Mott insulators appearing at a thickness period corresponding to nesting in CaRuO3

Abstract

In 2021, we discovered a novel size effect with a period of 25 Å on strongly correlated compound CaRuO3. The change in film thickness of only one nanometer leads to an increase in electrical resistivity at 4 K by a factor of several Billion. However, the excitation energy of 2.14 eV on insulating CaRuO3 is too large to explain the mechanism by conventional quantum well. In this study, we clarify that the increases in electrical resistivity are accompanied by lattice expansion and caused by the Mott transitions. We measured in-plane x-ray diffraction on CaRuO3 films and found a 24.8 Å periodic thickness oscillation of the lattice spacing d(004). We determined the nesting vector from the Fermi surface and revealed the spin density wave with a period equivalent to the size effect. We concluded that the antiferromagnetic correlation appearing in the boundary conditions triggers the periodic Mott insulators depending on the film thickness.