Evidence of Rotational Fr\"ohlich Coupling in Polaronic Trions

TL;DR

This study reveals that the binding energy of trions in 2D MoS₂ is significantly affected by rotational Fr"ohlich phonon coupling, depending on the substrate's crystallographic orientation, combining theoretical and experimental insights.

Contribution

It introduces the concept of rotational Fr"ohlich coupling in trions and demonstrates its impact on binding energy variation with substrate orientation.

Findings

Trion binding energy increases from 60 meV to 90 meV when changing substrate orientation.

Experimental and theoretical evidence supports rotational phonon coupling effects.

Binding energy is influenced by the interplay between trion rotation axis and phonon modes.

Abstract

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 MoS$_2$ and the bulk transition metal oxide 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 counter-intuitive interplay between the rotational axis of the MoS$_2$ trion and that of the SrTiO$_3$ phonon mode.