  Layered two-dimensional (2D) materials MoTe2 have been paid special attention
due to the rich optoelectronic properties with various phases. The
nonequilibrium carrier dynamics as well as its temperature dependence in MoTe2
are of prime importance, as it can shed light on understanding the anomalous
optical response and potential applications in far infrared (IR)
photodetection. Hereby, we employ time-resolved terahertz (THz) spectroscopy to
study the temperature dependent nonequilibrium carrier dynamics in MoTe2 films.
After photoexcitation of 1.59 eV, the 1T'-phase MoTe2 at high temperature
behaves only THz positive photoconductivity (PPC) with relaxation time of less
than 1 ps. In contrast, the Td-phase MoTe2 at low temperature shows ultrafast
THz PPC initially followed by emerging THz negative photoconductivity (NPC),
and the THz NPC signal relaxes to the equilibrium state in hundreds of ps time
scale. Small polaron formation induced by hot carrier has been proposed to be
ascribed to the THz NPC in the polar semimetal MoTe2 at low temperature. The
polaron formation time after photoexcitation increases slightly with
temperature, which is determined to be ~0.4 ps at 5 K and 0.5 ps at 100 K. Our
experimental result demonstrates for the first time the dynamical formation of
small poalron in MoTe2 Weyl semimetal, this is fundamental importance on the
understanding the temperature dependent electron-phonon coupling and quantum
phase transition, as well as the designing the MoTe2-based far IR
photodetector.
