Ionization-sensitive atomic force microscopy for the oxide surface structure of Nb(110)

Regarding applications for superconducting junctions, Nb is one of the most promising materials as it has the highest superconducting transition temperature among elemental metals. Although the interfacial properties are important for such junctions, the surface of Nb is covered with oxide layers even in a vacuum condition, preventing the preparation of well-defined junctions. A method to prepare a clean surface was recently established [1]. In this method, however, the crystal needs to be annealed at 2410℃. Such sophisticated methods may not be preferred for applicational purposes. Thus, we aim to clarify the oxide structure for an alternative interface by using the atomic force microscopy (AFM) technique.

Scanning tunneling microscopy (STM) measurements for the oxygen-induced NbO surfaces on Nb(110) single crystals reproduced the previous reports [2]; a quasi-periodic structure of topmost atoms, referred to as the Nb^* chain structure, was confirmed. Furthermore, AFM is capable of detecting O atoms and possibly low-lying Nb atoms, and it revealed that previously proposed models may not be sufficient to fully explain the present observations. We propose a new structure model based on the density functional theory calculations with further analysis of the density of states as well as the Bader charge.

[1] A. B. Odobesko et al., Phys. Rev. B 99, 25335 (2016).

[2] I. Arfaoui et al., Phys. Rev. B 65, 115413 (2002).