Unraveling the atomic and electronic structure of nanocrystals on superconducting Nb(110): Impact of the oxygen monolayer

The niobium surface is almost always covered by a native oxide layer which greatly influences the performance of superconducting devices. Here we investigate the highly stable niobium oxide overlayer of Nb(110), which is characterized by its distinctive nanocrystal structure as observed by scanning tunneling microscopy (STM). Our ab initio density functional theory (DFT) calculations show that a subtle reconstruction in the surface niobium atoms gives rise to rows of fourfold coordinated oxygen separated by regions of threefold coordinated oxygen. The fourfold oxygen rows are determined to be the source of the nanocrystal pattern observed in STM and the two chemical states of oxygen observed in core-level x-ray photoelectron spectroscopy (XPS) are ascribed to the threefold and fourfold oxygens. Furthermore, we find excellent agreement between the DFT calculated electronic structure with scanning tunneling spectroscopy and valence XPS measurements.

Automated summary

In this study, the highly stable niobium oxide overlayer of Nb(110) with a distinctive nanocrystal structure was investigated by scanning tunneling microscopy (STM). Ab initio density functional theory (DFT) calculations revealed that a subtle reconstruction in the surface niobium atoms resulted in rows of fourfold coordinated oxygen, which were found to be responsible for the nanocrystal pattern observed in STM. The two chemical states of oxygen observed in core-level x-ray photoelectron spectroscopy (XPS) were attributed to the threefold and fourfold oxygens. Moreover, excellent agreement was found between the DFT calculated electronic structure and the scanning tunneling spectroscopy and valence XPS measurements.