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Electronic structure measurements of twisted graphene and 2D magnetic systems
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Nunn, James Edward (2023) Electronic structure measurements of twisted graphene and 2D magnetic systems. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3985217
Abstract
The field of two-dimensional (2D) materials has grown significantly in recent years, motivated by the continued discovery of emergent phenomena and exotic phases in fabricated 2D homo- and heterostructures. This is particularly evident in correlated 2D systems such as the magic-angle twisted graphenes and 2D magnets. Many of the phenomena observed in these correlated materials are intimately tied to their electronic structures. Due to their complexity, simplified electronic structure models have been developed, whose predictions require testing against experimental results. In this thesis, we use angle-resolved photoemission spectroscopy (ARPES) with micrometre spatial resolution (μARPES) to directly visualise the electronic structure of twisted graphenes and 2D magnets and study their interlayer interactions, comparing measurements to theoretical predictions through both qualitative and quantitative analysis of spectral features.
Three twisted graphene systems are examined: twisted bilayer graphene (tBG), twisted monolayer-bilayer graphene (tMBG) and twisted double bilayer graphene (tDBG), allowing a systematic study of the electronic structure within these structures as a function of twist angle and number of layers. Results are compared to predictions from a hybrid k・p-tight-binding (HkpTB) model, beginning with a qualitative comparison through simulation of the photoemission spectra, before extending this to a quantitative comparison of measured band parameters including hybridisation gap sizes and Dirac points shifts from an applied gate voltage. Good agreement with the HkpTB model is found across all stacking geometries for twist angles above 2◦, however, characterisation of a flat band observed in 1.5◦ tDBG highlights the need to include lattice relaxation effects in theoretical models of small twist angle systems.
Similarly, we study the model 2D magnetic system CrSBr using μARPES. By exfoliating flakes onto a fresh metal surface, low-temperature charging effects can be overcome, allowing ARPES measurements of bulk CrSBr in the antiferromagnetic phase. These are compared to measurements above the N´eel temperature, revealing the effect that magnetic order has on the electronic structure. This technique could be readily applied to other semiconducting van der Waals magnetic materials, allowing measurement of their electronic structure at low-temperatures.
These results demonstrate the effectiveness of ARPES in studying complex 2D systems and how improvements in instrumentation and fabrication methods will allow for deeper comparison to theoretical predictions.
Item Type: | Thesis (PhD) | ||||
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Subjects: | Q Science > QC Physics | ||||
Library of Congress Subject Headings (LCSH): | Graphene -- Electric properties, Graphene -- Structure, Two-dimensional magnets -- Structure, Electronic structure, Photoelectron spectroscopy, Two-dimensional materials | ||||
Official Date: | September 2023 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | Department of Physics | ||||
Thesis Type: | PhD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Wilson, Neil R. ; Cephise, Cacho | ||||
Format of File: | |||||
Extent: | xii, 137 pages : illustrations | ||||
Language: | eng |
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