Characterization of structure and superconducting properties of low-temperature phase of Pb-Bi alloy films

Abstract

Lead-bismuth (Pb-Bi) alloys, as a superconducting material, have been widely studied at their superconducting transition temperatures and the critical magnetic fields for different composition ratios. Most of experimental studies focused on the stable ε phase formed at high temperatures, but less on the Pb-Bi alloys grown at low temperatures. So far, the structural and superconducting properties of the low-temperature Pb-Bi phases are far from good understanding. Here, we report our investigation of structural and superconducting properties of a low-temperature phase of Pb-Bi alloy. The Pb-Bi alloy films with a nominal thickness of about 6 nm are prepared by co-depositing Bi and Pb on Bi(111)/Si(111)-(7 × 7) substrates at a low temperature of 100 K followed by annealing at a treatment of 200 K for 2 h. The structural and superconducting properties of the Pb-Bi alloy films are characterized in situ by using low-temperature scanning tunneling microscopy/spectroscopy (STM/STS). It is observed that the spatially separated phases of nearly pure Bi(111) domains and Pb_1–<i>x</i>Bi<i>_x</i> alloy domains are formed in the films, where these phases can be identified by their distinct differences in the atomic structure and the distributions of step heights in the atomically resolved STM images, as well as by their distinguished STS spectra. The Pb_1–<i>x</i>Bi<i>_x</i> alloy phase presents the structure of Pb(111), in which about <i>x</i> ≈ 0.1 Bi is substituted for Pb. The STS spectra show that the Pb_1–<i>x</i>Bi<i>_x</i> alloy phase is superconducting, with a transition temperature <i>T</i>_c = 7.77 K derived from the variable-temperature measurements. This transition temperature is higher than that in pure Pb film (6.0–6.5 K), which can be well explained by the Mattias rules, with considering the fact that the average number of valance electrons increases after Bi atoms with five valance-electrons have been substituted for Pb atoms with four valance-electrons. The analysis shows that the ratio <inline-formula><tex-math id="M1">\begin{document}$ 2\Delta (0)/{k_{\rm{B}}}{T_{\rm{C}}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210482_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210482_M1.png"/></alternatives></inline-formula> is about 4.94 with the superconducting gap <inline-formula><tex-math id="M2">\begin{document}$ \varDelta (0) = 1.66$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210482_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="17-20210482_M2.png"/></alternatives></inline-formula> meV at 0 K, indicating that the Pb_1–<i>x</i>Bi<i>_x</i> alloy is a strongly-coupled superconductor. The non-superconducting Bi(111) and the superconducting Pb_1–<i>x</i>Bi<i>_x</i> alloy domains form an in-plane superconductor-normal metal-superconductor (S-N-S) Josephson junction. The proximity effect in the Bi(111) domains is measured at different N-S junctions, which suggests that the lateral superconducting penetration length in Bi(111) might be affected by the area of the quasi-two-dimensional interface. The superconducting gap in the Bi(111) region with a narrow width of 23 nm in an S-N-S Josephson junction is found to be greatly enhanced due to the existence of multiple Andreev reflections. Since Bi can host potential topological properties, the lateral Bi(111)-Pb_1–<i>x</i>Bi<i>_x</i> heterostructures, because of the existing proximity effect, could have potential applications in exploring the novel topological and superconducting phenomena.