Broadband Transient THz Conductivity of the Transition-Metal Dichalcogenide MoS_2

摘要

The transient dynamics of transition-metal dichalcogenides is of significant interest for clarifying fundamental many-particle interactions at the nanoscale as well as for novel applications. We report an ultrafast terahertz study up to 7 THz of the lamellar semiconductor MoS_2 to access the non-equilibrium conductivity of photo-excited indirect e-h pairs in this multi-layered parent compound. While the equilibrium transport is Drude-like, near-IR optical excitation results in a complex photo-induced conductivity that consists of two components. Mobile charge carriers dominate the low frequency response below 2 THz, while at low temperatures an additional excess conductivity is observed that is enhanced around 4 THz. Two time scales appear in the dynamics: a slow ns relaxation due to non-radiative recombination and a faster sub-100 ps decay connected to the high-frequency THz feature. We discuss the broad THz peak within a model of intra-excitonic transitions in MoS_2. It agrees well with the expected binding energy and oscillator strength, yet results in an anomalous temperature dependence of the exciton fraction requiring an electronically inhomogeneous phase.