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 55K (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 similar to 0.35 eV. In the atomic limit, this value of J(Hund) leads to the critical ratio of the exchange constants J(1)/J(2) similar to 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.