<p>Nb has the highest superconducting transition temperature among elemental metals. That facilitates applications using superconducting junctions such as the superconducting quantum interference device (SQUID). Although the interfacial properties are important for such junctions [1], the surface of Nb is covered with oxide layers even in a vacuum condition, preventing preparation of well-defined junctions. A method to prepare a clean surface was recently established [2]. In this method, however, the crystal needs to be annealed at 2410℃, only 70℃ below its melting point. Such sophisticated methods may not be preferred for applicational purposes. Thus, we aim to clarify the oxide structure to utilize it as a well-defined interface.</p><p>We performed atomic force microscopy (AFM) and scanning tunneling microscopy (STM) measurements for the oxygen-induced NbO surface on Nb(110). The STM image in Fig. 1 represents the topmost atoms, which are referred to as the Nb^* chain atoms [3-5]. The observed chain structure is consistent with the previously proposed models. Furthermore, AFM is capable of detecting O atoms and possibly low-lying Nb atoms, and it clarified that the previously proposed models are not enough to fully explain the present observations. We propose a new structure model based on the density functional theory calculations.</p><p></p><p>References</p><p>[1] J. Brand et al., Phys. Rev. Lett. 118, 107001 (2017).</p><p>[2] A. B. Odobesko et al., Phys. Rev. B 99, 25335 (2016).</p><p>[3] I. Arfaoui et al., Surf. Sci. 557, 119 (2004).</p><p>[4] A. S. Razinkin and M. V. Kuznetsov, Phys. Met. Metallogr. 110, 531 (2010).</p><p>[5] S. Berman et al., Phys. Rev. B 107, 165425 (2023).</p>