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Beschreibung:
We investigate the magnetic couplings in Sr2IrO4 in the Mott-insulating picture, combining density-functional theory, dynamical mean-field theory, and many-body perturbation theory. We first determine the form of the jeff=1/2 pseudospin via the local-density-approximation + dynamical mean-field theory approach. Next we study the magnetic interactions in the strong-to-intermediate coupling regime. To this end, we calculate the superexchange pseudospin tensors Γ1, Γ2, and Γ3 up to fourth order and analyze their dependence on the screened Coulomb interaction integrals U and J. We show that, due to term cancellations, the experimental nearest-neighbor coupling Γ1 is reasonably well reproduced for a whole range of realistic (U,J) values. We show that increasing the Hund's rule coupling J (within the window of realistic values) can lead to large fourth-order contributions, which could explain the ferromagnetic next-nearest-neighbor coupling Γ2 extracted from the spin-wave dispersion. This regime is characterized by a sizable ring exchange K. For (U,J) values that yield a Mott insulator with a half-filled jeff=1/2 state, however, fourth-order terms remain minor even if the gap is small. For no realistic parameters, we find a sizable next-next-nearest-neighbor coupling Γ3∼|Γ2|. Possible implications are discussed.