Phonon-drag thermopower in a monolayer MoS2

The theory of phonon-drag thermopower Sg is developed in a monolayer MoS2, considering the electron-acoustic phonon interaction via deformation potential (DP) and piezoelectric (PE) coupling, as a function of temperature T and electron concentration ns. DP coupling of TA (LA) phonons is taken to be unscreened (screened) and PE coupling of LA and TA phonons is taken to be screened. Sg due to DP coupling of TA phonons is found to be dominant over all other mechanisms and in the Bloch-Grüneisen regime it gives power law Sg ∼ T3. All other mechanisms give Sg ∼ T5. These power laws are characteristic of two-dimensional (2D) phonons with linear dispersion. Screening enhances the exponent of T by 2 and strongly suppresses Sg due to the large effective mass of the electrons. We find that Sg, due to screened DP and PE couplings is nearly the same in contrast to the results in GaAs heterojunctions. Also, we predict that , a characteristic of 2D electrons with parabolic relation. With the increasing (decreasing) T(ns) its exponent decreases. For comparison, we give diffusion thermopower Sd. At very low T and high ns, Sd ∼ T and . Sd is found to be greater than Sg for about T < 2-3 K. The results are compared with those in conventional 2D electron gas and graphene.