Superconductivity near the saddle point in the two-dimensional Rashba system Si(111)-$\sqrt{3}\times\sqrt{3}$-(Tl,Pb)

TL;DR

This study uses high-resolution spectroscopic imaging to investigate the superconducting gap in Si(111)-(Tl,Pb), finding evidence for conventional spin-singlet s-wave pairing and suggesting that the superconducting nature varies with the Fermi energy relative to saddle points.

Contribution

The paper provides detailed spectroscopic data showing conventional s-wave superconductivity in Si(111)-(Tl,Pb), challenging previous indications of triplet components and linking superconductivity variation to saddle-point proximity.

Findings

Spectroscopic features are consistent with spin-singlet s-wave superconductivity.

No evidence of triplet superconductivity was observed.

Superconductivity nature depends on the Fermi energy position near saddle points.

Abstract

Two-dimensional Rashba superconductor Si(111)-$\sqrt{3}\times\sqrt{3}$-(Tl,Pb) is a candidate platform of mixed spin-singlet and -triplet superconductivity. A recent scanning tunneling microscope (STM) experiment revealed a pseudogap at the vortex core, suggesting the finite triplet component [T. Nakamura $\textit{et al.}$, Phys. Rev. B $\bf{ 98}$, 134505 (2018)]. Detailed spectroscopic information of the superconducting gap and the low-energy band structure is necessary to establish the putative triplet superconductivity. Here, we performed high-energy-resolution spectroscopic imaging experiments on Si(111)-$\sqrt{3}\times\sqrt{3}$-(Tl,Pb) using an ultra-low temperature STM. We found that various spectroscopic features, including the vortex-core spectrum, are consistent with spin-singlet $s$-wave superconductivity, having no sign of the triplet component. The apparent contradiction with the previous STM result suggests that the nature of superconductivity changes within the same system. From the analysis of the quasiparticle interference patterns, we found that the Fermi energy is in the close vicinity of the saddle point near the $\overline{\rm{M}}$ point. We speculate that the nature of superconductivity varies depending on the saddle-point energy with respect to the Fermi energy, which is sample-dependent due to different band filling.