Rare earth elements are highly reactive, thus are easily oxidized in air. Such rare earth oxides are expressed as chemical formulae R2O3 (R: rare earth), that are highly insulating compounds. Recently, we developed synthetic method of rock-salt structure RO by using state-of-the-art thin film epitaxy, although most of RO have been recognized as a gas phase, so far, except for several RO for nearly 40 years. We revealed that those RO are quite different from electrically insulating R2O3: YO and LuO are semiconductors, YbO is high electron mobility semiconductor, NdO is low TC ferromagnetic metal, SmO is heavy fermion metal, and LaO is superconductor. It is noted that those various functionalities might be governed by the number of f-electron in their rare earth ions.
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