Anisotropic superconductivity and elongated vortices with unusual bound states in quasi-one-dimensional nickel-bismuth compounds

We report low-temperature scanning tunneling microscopy and spectroscopy studies of Ni-Bi films grown by molecular beam epitaxy. Highly anisotropic and twofold symmetric superconducting gaps are revealed in two distinct composites, Bi-rich ${\mathrm{NiBi}}₃$ and near-equimolar ${\mathrm{Ni}}_x\mathrm{Bi}$, both sharing quasi-one-dimensional crystal structure. We further reveal axially elongated vortices in both phases, but Caroli--de Gennes--Matricon states solely within the vortex cores of ${\mathrm{NiBi}}₃$. Intriguingly, although the localized bound state splits energetically off at a finite distance $\ensuremath{\sim}10$ nm away from a vortex center along the minor axis of elliptic vortex, no splitting is found along the major axis. We attribute the elongated vortices and unusual vortex behaviors to the combined effects of twofold superconducting gap and Fermi velocity. The findings provide a comprehensive understanding of the electron pairing and vortex matter in quasi-one-dimensional superconductors.