Violation of Spin Paramagnetic limit in Bi/Ni Bilayer

Abstract: We report a violation of the spin-paramagnetic limit in the in-plane upper critical field ($B_{C2}<sup>{\parallel}$) in Bi(10 nm)/Ni(1 nm) bilayers grown by molecular beam epitaxy (MBE). Superconductivity emerges at the interface through the formation of an ultrathin NiBi$<em>3$ layer, which hosts a remarkably robust superconducting state under in-plane magnetic fields. To account for the $B</em>{C2}<sup>{\parallel}$ enhancement, we evaluate two-dimensional (2D) superconductivity models incorporating spin-orbit scattering and Rashba-type spin-orbit coupling. However, none of these mechanisms fully capture the observed behavior. We discuss the potential role of unconventional pairing, possibly linked to spin fluctuations. Our findings suggest that, in the 2D limit, NiBi$_3$ may support a superconducting state beyond the standard spin-singlet framework.