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

TL;DR

This study explores how varying Bi content in Bi$_x$Ni$_{1-x}$ thin films influences their superconducting properties, revealing a dome-shaped phase diagram driven by impurity scattering and carrier doping effects.

Contribution

It provides the first systematic analysis of Bi stoichiometry effects on superconductivity in Bi/Ni thin films, highlighting the interplay of impurity scattering and doping.

Findings

Maximum T_c of 4.2 K at x ≈ 0.79

Unusual superconducting dome observed with Bi impurities

Carrier doping dominates in mild doping regime

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$\colon$ impurity scattering and carrier doping. We found that the carrier doping effect is dominant in the mild doping regime (0.74 $\leq {x} \leq$ 0.79), while impurity scattering becomes more pronounced at larger Bi stoichiometry.