Evidence of Rotational Fröhlich Coupling in Polaronic Trions

Electrons commonly couple through Fr\"ohlich interactions with longitudinal optical phonons to form polarons. However, trions possess a finite angular momentum and should therefore couple instead to rotational optical phonons. This creates a polaronic trion whose binding energy is determined by the crystallographic orientation of the lattice. Here, we demonstrate theoretically within the Fr\"ohlich approach and experimentally by photoluminescence emission that the bare trion binding energy (20 meV) is significantly enhanced by the phonons at the interface between the two-dimensional semiconductor ${\mathrm{MoS}}_{2}$ and the bulk transition metal oxide ${\mathrm{SrTiO}}_{3}$. The low-temperature binding energy changes from 60 meV in [001]-oriented substrates to 90 meV for [111] orientation, as a result of the counterintuitive interplay between the rotational axis of the ${\mathrm{MoS}}_{2}$ trion and that of the ${\mathrm{SrTiO}}_{3}$ phonon mode.