  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.
