Enhancement of Extraordinary Size Effect on CaRuO₃ Ultrathin Films

Abstract

In a previous report, 25 Å periodic metal–insulator transitions dependent on the thickness of CaRuO3 ultrathin films were discovered. Compared to the conventional size effect caused by quantum wells, the rate of change is a few billions of times greater at low temperatures. Although the electrical resistivity of insulating state differs by several orders with the film thickness, the enhancement trigger was not clarified. In this study, it is clarified that the magnitude of the extraordinary size effect varies with the element supply ratio owing to precise control of the molecular beam rates of calcium and ruthenium. The supplied calcium over ruthenium ratio Ca/Ru = 1.2–1.8 during CaRuO3 deposition results in a stoichiometric ratio of Ca:Ru = 1:1 on the CaRuO3 films. Calcium‐rich elemental supply conditions result in large electrical resistivity variations. Electrical resistivity oscillations with periods of T = 23.4 and 26.3 Å are observed for the growth conditions of Ca/Ru = 1.2 and 1.6, respectively. Furthermore, a flat surface with a roughness of ≈2 Å is required to observe the size effect. Thus, the insulation of an extraordinary size effect is enhanced in CaRuO3 ultrathin films grown under the condition of calcium‐rich Ca/Ru ≈ 1.6.