin films with varying pump fluence, temperature, and film thickness. Upon
photoexcitation the THz photoconductivity (PC) of 5 nm PtTe2 film shows abrupt
increase initially, while the THz PC changes into negative value in a
subpicosecond time scale, followed by a prolonged recovery process that lasted
hundreds of picoseconds (ps). This unusual THz PC response observed in the 5 nm
PtTe2 film was found to be absent in a 2 nm PtTe2 film. We assign the
unexpected negative THz PC as the small polaron formation due to the strong
electron-Eg-mode phonon coupling, which is further substantiated by pump
fluence- and temperature-dependent measurements as well as the Raman
spectroscopy. Moreover, our investigations give a subpicosecond time scale of
sequential carrier cooling and polaron formation. The present study provides
deep insights into the underlying dynamics evolution mechanisms of photocarrier
in type-II Dirac semimetal upon photoexcitation, which is fundamental
importance for designing PtTe2-based optoelectronic devices.
