Origin of orbital polarization in LaCoO₃+LaTiO₃: DFT+U and DMFT study

Orbital polarization, i.e., the degree of broken orbital degeneracy, can play a crucial role in the electronic and magnetic properties of transition metal oxides. Recently, we observed strong orbital polarization of Co²⁺ in LaCoO₃+LaTiO₃ (LCO+LTO) superlattices, but the underlying origin of this orbital polarization was not fully understood. The orbital polarization of the Co²⁺ cation is particularly interesting since it has multiple spin states, and the dominant t_2g or e_g character is determined by the spin state. We systematically study the origin of the orbital polarization of Co²⁺ by considering the various structural phases of (LCO)₁+(LTO)₁ superlattices and the La₂CoTiO₆ double perovskite, using density functional theory (DFT)+U and dynamical mean-field theory (DMFT) calculations. While we find that structural symmetry reduction is the primary origin of the orbital polarization, the Coulomb U causes spontaneously broken electronic symmetry and large polarizations in some of the DFT+U calculations. We show that this symmetry breaking is artificial as fluctuations between multiple configurations (i.e., Slater determinants) as captured by DMFT restore the symmetry: caution is required when only using DFT+U to compute such observables.