heterostructures: Effect of local correlation and nonlocal exchange*

Motivated by the puzzling report of a ferromagnetic insulating state in LaMnO₃/SrTiO₃ heterostructures, we calculate the electronic and magnetic state of LaMnO₃, strained to a SrTiO₃ square substrate. We use 3 different computational approaches: (a) density functional theory (DFT) with Hubbard U(DFT+U), (b) DFT + dynamical mean-field theory (DMFT), and (c) DFT + Hartree Fock (HF) as a hybrid functional. While the first two approaches include local correlations and exchange at Mn sites, treated in a static and dynamic manner, respectively, the last one takes into account the effect of nonlocal exchange at all sites. We find in all three approaches that the compressive strain induced by the square substrate of SrTiO₃ turns LaMnO₃ to a ferromagnet with suppressed Jahn-Teller distortion, in agreement with experiment. The hybrid calculations result in a ferromagnetic insulating solution, as observed in experiment. This insulating behavior is found to originate from an electronic charge disproportionation. Our conclusions remain valid when we investigate LaMnO₃/SrTiO₃ within the experimental setup of a superlattice geometry using hybrid functionals. DMFT calculations show the presence of a paramagnetic insulating state.

Ref.: Phys. Rev. B 100, 115143 (2019)