Unconventional superconductivity in the single-atom-layer alloy Si(111)-√3×√3-(Tl, Pb)

We present results of low-temperature scanning tunneling microscopy/spectroscopy measurements on the one-atom-layer superconductor, Si(111)-√3×√3-(Tl,Pb) which has a spin-split band structure due to the Rashba-Bychkov effect. It was revealed that it has a multiple superconducting gap Δ with significant anisotropy and sizable magnitude of 2Δ/kBTC∼8.6(kB is the Boltzmann constant and TC is the critical superconducting temperature). Under the magnetic field, a dip structure was observed even at the core of the vortex. The dip structure was like a "pseudogap" because it remained up to 2 T, well above the upper critical magnetic field determined in the transport measurements (∼0.7 T). This unconventional behavior suggests the possibility of spin-triplet Cooper pairs related to parity-broken superconductors.