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Description
Superconducting properties can be controlled by various external parameters, such as doping and pressure, which modify low-energy electronic states. In particular, materials hosting singularities in the density of states near the Fermi level are expected to be susceptible to small changes in external parameters. Here, we employ ultralow-temperature scanning tunneling microscopy and spectroscopy to investigate artificial two-dimensional systems that offer unique routes for controlling superconductivity. In the Rashba surface superconductor Si(111)-√3 × √3-(Tl, Pb), both conventional and unconventional pairing states have been proposed. High-resolution quasiparticle interference imaging reveals a van Hove singularity (VHS) in close proximity to the Fermi level. We speculate that this VHS plays a crucial role in the pairing interaction, which can be modified by slight shifts of the Fermi level relative to the VHS through doping of the Si(111) substrate [1].
We also investigate monolayer NbSe2 on graphene, where the superconducting gap can be controlled by the twist angle, which determines the degree of Fermi surface overlap between NbSe2 and graphene [2]. By developing an in-situ twist-angle tuning technique, we reveal the detailed angular dependence of the superconducting gap.
[1] T. Machida, et al., Phys. Rev. B 105, 064507 (2022).
[2] M. Naritsuka et al., Nature Phys. 21, 746-753 (2025).