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

Physical Review B: Condensed Matter and Materials Physics. 2023. Vol. 107. No. 16. Article 165425.

Berman S., Zhussupbekova A., Walls B., Walshe K., Bozhko S., Ionov A. M., O'Regan D.

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.

Keywords: niobium, ниобий, СТМ, XPS, РФЭС, ТФП расчеты, Scanning tunneling microscopy (STM), Density functional theory (DFT)