Thermoelectric properties of a molecular junction with strong vibrational coupling

We investigate the thermoelectric properties of a molecular quantum dot strongly coupled to a local phonon and tunnel-connected to two metallic electrodes. By employing the extended nonequilibrium Green function method and the Lang-Firsov transformation, we derive explicit expressions for the elastic and inelastic contributions to the electric conductance, thermopower, and electron thermal conductance in the linear response. We find that the figure of merit ZT can be enhanced with increasing the electron-phonon interaction (EPI) strength at low temperature, T ≤ 2Γ (Γ is the tunnel coupling strength of the molecular junction), while be suppressed rapidly with increasing temperature in the case of strong EPI, which is ascribed to the remarkable increase of the inelastic part of electron thermal conductance when T > 2Γ.