A divalent rare earth oxide semiconductor: Yttrium monoxide*

Rare earth sesquioxides like Y$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$are known as widegap insulators with the highly stable closed shell trivalent rare earth ions. On the other hand, rare earth monoxides such as YO have been recognized as gaseous phase, and only EuO and YbO were thermodynamically stable solid-phase rock salt monoxides. In this study, solid-phase rock salt yttrium monoxide, YO, was synthesized in a form of epitaxial thin film by pulsed laser deposition method [1]. YO possesses unusual valence of Y$^{\mathrm{2+}}$ ([Kr] 4$d^{\mathrm{1}})$. In contrast with Y$_{\mathrm{2}}$O$_{\mathrm{3}}$, YO was narrow gap semiconductor with dark-brown color. The electrical conductivity was tunable from 10$^{\mathrm{-1}}$ to 10$^{\mathrm{3\thinspace }}\Omega ^{\mathrm{-1}}$cm$^{\mathrm{-1}}$ by introducing oxygen vacancies as electron donor. Weak antilocalization behavior was observed indicating significant spin-orbit coupling owing to 4$d$ electron carrier. The absorption spectral shape implies the Mott-Hubbard insulator character of YO. Rare earth monoixdes will be new platform of functional oxides. [1] K. Kaminaga et.al., Appl. Phys. Lett. \textbf{108}, 122102 (2016). (Selected as Editor's Picks)