Circular photogalvanic effect of layered type-II Weyl semimetal MoTe₂ in mid-infrared band

T_d-Phase Molybdenum ditelluride (MoTe2) has attracted enormous research interests as a Type-II Weyl semimetallic state and the fascinating properties it exhibits. By applying circularly polarized 4.0-μm and 10.6-μm excitation at normal incidence, circular photogalvanic effect (CPGE) has been observed in topological T_d-phase MoTe₂. The CPGE signal is not allowed by the C_2V symmetry by second order nonlinear effect under our experimental geometry. This work is one of the first experimental realizations of this effect in type-II Weyl semimetals with the MIR spectral regime, which is suitable to unveil the physics related to Weyl cones. The occurrence of CPGE in MoTe₂ is attributed to a built-in electric field, which can remove full C_2v symmetry, resulting in nonzero injection current via an third order nonlinear optical effect. The field is produced by the work function difference between samples and electrodes or photothermoelectric effect after photo-excitation. Additionally, the existence of CPGE in our experiments is an unique signature of the T_d phase，which is in consistence with the recently-reported Barkhausen effect during phase transition of MoTe₂.