Magic thickness of 25 Å alternately makes metal-insulator transitions

Interesting physical properties, e.g., high-temperature superconductivity, topological properties, and charge/spin density waves, are observed in low-dimensional conductive materials. It is also possible to artificially create low-dimensional systems by fabricating ultrathin films, quantum wires, or quantum dots with flat interfaces. We develop the 2-dimensional electronic systems to grow ultrathin films on CaRuO₃. We have succeeded in fabrication of high-crystalline CaRuO₃ ultrathin films, whose surface roughness is controlled at 199 pico-meters, by molecular beam epitaxy. We observe that magnitude of resistivity oscillates with a ‘magic’ thickness of 25 Å, and changes by 3 and 9 orders at 300 K and at 4.2 K, respectively. These changes are much larger than conventional size effects accompanied with quantum well. We also confirm the same periodicity to perform photoelectron spectroscopy by etching the ultrathin film. Considering the large excitation energy, periodicity of 25 Å, and crystal anisotropy, we will discuss 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.