Proximity-Effect-Induced Anisotropic Superconductivity in a Monolayer Ni-Pb Binary Alloy

A proximity effect facilitates the penetration of Cooper pairs that permits superconductivity in a normal metal, offering a promising approach to turn heterogeneous materials into superconductors and develop exceptional quantum phenomena. Here, we have systematically investigated proximity-induced anisotropic superconductivity in a monolayer Ni-Pb binary alloy by combining scanning tunneling microscopy/spectroscopy (STM/STS) with theoretical calculations. By means of high-temperature growth, the $(3\sqrt{3}\times 3\sqrt{3})R{30}^o$ Ni-Pb surface alloy has been fabricated on Pb(111) and the appearance of a domain boundary as well as a structural phase transition can be deduced from a half-unit-cell lattice displacement. Given the high spatial and energy resolution, tunneling conductance (dI/dU) spectra have resolved the reduced but anisotropic superconducting gap Δ_NiPb ≈ 1.0 meV, in stark contrast to the isotropic Δ_Pb ≈ 1.3 meV. In addition, the higher density of states at the Fermi energy (D(E_F)) of the Ni-Pb surface alloy results in an enhancement of coherence peak height. According to the same T_c ≈ 7.1 K with Pb(111) from the temperature-dependent Δ_NiPb and the short decay length L_d ≈ 3.55 nm from the spatially monotonic decrease of Δ_NiPb, both results are supportive of a proximity-induced superconductivity. Despite a lack of a bulk counterpart, the atomically thick Ni-Pb bimetallic compound opens a pathway to engineer superconducting properties down to the two-dimensional limit, giving rise to the emergence of anisotropic superconductivity via a proximity effect.