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

Abstract

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 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 ΔNiPb ≈ 1.0 meV, in stark contrast to the isotropic ΔPb ≈ 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 ΔNiPb and the short decay length L d ≈ 3.55 nm from the spatially monotonic decrease of ΔNiPb, 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.

Lead overlayer dynamics on Ni(111)

“…32–35 The interest in the low-dimensional Pb/Ni binary system is determined by the fact that it is one of the promising and technologically demanded materials, despite the absence of mutual solubility of lead and nickel in the bulk state. In particular, anisotropic superconductivity induced by a proximity effect was discovered in the single-layer binary Ni–Pb alloy 36 while proximity effects in superconductor-ferromagnet Pb/Ni bilayer structures were actively discussed. 37,38…”

Fabrication and Imaging Monatomic Ni Kagome Lattice on Superconducting Pb(111)

“…Figure a shows an overview STM topography of the as-prepared sample surface after first depositing Ni onto Pb(111) at low temperature about 80 K, and then post annealing to 200 K. Atomically flat Ni nanostructures have been grown in contrast to the NiPb surface alloy with a bilayer stacking as results of a high temperature growth . Two white arrows marked on the Pb(111) (left) and on top of Ni structure (right) indicate surface features from the nanocavities of Ar-ions buried underneath Pb(111) after the sputtering and annealing process. , The corresponding atomic resolution image on Pb(111) surface (black square frame in Figure a) is shown in Figure b, where not only the directions of high symmetry crystalline axes can be determined, but also a lattice constant of 3.52 Å, has been extracted.…”