Abstract
In this thesis, the metallic 2H transition metal dichalcogenides TaSe2 and NbSe2 are studied in monolayer, bulk, and twisted bilayer form. In particular, the properties of the charge density wave present in 2H-TaSe2 are studied and how it can be modelled in a way consistent with experimental measurements, showing the importance of a saddle point present in the bandstructure.An effective two-band tight-binding model of 2H-TaSe2 is proposed, describing the bands around the Fermi level with a minimal set of parameters. This model is consistent with ARPES measurements of the Fermi surface as well as with ab initio density functional theory calculations. This effective model is then used to explain novel experimental measurements of the Fermi surface reconstruction in potassium-doped 2H-TaSe2, where at high doping the reconstruction is characteristic of an (2 × 2) charge ordering instead of the usual (3 × 3) observed in pristine crystals. The observations are explained as a consequence of the single-particle Lifshitz transition, modifying the topology of the Fermi surface and during which the Fermi energy passes through a van Hove singularity (vHS). The high electronic density of states associated with the vHS induces a change in the periodicity of the charge density wave from the known (3 × 3) to a new (2 × 2) superlattice.
A more detailed field theory-based description of the charge density wave gap in these materials is then developed, with the gap an energy- and momentum-dependent complex function rather than a constant real value. Several improvements on previous work are made with the entire gap matrix calculated self-consistently.
Descriptions of twisted 2H-TaSe2 and 2H-NbSe2 bilayers are made using a novel tight-binding model and their bandstructures and density of states (DOS) calculated. A suppression of the (2 × 2) order in doped 2H-TaSe2 is predicted as a function of twist angle. A large feature in the DOS for twisted 2H-NbSe2 is found, which could incentivise the formation of correlated states in doped, twisted samples.
| Date of Award | 28 Jun 2023 |
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| Original language | English |
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| Supervisor | Marcin Mucha-Kruczynski (Supervisor) & Simon Crampin (Supervisor) |