# On the preparation and electronic properties of clean superconducting Nb(110) surfaces

## TL;DR

This study investigates the cleaning procedures and electronic properties of Nb(110) surfaces, revealing how high-temperature annealing removes oxygen impurities, and demonstrating the surface's superconductivity and vortex behavior.

## Contribution

It provides a detailed analysis of Nb(110) surface cleaning, electronic structure, and superconducting properties, combining experimental and theoretical approaches.

## Key findings

- Oxygen impurities are removed at annealing temperatures up to 2400°C.
- A sharp conductance peak at -450 meV is observed on clean Nb(110).
- The surface exhibits superconductivity with vortex structures in magnetic fields.

## Abstract

We have studied cleaning procedures of Nb(110) by verifying the surface quality with low-energy electron diffraction, Auger electron spectroscopy, and scanning tunneling microscopy and spectroscopy. Our results show that the formation of a surface-near impurity depletion zone is inhibited by the very high diffusivity of oxygen in the Nb host crystal which kicks in at annealing temperatures as low as a few hundred degree Celsius. Oxygen can be removed from the surface by heating the crystal up to $T = 2400^\circ$C. Tunneling spectra measured on the clean Nb(110) surface exhibit a sharp conductance peak in the occupied states at an energy of about $-450$\,meV. Density functional theory calculations show that this peak is caused by a $d_{z^2}$ surface resonance band at the $\bar{\Gamma}$ point of the Brillouin zonewhich provides a large density of states above the sample surface. The clean Nb(110) surface is superconducting with a gap width and a critical magnetic field strength in good agreement to the bulk value. In an external magnetic field we observe the Abrikosov lattice of flux quanta (vortices). Spatially resolved spectra show a zero-bias anomaly in the vortex core.