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

The article focuses on the discovery and elucidation of an extraordinary size effect with a 25 Å period in ultrathin films of the strongly correlated compound calcium ruthenate (CaRuO₃). It demonstrates that small changes in film thickness induce large oscillations in lattice spacing and electrical resistivity at low temperatures, linked to periodic Mott transitions triggered by spin density waves (SDW) arising from Fermi surface nesting. Using molecular beam epitaxy, in-plane x-ray diffraction, scanning transmission electron microscopy, and magnetotransport measurements, the study reveals that lattice expansions correspond to insulating states with antiferromagnetic correlations, while other thicknesses exhibit metallic behavior. This thickness-dependent quantum phenomenon, unique to CaRuO₃ films grown along the [110] direction, highlights the interplay between lattice structure, electron correlation, and magnetic ordering in controlling electronic phases at the nanoscale.