Photocarrier-Driven Emergence of Negative Photoconductivity in Semimetal MoTe₂ Films Revealed with Terahertz Spectroscopy

Abstract

The nonequilibrium carrier dynamics and its temperature dependence in MoTe2 are of prime importance, which sheds light on understanding the anomalous optical response. Here, we employ transient terahertz (THz) spectroscopy to study the temperature-dependent photocarrier dynamics in MoTe2 films. After photoexcitation at 1.59 eV, 1T’-phase MoTe2 at high temperatures exhibits only THz positive photoconductivity (PPC) with a relaxation time of less than 1 ps. In contrast, Td-phase MoTe2 at low temperatures shows an ultrafast THz PPC initially followed by emerging THz negative photoconductivity (NPC), and the THz NPC signal relaxes to the equilibrium state in a hundreds-of-ps timescale. Small-polaron formation induced by hot carriers has been proposed to be ascribed to the THz NPC in the polar semimetal MoTe2 at low temperatures. 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 dynamic formation of small polarons in a MoTe2 Weyl semimetal; this is of fundamental importance on understanding the optically driven enhancement of electron–phonon coupling and the temperature-induced quantum phase transition, as well as the designing of a MoTe2-based far-infrared photodetector.