Phonon-drag thermopower in a monolayer MoS2

The theory of phonon-drag thermopower S^g 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 n_s. 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. S^g 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 S^g ∼ T³. All other mechanisms give S^g ∼ 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 S^g due to the large effective mass of the electrons. We find that S^g, due to screened DP and PE couplings is nearly the same in contrast to the results in GaAs heterojunctions. Also, we predict that S^g∼ n_s^-3/2 , a characteristic of 2D electrons with parabolic relation. With the increasing (decreasing) T(n_s) its exponent decreases. For comparison, we give diffusion thermopower S^d. At very low T and high n_s, S^d ∼ T and n_s^-1 . S^d is found to be greater than S^g for about T < 2-3 K. The results are compared with those in conventional 2D electron gas and graphene.