Electronic and spin dynamics in the insulating iron pnictide NaFe0.5Cu0.5As

NaFe0.5Cu0.5As represents a rare exception in the metallic iron pnictide family, in which a small insulating gap is opened. Based on first-principles study, we provide a comprehensive theoretical characterization of this insulating compound. The Fe3+ spin degree of freedom is quantified as a quasi-one-dimensional (1D) S = 5/2 Heisenberg model. The itinerant As hole state is downfolded to a p(xy)-orbital hopping model on a square lattice. An orbital-dependent Hund's coupling between the spin and the hole is revealed. Several important material properties are analyzed, including (a) the factors affecting the small p - d charge-transfer gap; (b) the role of extra interchain Fe atoms; and (c) quasi-1D spin excitation in the Fe chains. The experimental manifestations of these properties are discussed.