Many-Body Effects in Optical Excitations of Transition Metal Dichalcogenides

Abstract

This dissertation treats a quantum impurity problem in a semiconductor system. Quantum impurity problems describe the interaction between a single quantum object and a complex environment. They are ubiquitous in physical systems and represent a fundamental eld of research in many-body physics. Prominent examples are the Anderson orthogonality catastrophe and the Kondo effect. In both cases, the impurity is much heavier than the constituents of the interacting environment. If the mass of the impurity is comparable to the surrounding particles, we have a mobile impurity. These systems are usually harder to solve, as evident in the case of lattice polarons, which were rst proposed in 1933 by Lev Landau. A complex but accurate description was found years later in 1955 by Richard Feynman. In recent years, strong coupling between single, mobile quantum impurities and a fermionic bath was realized in cold atoms. The interaction results in the formation of new quasiparticles called Fermi polarons. In contrast to other mobile quantum impurities such as lattice polarons, Fermi polarons can be described with a simple and quantitatively accurate model, which renders them an especially attractive eld of research of many-body physics. In this work, we report the observation of Fermi polarons in a solid state environment, namely a new class of semiconductors called transition metal dichalcogenides (TMDs). TMDs consisting of the transition metal Tungsten or Molybdenum and the chalcogenide Sulphur or Selenium are semiconductors. In the monolayer limit, they feature a direct bandgap, a large Coulomb interaction and a large effective electron and hole mass as compared to GaAs. In combination with the two-dimensional con nement, these result in a large binding energy of the exciton. As a consequence, the exciton remains a rigid particle even when it is surrounded by a two-dimensional electron system (2DES) with a large electron density. When the exciton is surrounded by a 2DES, a second resonance emerges in the optical spectrum. Previously, this resonance was attributed to the trion, a bound state of two electrons and a hole. In this dissertation, we demonstrate that this emerging red-shifted resonance has to be described as a Fermi polaron. Thanks to the large binding energy of excitons in TMDs, we can test the predictions of our model qualitatively and quantitatively for a large range of electron densities. For our experimental investigations, we employ cavity quantum electrodynamics in a zero-dimensional, tunable micro-cavity to investigate the optical spectrum of the TMD monolayer for different electron densities. The possibility to reduce the cavity length to a few wavelengths allows the formation of polaron-polariton modes. The strong light-matter coupling cannot be explained with the trion model, and provides solid evidence for the validity of the Fermi polaron model to describe optical resonances in a 2DES.