Superconducting phase diagram in BixNi1–x thin films: The effects of Bi stoichiometry on superconductivity

The Bi-Ni binary system has been of interest due to possible unconventional superconductivity aroused therein, such as time-reversal symmetry breaking in Bi/Ni bilayers or the coexistence of superconductivity and ferromagnetism in ${\mathrm{Bi}}_3\mathrm{Ni}$ crystals. While Ni acts as a ferromagnetic element in such systems, the role of the strong spin-orbit coupling element Bi in superconductivity has remained unexplored. In this work, we systematically studied the effects of Bi stoichiometry on the superconductivity of ${\mathrm{Bi}}_x{\mathrm{Ni}}_{1–x}$ thin films ($x\approx 0.5–0.9$) fabricated via a composition-spread approach. The superconducting phase map of ${\mathrm{Bi}}_x{\mathrm{Ni}}_{1–x}$ thin films exhibited a superconducting composition region attributable to the intermetallic ${\mathrm{Bi}}_3\mathrm{Ni}$ phase with different amounts of excess Bi, revealed by synchrotron x-ray diffraction analysis. Interestingly, the mixed-phase region with ${\mathrm{Bi}}_3\mathrm{Ni}$ and Bi showed unusual increases in the superconducting transition temperature and residual resistance ratio as more Bi impurities were included, with the maximum $T_{\mathrm{c}}$ ($=4.2\mathrm{K}$) observed at $x\approx 0.79$. A correlation analysis of structural, electrical, and magneto-transport characteristics across the composition variation revealed that the unusual superconducting “dome” is due to two competing roles of Bi: impurity scattering and carrier doping. We found that the carrier doping effect is dominant in the mild doping regime $(0.74\le x\le 0.79)$, while impurity scattering becomes more pronounced at larger Bi stoichiometry.