Longer-range exchange interaction in iron-based superconductors: First-principles studies

The nature of strong electronic correlations in iron-based superconductors (FeSCs) was shown to be kin to manganites where localized spins and itinerant electrons coexist and are coupled by Hund’s rule interaction, leading to fierce competition between superexchange antiferromagnetism and orbital-degenerate double-exchange ferromagnetism [1,2]. This implies that in a low-energy spin-only description of FeSCs where the itinerant-electron degree of freedom were integrated out, the effective exchange interactions would have been long ranged. The spin dynamics in FeSCs has been interpreted by using such spin-only Heisenberg models with exchange couplings extended to the third nearest neighbors, i.e., the J1-J2-J3 model. Here, we use first-principles calculations on the K2Fe4Se5 and KFeAgTe2 insulators to demonstrate the importance of J4 the effective exchange between the fourth nearest neighbors. We further reveal that the band gap of K2Fe4Se5 or KFeAgTe2 is not of Mott type but the gap of Hund's metal. Our findings shed a new light on the understanding of FeSCs and the puzzling strong magnetic fluctuations in FeSe in particular. [1] PRL 105, 107004 (2010); [2] PRB 86, 081106(R) (2012).