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Magic thickness of 25 Å makes periodic metal-insulator transitions

Novel quantum phenomena, including high-temperature superconductivity, topological properties, and charge/spin density waves, appear in low-dimensional conductive materials. It is possible to artificially create low-dimensional systems by fabricating ultrathin films, quantum wires, or quantum dots with flat interfaces. Some experiments have been performed on ultrathin compounds of strongly correlated electron systems. However, since it is technically difficult to control multiple elements precisely, most of the properties of artificially fabricated low-dimensional compounds fall into uncharted territory. Here we show that extraordinary metal-insulator transitions that oscillate depending on the scale occur in CaRuO_3 films with a thickness of around several unit cells. We grow high-crystalline CaRuO_3 ultrathin films, whose surface roughness is controlled at 199 pm, by molecular beam epitaxy. We observe that resistivity oscillates with a magic thickness of 25 Å, which changes by 3 and 9 orders of magnitude at room temperature and at low temperature, respectively. These changes are much larger than quantum size effects. We also confirm the same periodicity with photoelectron spectroscopy by etching the ultrathin film. Considering the large energy, periodicity and anisotropy, we conclude that the oscillating transitions originate from the commensurability of Mott insulation triggered by Peierls instability arising from a dual restriction on the dimensions in wavenumber space and real space. We have shown the possibility of producing new functional materials by controlling film thickness on electron correlated compounds at the picometer level.