Element-selective structural visualization for the oxygen-induced surface of Nb(110)

The highest superconducting transition temperature in Nb among elemental materials facilitates applications of superconducting junctions. Nonetheless, its surface forms self-organized oxide structures, which hinders the preparation of well-defined interfaces. We investigated the atomic structure of oxygen-induced Nb(110) surfaces, aiming to define the substrates for such interfaces. The atomic force microscopy and scanning tunneling microscopy measurements visualized two rows of apparently low-lying Nb and three rows of O between Nb chains. An optimized model structure is proposed based on density-functional theory calculations. Analysis of the Bader charge and the density of states clarified how the atoms appear in the microscopies.