Many-Body Effects in Optical Excitations of Transition Metal Dichalcogenides

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Author
Date
2018-02Type
- Doctoral Thesis
ETH Bibliography
yes
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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. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000290909Publication status
publishedExternal links
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Publisher
ETH ZurichSubject
Quantum Optics; Solid state physics; Transition metal dichalcogenides (TMD); Polaritons; Quantum dotsOrganisational unit
03636 - Imamoglu, Atac / Imamoglu, Atac
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ETH Bibliography
yes
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