Multiband model for the electronic structure of Sr2TiO4

We introduce and investigate the multiband d-p model for a TiO4 layer, such as realized in Sr2TiO4, with all t(2g) and e(g) orbitals (at titanium ions) and 2p orbitals (at oxygen ions). Complementary density functional theory ab initio computations are employed to determine the actual electron number per TiO4 unit and one finds perfect Sr ionization with Sr+2 ions and charged (TiO4)(-2) layer. This system is predicted to be a robust nonmagnetic insulator, in agreement with experiment. The above charge distribution is crucial and when we deviate from it, even by a small amount, the system becomes conducting or very close to conducting and various magnetic structures compete with one another. This finding is generic, that is, it holds in a broad range of d-p Hamiltonian parameters. As expected, d-p hybridization strongly redistributes electrons and leads to titanium ions between d(1) and d(2) ionic configurations. Surprisingly, Sr-2(2+) (TiO4)(2-) is not a simpleminded t(2g) system but instead electron densities are finite and roughly the same in all different 3d orbitals ( of t2g and e(g) symmetry) and the electron densities within O(2p) orbitals are within the range 5.4-5.9. By selecting the charge-transfer energy, Delta = epsilon(d) - epsilon(p), we reproduce the experimental band gap of 3.8 eV. We emphasize that a realistic treatment of electronic distribution requires local Coulomb interactions at 2p oxygen orbitals and we show that the Coulomb interactions at titanium ions are strongly renormalized when the Coulomb interactions at oxygen ions are neglected. (C) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim