Observation of Negative THz Photoconductivity Induced by the Formation of Polaron in Low Temperature Phase of Weyl Semimetal MoTe2

By using optical pump THz probe spectroscopy, the transient photoconductivity (Δσ) of semimetallic MoTe2 thin film was investigated in the temperature range from 5K to 290 K. For 1T’-MoTe2, at room temperature, photoexcitation induces positive THz Δσ that decays within a few picoseconds. In contrast, the negative THz Δσ of MoTe2 appears at low temperature following the initial positive Δσ. The negative Δσ relaxes to the equilibrium within time scale of nanoseconds. The negative Δσ in MoTe2 at low temperature is ascribed to the formation of small polaron in Weyl semimetal phase.