Dimensionality control of d-orbital occupation in oxide superlattices

Manipulating the orbital state in a strongly correlated electron system is of fundamental and technological importance for exploring and developing novel electronic phases. Here, we report an unambiguous demonstration of orbital occupancy control between t 2g and e g multiplets in quasi-two-dimensional transition metal oxide superlattices (SLs) composed of a Mott insulator LaCoO 3 and a band insulator LaAlO 3. As the LaCoO 3 sublayer thickness approaches its fundamental limit (i.e. one unit-cell-thick), the electronic state of the SLs changed from a Mott insulator, in which both t 2g and e g orbitals are partially filled, to a band insulator by completely filling (emptying) the t 2g (e g) orbitals. We found the reduction of dimensionality has a profound effect on the electronic structure evolution, which is, whereas, insensitive to the epitaxial strain. The remarkable orbital controllability shown here offers a promising pathway for novel applications such as catalysis and photovoltaics, where the energy of d level is an essential parameter.