+Cerenkov emission of terahertz acoustic phonons generated electrically from monolayers of transition metal dichalcogenides

Abstract

We have studied theoretically the Cerenkov acoustic phonon emission from monolayers of transition metal dichalcogenides (TMDs) driven by an electric field. The features of emitted phonon spectrum P(ωq,θ) are investigated as a function of the phonon frequency ωq, electron concentration ns, electric field E, and phonon emission angle θ. Numerical evaluations are made for n-MoS2. Acoustic phonons in the terahertz (THz) frequency region are found to be generated when the Cerenkov emission condition is satisfied. Frequency dependence of P(ωq,θ) shows a maximum at a frequency ωqm, position which shifts to larger frequency for larger E and ns. We predict ωqm/ns1/2 and ωqm/E1/4 to be nearly constant. The angular dependence of P(ωq,θ) and the phonon intensity P(θ) show emission to be maximum along the direction of electric field, i.e., for θ = 0. Both P(ωq,θ) and P(θ) are found to be larger for larger E. In the Bloch-Gruneisen regime P(ωq,θ) and P(θ) ∼ ns−3/2 and P(θ) ∼ T4. The results are compared with those in the bulk and conventional two-dimensional semiconductors and graphene. It is suggested that monolayers of TMDs can find potential applications as a new source of electrical generation of acoustic phonons in the THz frequency region.