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-Gruneisen regime it gives power law S-g similar to T-3. All other mechanisms give S-g similar to T-5. 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 similar to 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 similar to T and n(s)(-1). S-d 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.