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Optical and magnetic control of exciton valley pseudospin in layered two-dimensional semiconductors

Lyons, Thomas (2019) Optical and magnetic control of exciton valley pseudospin in layered two-dimensional semiconductors. PhD thesis, University of Sheffield.

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The experiments detailed in this thesis bring together two well established fields within solid state physics, namely the optical properties of Coulomb-bound complexes known as excitons, and the application of novel quantum degrees of freedom towards information processing, storage and communication. The material basis for these investigations are the monolayer transition metal dichalcogenides (TMDs), atomically thin direct band gap semiconductors which exhibit exceptional coupling to light, via tightly bound exciton states which are stable beyond room temperature. These materials may be easily thinned down to single atomic monolayers, in which a unique regime of broken crystal inversion symmetry, time reversal symmetry, and strong spin-orbit coupling together give rise to a valley pseudospin, which acts as a spin-1/2 degree of freedom for charge carriers and excitons alike. Robust optical selection rules allow selective addressability of the exciton valley pseudospin via circularly polarised light, paving the way towards all-optical valley computation or memory. Here, various ways in which the exciton valley pseudospin may be enhanced or controlled are explored. In Chapter 4, the interaction between negatively charged exciton valley pseudospin and strong magnetic fields is discussed, where lifting of degeneracy of opposite valley states is demonstrated, leading to new understanding of the valley magnetism. In Chapter 5, monolayer TMDs are embedded in energy-tunable zero dimensional optical microcavities, in which the TMD excitons strongly couple to the confined photonic modes, forming composite quasiparticles known as exciton-polaritons. By control of the optical density of states via tuning of cavity length, the signatures of exciton valley pseudospin in emission may be enhanced by several times relative to the uncoupled monolayer. Finally, in Chapter 6, a monolayer TMD is placed in direct proximity to a ferromagnetic van der Waals layered material, realising an exciton valley pseudospin switch controllable via milliTesla range magnetic fields.

Item Type: Thesis (PhD)
Additional Information: First submission date: August 2019 Corrected version: December 2019
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > Physics and Astronomy (Sheffield)
Depositing User: Mr Thomas Lyons
Date Deposited: 24 Dec 2019 10:02
Last Modified: 24 Dec 2019 10:02
URI: http://etheses.whiterose.ac.uk/id/eprint/25646

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