Chiral d-wave superconductivity in Sn/Si(111) studied with tunneling Andreev reflection

Chiral d-wave superconductivity features a momentum-dependent complex order parameter in which Cooper pairs acquire a well-defined handedness, spontaneously breaking time-reversal symmetry. Spectroscopic STM studies recently indicated that the Sn/Si(111) surface is a chiral d-wave superconductor. Despite compelling evidence from momentum-space [1] and real-space [2] quasiparticle interference images, determining the pairing symmetry remains challenging because spectroscopic STM probes the single-electron spectral function and thus accesses the superconducting order parameter indirectly. Here, we employ a new STM-based methodology, tunneling Andreev reflection (TAR), to directly probe the superconducting order parameter of Sn/Si(111). Spatial mapping of TAR reveals atomic-scale variations in the superconducting order parameter, offering new insight into the nature of the chiral superconducting state. Our results establish TAR as a powerful microscopic probe that unveils the complex order underlying chiral superconductivity in real space.

[1]  F. Ming et al., Nat. Phys. 19 500 (2023)

[2]  X. Wu et al., https://arxiv.org/abs/2507.18693