Ni/Bi bilayers: The effect of thickness on the superconducting properties

TL;DR

This study investigates how the thickness of Ni/Bi bilayers influences their superconducting properties, revealing structural changes and varying superconducting behavior, including critical field anisotropy and spin-orbit effects.

Contribution

It provides a detailed analysis of the thickness-dependent superconducting properties of Ni/Bi bilayers, highlighting the formation of NiBi$_{3}$ and the transition from conventional to anisotropic superconductivity.

Findings

Superconducting transition temperature varies with Bi thickness.

Thicker samples exhibit isotropic properties similar to NiBi$_{3}$ bulk.

Thin samples show increased Maki parameter and anisotropic coherence length.

Abstract

Nickel/Bismuth (Ni/Bi) bilayers have recently attracted attention due to the occurrence of time-reversal symmetry breaking in the superconducting state. Here, we report on the structural, magnetic and electric characterization of thin film Ni/Bi bilayers with several Bi thicknesses. We observed the formation of a complex layered structure depending on the Bi thickness caused by the inter-diffusion of Bi and Ni which leads to the stabilization of NiBi$_{3}$ at the Bi/Ni interface. The superconducting transition temperature and the transition width are highly dependent on the Bi thickness and the layer structure. Magnetoelectric transport measurements in perpendicular and parallel magnetic fields were used to investigate the temperature-dependent upper critical field within the framework of the anisotropic Ginzburg-Landau theory and the Werthamer Helfand Hohenberg model. For thicker samples, we observed a conventional behavior, similar to that shown by NiBi$_{3}$ bulk samples, including a small Maki parameter ($\alpha_{M}$ = 0), no spin-orbit scattering ($\lambda_{SO}$= 0) and nearly isotropic coherence length ($\gamma$ = $\xi_{\perp}$(0)/$\xi_{\parallel}$(0) $\approx$ 1). The values obtained for these properties are close to those characterizing NiBi$_{3}$ single crystals. On the other hand, in very thin samples the Maki parameter increases to about $\alpha_{M}$ = 2.8. In addition, the coherence length becomes anisotropic ($\gamma$ = 0.32) and spin-orbit scattering ($\lambda_{SO}$= 1.2) must be taken into account. Our results unequivocally show that the properties characterizing the superconducting state in the Ni/Bi are strongly dependent on the sample thickness.