Two-dimensional electron systems in ATiO3 perovskites ( A=Ca , Ba, Sr): Control of orbital hybridization and energy order

We report the existence of a two-dimensional electron system (2DES) at the $(001)$ surface of ${\mathrm{CaTiO}}_{3}$. Using angle-resolved photoemission spectroscopy, we find a hybridization between the ${d}_{xz}$ and ${d}_{yz}$ orbitals, not observed in the 2DESs at the surfaces of other $A{\mathrm{TiO}}_{3}$ perovskites, e.g., ${\mathrm{SrTiO}}_{3}$ or ${\mathrm{BaTiO}}_{3}$. Based on a comparison of the 2DES properties in these three materials, we show how the electronic structure of the 2DES (bandwidth, orbital energy order, and electron density) is coupled to different typical lattice distortions in perovskites. The orbital hybridization in orthorhombic ${\mathrm{CaTiO}}_{3}$ results from the rotation of the oxygen octahedra, which can also occur at the interface of oxide heterostructures to compensate strain. More generally, the control of the orbital energy order in 2DES by choosing different $A$-site cations in perovskites offers a gateway toward 2DESs in oxide heterostructures beyond ${\mathrm{SrTiO}}_{3}$.