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 Bi₃Ni 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 BiₓNi₁₋₋ₓ thin films (x≈0.5--0.9) fabricated via a composition-spread approach. The superconducting phase map of BiₓNi₁₋₋ₓ thin films exhibited a superconducting composition region attributable to the intermetallic Bi₃Ni phase with different amounts of excess Bi, revealed by synchrotron x-ray diffraction analysis. Interestingly, the mixed-phase region with Bi₃Ni and Bi showed unusual increases in the superconducting transition temperature and residual resistance ratio as more Bi impurities were included, with the maximum Tc (=4.20.28em0exK) observed at x≈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≤x≤0.79), while impurity scattering becomes more pronounced at larger Bi stoichiometry.