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Understanding the magnetic properties of undoped
iron-pnictides can be crucial to clarify the role played by electronic correlations in these materials and to define the playground from which
superconductivity eventually emerges upon hole or electron doping. The theoretical analysis of the magnetic properties in iron-pnictides, however, is
in itself very puzzling. In fact, while the symmetry of the magnetic long-range ordered phase is easily captured already at the density functional level, the predicted magnetic moment is
surprisingly much larger than that experimentally observed.
We have shed light[1] on this problem by studying a multi-band model[2] for LaFeASO within the local density approximation combined with dynamical mean-field theory (LDA+DMFT). Our findings reveal that the puzzling features of the magnetic phase are, in fact, a consequence of electronic correlation whose effects in multi-band systems can be much more subtle than expected. In particular, our results demonstrated that the new superconductors are indeed more strongly correlated than their single-particle properties suggest. Correlation effects appear in fact more clearly at the two-particle level. As for the magnetic properties, we could show that on short-time scales the magnetic moment is indeed large, whereas on long-time scales (as in experiment) the magnetic moment is small. This dichotomy explains the puzzling discrepancy between previous theoretical calculations and experiments. Noteworthy, recent x-ray absorption experiments resolving the moment on short time scales confirm this scenario. The impact of our analysis, showing the importance of the interplay between multi-band and many body physics, goes beyond the specific case of iron pnictides, since precisely this interplay can be exploited in engineering new superconducting materials. In this context, Nickel based heterostructures seem particularly promising[3]. In fact, for such systems, magnetic and spectral properties can be tuned with relatively high precision to engineer the basic conditions for the appearance of high-temperature superconductivity[4]. [1] P. Hansmann, et al., Phys. Rev. Lett. 104, 197002 (2010). [2] R. Arita and H. Ikeda, J. Phys. Soc. Jap. 78, 113707 (2009). [3] J. Chaloupka and G. Khaliullin, Phys. Rev. Lett. 100, 016404 (2008). [4] P. Hansmann, et al., Phys. Rev. Lett. 103, 016401 (2009). |
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