Carroll, Edward Robert;
(2025)
Localisation, Hall effect and electron-phonon decoupling in GaAs and graphene.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
In the context of electron transport in two-dimensional disordered systems, an Arrhenius Hall carrier density has never been observed alongside an Arrhenius conductivity when the Fermi level is below a mobility edge. An Arrhenius quantity decreases exponentially with inverse temperature. For over half a century, it is the Hall mobility that has been consistently reported as being activated as opposed to the Hall carrier density. This has historically been a significant issue with respect to claiming transport via activation to a mobility edge. In this work, for the first time, an Arrhenius Hall carrier density and an Arrhenius conductivity have been observed together. The system used is a two-dimensional electron gas hosted in a gated GaAs/AlₓGa₁₋ₓAs heterostructure. Furthermore, the Hall mobility is shown to be independent of Fermi level. This itself is striking and strong evidence of transport via activation to a mobility edge. A transition between carrier density and mobility dominating the conductivity temperature dependence has also been observed in this work. This could explain the historical results in the literature. Additionally, transport within disorder broadened Landau levels has been investigated. The breakdown of the quantum Hall effect is shown to agree well with a phenomenological electron overheating model and is subsequently ascribed to electron-phonon decoupling. This decoupling manifests itself as large, hysteretic current jumps in current-voltage characteristics. The overheating model has previously been applied to transport in disordered thin films. The work presented in this thesis is the first in which it has been successfully applied to the breakdown of the quantum Hall effect. Finally, an unusual low temperature Arrhenius resistivity regime has been observed in helium focused ion beam damaged graphene. It is suggested that current theories in the literature cannot explain the behaviour.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Localisation, Hall effect and electron-phonon decoupling in GaAs and graphene |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2025. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author’s request. |
| UCL classification: | UCL > Provost and Vice Provost Offices > UCL BEAMS UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Engineering Science > Dept of Electronic and Electrical Eng UCL |
| URI: | https://discovery.ucl.ac.uk/id/eprint/10210176 |
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