Thinning a bulk material down to a few atomic layers or even to a monolayer influences its physical properties considerably. Such two-dimensional materials can also be used to engineer new correlated phases by stacking them on top of each other. In the present thesis, I present my work on twisted bilayer transition metal dichalcogenides and on few-layer T_d-MoTe₂.

Stacking two transition metal dichalcogenide monolayers with a small twist angle or a small lattice mismatch leads to a long-wavelength moiré superlattice. We study the influence of the moiré potential on the electronic structure of twisted WSe₂ homo-bilayers and of twisted WSe₂/ WS₂ hetero-bilayers by means of micro-focused angle-resolved photoemission. Our results on a twisted 57.4° WSe₂ homo-bilayer reveal a nearly dispersionless flat band at Γ separated by a small gap from the higher mini-bands emerging in the moiré superlattice. From our ARPES study of WSe₂/ WS₂ hetero-bilayers, we deduce that the topmost valence states at the K points originate from WSe₂ monolayer states. We observe multiple moiré replica bands throughout the Brillouin zone emerging from the moiré superlattice. However, we cannot resolve the moiré mini-bands underlying the correlated phases observed in WSe₂/WS₂ hetero-bilayers. We further observe that the relative spectral weight of the moiré replica bands to the main band depends strongly on the polarisation of the light used in the photoemission experiment. This suggests that the moiré potential cannot be directly deduced from the spectral weight distribution.

Finally, we study the electronic structure of mono-, bi-, and trilayer T_d-MoTe₂. We show that high-quality ARPES can be achieved by encapsulating MoTe₂between a bottom graphite flake and a single graphene layer on top. Our ARPES study reveals that mono-, bi- and trilayer MoTe₂ are semimetallic and all have a similar charge carrier density of ~0.02e^-/Mo, corresponding to approximately ≈2·10¹³cm^-2 per layer. For monolayer MoTe₂, a single spin-degenerate electron-like band and a spin-degenerate hole-like band crosses the Fermi level. The electron pocket of monolayer MoTe₂ shows signatures of strong coupling to phonon modes with frequencies ≈10-20 meV. Analysing the mass enhancement, we determine a coupling strength of λ≈ 1.2-1.5. In bilayer T_d-MoTe₂, we observe a large lifting of the spin degeneracy, consistent with the strong inversion symmetry breaking of the crystal structure. In trilayer MoTe₂, the lifting of the spin degeneracy is below our resolution limit. We show that this is consistent with either a T_d structure, which has a weak inversion symmetry breaking for a thickness of 3 layers, or an inversion symmetric 1T' structure. Our data show that the electron-phonon coupling strength in bi- and trilayer MoTe₂ decreases significantly with respect to the monolayer. This suggests that the strong increase of the critical temperature for superconductivity in monolayer MoTe₂ observed in recent transport experiments can be attributed to an enhanced electron-phonon coupling in the monolayer.

Title	Published in	Year	Views	Downloads

Nano-ARPES study of moiré systems and T_d-MoTe₂: from flat bands to electron-phonon coupling		2024	772	1,034
Observation of flat Γ moiré bands in twisted bilayer WSe₂	Physical review letters	2023	775	528
Electronic structure of the highly conductive perovskite oxide SrMoO₃	Physical review materials	2022	640	366

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Issing, Julia

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