Superconducting phase diagram in Bi${}_x$Ni${}_{1-x}$ thin films$:$ the effects of Bi stoichiometry on superconductivity

Abstract

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 Bi${}_3$Ni crystals. While Ni acts as a ferromagnetic element in such systems, the role of 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 Bi${}_x$Ni${}_{1-x}$ thin films ($x\approx$ 0.5 to 0.9) fabricated via a composition-spread approach. The superconducting phase map of Bi${}_x$Ni${}_{1-x}$ thin films exhibited a superconducting composition region attributable to the intermetallic Bi${}_3$Ni phase with different amount of excess Bi, revealed by synchrotron X-ray diffraction analysis. Interestingly, the mixed phase region with Bi${}_3$Ni and Bi showed unusual increases in the superconducting transition temperature and residual resistance ratio as more Bi impurities were included, with the maximum $T_c$ ($=$ 4.2 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.