We report the existence of a two-dimensional electron system (2DES) at the (001) surface of CaTiO3. 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 ATiO3 perovskites, e.g. SrTiO3 or BaTiO3. Based on a comparison of the 2DES properties in these three materials, we show how the electronic structure of the 2DES (bandwidth, orbital order and electron density) is coupled to different typical lattice distortions in perovskites. The orbital hybridization in orthorhombic CaTiO3 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 order in 2DES by choosing different A-site cations in perovskites offers a new gateway towards 2DESs in oxide heterostructures beyond SrTiO3.
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