Extraordinary size effect on CaRuO₃ ultrathin films

The physical phenomena, e.g., high-temperature superconductivity, topological properties, and charge/spin density waves, are discovered in two-dimensional materials. It is also possible to artificially create two-dimensional systems by fabricating ultrathin films with flat interfaces. We develop the 2-dimensional electronic systems to grow CaRuO₃ ultrathin films. We are successful to fabricate the high-crystalline CaRuO₃ ultrathin films, whose surface roughness is 199 pico-meters, by using molecular beam epitaxy. We observe that electrical resistivity oscillates to 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 to etch the ultrathin film by Ar ions. Considering the large excitation energy, periodicity of 25 Å, and anisotropy, we assume that the oscillating transitions originate from the commensurability of Mott insulation triggered by Peierls instability.