Coherence–incoherence crossover in the normal state of iron oxypnictides and importance of Hund's rule coupling

A new class of high-temperature superconductors based on iron and arsenic was recently discovered (Kamihara et al 2008 J. Am. Chem. Soc. 130 3296), with the superconducting transition temperature as high as 55 K (Ren et al 2008 Chin. Phys. Lett. 25 2215). Here we show, using microscopic theory, that the normal state of the iron pnictides at high temperatures is highly anomalous, displaying a very enhanced magnetic susceptibility and a linear temperature dependence of the resistivity. Below a coherence scale T^*, the resistivity sharply drops and susceptibility crosses over to Pauli-like temperature dependence. Remarkably, the coherence–incoherence crossover temperature is a very strong function of the strength of Hund's rule coupling J_Hund. On the basis of the normal state properties, we estimate J_Hund to be ∼0.35 eV. In the atomic limit, this value of J_Hund leads to the critical ratio of the exchange constants J₁/J₂∼2. While normal state incoherence is common to all strongly correlated superconductors, the mechanism for emergence of the incoherent state in iron oxypnictides is unique due to its multiorbital electronic structure.