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

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 NiBi3 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 NiBi3 bulk samples, including a small Maki parameter (αM=0), no spin–orbit scattering (λSO=0) and nearly isotropic coherence length (γ=ξ⊥(0)/ξ∥(0)≈1). The values obtained for these properties are close to those characterizing NiBi3 single crystals. On the other hand, in very thin samples, the Maki parameter increases to about αM=2.8. In addition, the coherence length becomes anisotropic (γ=0.32) and spin–orbit scattering (λSO=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.