Scanning Tunneling Microscopy and Spectroscopy Studies of Niobium Single Crystals*

Several mechanisms have been identified as possible limiting factor for coherence in quantum systems. Nb is a key component of superconducting qubits and resonators. Any dissipation mechanism present in Nb can therefore affect the decoherence time of these devices. One of the main drawbacks of Nb is the affinity to oxygen and the presence of a few nm thick native oxide that it is known to be very difficult to remove. Here we use scanning tunnelling microscopy and spectroscopy to investigate Nb single crystals and disentangle the effect of the native oxide and interstitial oxygen or OH^- from the microstructure present in Nb thin films. We use Ar-ion milling and high temperature annealing, and we study the evolution of the native Nb oxide and its effect on the quasiparticle density of states at the Nb surface.  Measurements performed at 1.5 K and 400 mK will be compared with measurements on Nb thin films.