High-temperature epitaxy of vacancy-ordered rock salt NbO thin films

Oxides of the early column members of d-block transition metals are known for strong electronic correlations and a variety of correlated phenomena, including metal-insulator transitions and superconductivity. However, as most of them are classified as refractory metals, it is challenging to realize ideal thermodynamic conditions for the growth of superior-quality crystals. In addition, synthesis pathways to the target oxidation state with precise stoichiometry are often limited. NbO, one such difficult material, is a vacancy-ordered rock salt superconductor with a transition temperature of 1.61 K in the bulk¹. To date, the growth of high-quality NbO thin films has been underreported due to these limitations.

Here, we synthesize single-phase, epitaxial NbO films in a molecular-beam epitaxy chamber equipped with a Nb electron-beam evaporation source and a CO₂ laser heating apparatus that enables substrate temperatures in excess of 1,000 °C. We explore the growth phase diagram by varying temperature and pressure and report thickness-dependent electrical transport measurements. These measurements allow us to study the effects of dimensionality and strain on the superconducting properties of NbO.

1. Okaz, A. M. & Keesom, P. H. Specific heat and magnetization of the superconducting monoxides: NbO and TiO. Phys. Rev. B 12, 4917–4928 (1975).