Anisotropic vortices on superconducting Nb(110)

TL;DR

This study explores how the shape and electronic properties of vortices in superconducting Nb(110) change with energy and surface conditions, revealing anisotropic vortex structures linked to the material's electronic states.

Contribution

It combines experimental scanning tunneling spectroscopy with theoretical density functional and Bogoliubov-de-Gennes calculations to explain vortex anisotropy in Nb(110).

Findings

Vortex shapes evolve from circular to elliptical with energy.

Surface oxidation transitions vortices from anisotropic to isotropic.

Theoretical models accurately reproduce experimental vortex behavior.

Abstract

We investigate the electronic properties of type-II superconducting Nb(110) in an external magnetic field. Scanning tunneling spectroscopy reveals a complex vortex shape which develops from circular via coffee bean-shaped to elliptical when decreasing the energy from the edge of the superconducting gap to the Fermi level. This anisotropy is traced back to the local density of states of Caroli-de-Gennes-Matricon states which exhibits a direction-dependent splitting. Oxidizing the Nb(110) surface triggers the transition from the clean to the dirty limit, quenches the vortex bound states, and leads to an isotropic appearance of the vortices. Density functional theory shows that the Nb(110) Fermi surface is stadium-shaped near the \Gamma point. Calculations within the Bogoliubov-de-Gennes theory using these Fermi contours consistently reproduce the experimental results.