Jean-Maxime Schlachter
Laboratoire PHotonique ELectronique et Ingénierie QuantiqueS (PHELIQS)​​​​

Abstract
van der Waals materials can adopt different crystalline phases and can be stacked with an arbitrary angle. Despite the weak interlayer coupling, the stacking order, alongside the reduction of the dimensionality in the few-layer limit, modify their electronic properties. The study of van der Waals materials of different thicknesses, polytypes, and stacking angles can be conducted at the atomic scale by scanning tunneling microscopy and spectroscopy (STM/S) at low temperature (50mK to 4K).

This work aims at understanding the electronic properties of the in-house-grown prototypical van der Waals superconductor NbSe2 in the few-layer limit and of 30°-twisted bilayer graphene samples.

The growth of crystalline few-layer NbSe2 on graphene on silicon carbide is achieved by molecular beam epitaxy (MBE). Single-step growths lead to polycrystalline dendritic islands at low temperature but facetted, almost monocrystalline 1H monolayers at high temperature. However, the formation of 1T islands and defects on the 1H monolayer follows additional deposition at high temperature. The optimized growth protocol thus consists in a multi-step process, combining nucleation at high temperature, low-temperature expansion and formation of additional layers and intermediate annealings. The result are defect-free, crystalline self-intercalated NbSe2 layers.

​ The effects of the interlayer coupling and intercalation are explored by STM/S. For few-layer NbSe2, the intercalation weakens the electronic correlations, while the interlayer coupling between layers of different polytypes modifies the superconducting gap. Regarding 30°-twisted graphene, new states associated with the quasiperiodic order emerge although the interlayer coupling is expected to be the weakest among twisted graphene layers. The effect of tip-sample interaction, which could bring the layers closer, is suspected to play a role in this observation as the effect depends on the tunneling condition and the tunneling current deviates from the exponential decay law with tip-sample distance. This work therefore sheds light on the importance of interlayer coupling in van der Waals materials.