In Search of a Truly Two-Dimensional Metallic Oxide
The existence of a metallic state and metal–insulator transition in two-dimensional (2D) systems has been an area that has attracted considerable attention, especially in view of theoretical considerations developed in the late 1970s and early 1980s which asserted on firm grounds that one could not have a metallic state in two dimensions. Since the slightest amount of disorder usually drives the system to an Anderson insulator, the origin of the metallic state discovered in the 1990s is still a puzzle. This chapter discusses results of density functional theory and DFT + U calculations for the electronic and magnetic ground state favored by 2D films of a 4d oxide which is both ferromagnetic and metallic in the bulk. Although the 4d oxides are usually not associated with strong correlation effects, interesting effects arise in the ultrathin limit. In particular, SrRuO3 is found to become an antiferromagnetic insulator, which evolves into a ferromagnetic metal with thickness. The thickness dependent insulator–metal transition takes place at four monolayers. The ultrathin limit represents the rare realization of a high spin state for Ru. The strong changes in the properties at this limit occur because the system becomes strongly localized and consequently strongly Jahn–Teller active. As a result, even a seemingly uncorrelated 4d oxide does not favor a metallic ground state.
KeywordsTiO2 Anisotropy RuO2 LaNiO3
PM thanks the Department of Science and Technology for financial support and acknowledges an earlier collaboration with A. Janotti, F. Aryasetiawan, and T. Sasaki on a part of the work.
- 20.Toyota, D., Ohkubo, I., Kumigashira, H., Oshima, M., Lipmaa, M., Takizawa, M., Fujimori, A., Ono, K., Kawasaki, M., Koinuma, H.: Thickness-dependent electronic structure of ultrathin SrRuO3 films studied by in situ photoemission spectroscopy. Appl. Phys. Lett. 87, 162508 (2005)CrossRefGoogle Scholar