Laboratoire de Physique Théorique
Toulouse - UMR 5152

Recently emerged two-dimensional semiconductors based on transition metal dichalcogenides demonstrate spectacular properties and are considered as materials beyond graphene for optoelectronics, spintronics, valleytronics, and straintronics.

Optical response of these atom-thin semiconductors is dominated by the excitons — Coulomb bound electron hole pairs. Strong Coulomb interaction makes it also possible to observe charged excitons also known as trions : three particle complexes consisting of an electron and two holes or a hole and two electrons. These complexes are formed in the presence of resident holes or electrons in the sample.

In my talk, after a brief introduction into the physics of two-dimensional semiconductors, I will discuss theoretical approaches to describe the optical and transport properties of excitons interacting with the Fermi-sea of resident electrons. Two complementary approaches are presented : (i) the trion one based on the consideration of a free particle bound state and (ii) the Fermi-polaron one where an exciton is considered as a mobile impurity in the Fermi-sea. We demonstrate that in many relevant situations these approaches yield similar results. We also present the situations where both models contradict each other, enabling experimental distinction between the trion and Fermi-polaron approaches.

We focus on description of optical absorption and emission effects, fine structure of the energy spectrum and diffusion of the Coulomb-correlated complexes. Theoretical results will be illustrated by the experimental data.