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Electronic correlations in quasi two-dimensional (2D) materials, especially transition metal compounds, have been a central issue in condensed matter physics since the discovery of the cuprate superconductors. While electron-electron interactions are very substantial in 3d transition metal compounds [1], they become progressively weaker when going to heavier transition metal elements, i.e., 4d and 5d systems. In these systems the relativistic spin-orbit (SO) interactions are comparable to electron-electron interactions. In 5d transition metal compounds, e.g., iridates, the interplay between SO couplings, local multiplet physics, crystal-field effects, and inter-site hopping that subsequently emerges has opened up a new window of interest in strongly correlated compounds, offering novel types of correlated ground states and
excitations. The quasi 2D square-lattice iridates such as Sr2IrO4 and Ba2IrO4 are appealing because of their perceived structural and magnetic similarity to La2CuO4, the mother compound of the cuprate high-Tc (HTC) superconductors, which has promoted them to novel platforms on which HTC superconductivity may be designed [2,3,4]. To put such considerations on a firm footing it is essential to quantify the different coupling strengths and energy scales, as they for instance appear in effective Hamiltonian descriptions of these correlated systems [3,5].
In this context, we present and discuss the implications of first-principles many-body calculations for the two so-called 214 iridates, Sr2IrO4 and Ba2IrO4. Wave-function-based multiconfiguration self-consistent-field (MCSCF) and multireference configuration-interaction (MRCI) methods from modern quantum chemistry are used to this end. Multiorbital and multiplet physics, SO couplings, and O 2p to Ir 5d charge-transfer effects are all treated on equal footing, fully ab initio. Our
results for the d-d excitations compare very well with available experimental values. We also find that the dominant nearest-neighbor magnetic interaction is of Heisen-berg type and for Sr 214 the computed exchange constant J compares well with
resonant inelastic x-ray scattering mesurements. For Ba 214, we find a J that is even somewhat larger, which renders it roughly a factor 2-3 lower than the J's in isostructural cuprate HTC superconductors. This might in itself be encouraging for
a scenario of magnetic mediated superconductivity in doped 214 iridates.
[1] M. Imada, A. Fujimori, and Y. Tokura, Rev. Mod. Phys. 70, 1039 (1988) . [2] B. J. Kim, H. Ohsumi, T. Komesu, S. Sakai, T. Morita, H. Takagi, and R. Arima, Science 323, 1329 (2009) . [3] F. Wang, and T. Senthil, Phys. Rev. Lett. 106, 136402 (2011) . [4] J. Kim, D. Casa, M. H. Upton, T. Gog, Y. -J. Kim, J. F. Mitchell, M. van Veenen- daal, M. Daghofer, J. van den Brink, G. Khaliullin, and B. J. Kim, arXiv:1110.0769v1 (unpublished) . [5] H.Watanabe, T. Shirakawa, and S. Yunoki, Phys. Rev. Lett. 105,216410 (2010) . |
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