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

Yen-Hui Lin; Chia-Hsiu Hsu; Iksu Jang; Chia-Ju Chen; Pok-Man Chiu; Deng-Sung Lin; Chien-Te Wu; Feng-Chuan Chuang; Po-Yao Chang; Pin-Jui Hsu

doi:10.1021/acsami.2c03034

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

ACS Appl Mater Interfaces. 2022 May 16. doi: 10.1021/acsami.2c03034. Online ahead of print.

Authors

Yen-Hui Lin¹, Chia-Hsiu Hsu^{2

3}, Iksu Jang¹, Chia-Ju Chen¹, Pok-Man Chiu¹, Deng-Sung Lin^{1

4}, Chien-Te Wu⁵, Feng-Chuan Chuang^{1

2

3}, Po-Yao Chang¹, Pin-Jui Hsu^{1

4}

Affiliations

¹ Department of Physics, National Tsing Hua University, Hsinchu 300044, Taiwan.
² Department of Physics, National Sun Yat-sen University, Kaohsiung 80424, Taiwan.
³ Physics Division, National Center for Theoretical Sciences, Taipei 10617, Taiwan.
⁴ Center for Quantum Technology, National Tsing Hua University, Hsinchu 300044, Taiwan.
⁵ Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, 300093, Taiwan.

PMID: 35575457
DOI: 10.1021/acsami.2c03034

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 $(3 \sqrt{3} \times 3 \sqrt{3}) R 30^{o}$ 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.

Keywords: BCS theory; BdG Hamiltonian; Cooper pairs; anisotropic superconductivity; monolayer Ni-Pb binary alloy; proximity effect; scanning tunneling spectroscopy; self-consistent Usadel model.