Hanno Weitering : Hotspot nesting and intertwined density wave order near a van Hove singularity in the chiral d-wave superconductor Sn/Si(111)

The emergence of unconventional superconductivity from a landscape of competing electronic instabilities remains a central problem in condensed-matter physics. Sn/Si(111), a superconducting triangular-lattice surface system with strong electronic correlations and a nearby van Hove singularity, provides a promising platform for exploring this interplay. Recent evidence for chiral d-wave pairing^1,2 further places the system among a small class of candidate topological superconductors.

In this talk, I will present a doping-dependent study of the phase diagram of Sn/Si(111). At low doping levels, the system phase-separates into lightly doped and heavily doped regions. The lightly doped regime exhibits an intertwined phase in which chiral d-wave superconductivity coexists with density-wave order. Scanning tunneling spectroscopy reveals two superconductivity-induced energy scales, with coherence peaks at approximately ±2.2 meV and ±4.2 meV. Conductance maps acquired between these energies display a 2×2 modulation consistent with either charge-density-wave order or a triple-Q spin-density-wave state. The emergence of this order correlates with the proximity of a van Hove singularity at the M point to the Fermi level and is consistent with hotspot nesting of the M-point states. The simultaneous disappearance of both spectral features at the superconducting transition temperature, together with an enhanced superconducting energy scale inferred from in-gap bound states, supports a scenario of intertwined superconductivity and density-wave order.

These results reveal a previously unexplored low-doping regime of Sn/Si(111), where electronic reconstruction driven by the M-point van Hove singularity gives rise to competing and cooperative many-body orders. The findings establish Sn/Si(111) as a model platform for investigating the relationship between topology, strong correlations, and unconventional superconductivity on a triangular lattice.^{1 X. Wu et al., Phys. Rev. X 16, 011026 (2026).
2 F. Ming et al., Nature Phys. 19, 500-506 (2023)}