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

TL;DR

This study investigates anisotropic superconductivity and elongated vortices in quasi-one-dimensional Ni-Bi compounds, revealing unique bound states and vortex behaviors linked to their crystal structure and electronic properties.

Contribution

It provides the first detailed STM/STS analysis of Ni-Bi films, uncovering anisotropic gaps, elongated vortices, and unusual bound states in quasi-one-dimensional superconductors.

Findings

Anisotropic, twofold symmetric superconducting gaps in NiBi3 and NixBi.

Elongated vortices with bound states only in NiBi3.

Asymmetric vortex bound state splitting related to Fermi velocity effects.

Abstract

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 NiBi3 and near-equimolar NixBi, 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 NiBi3. Intriguingly, although the localized bound state splits energetically off at a finite distance ~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