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A new class of high temperature superconductors based on iron and arsenic wasrecently discovered, with superconducting transition temperature as high as 55K. Here we show, using microscopic theory, that the normal state of the ironpnictides at high temperatures is highly anomalous, displaying a Curie Weisssusceptibility and a linear temperature dependence of the resistivity. Below acoherence scale T*, the resistivity sharply drops and susceptibility crossesover to Pauli-like temperature dependence. Remarkably, thecoherence-incoherence crossover temperature is a very strong function of thestrength of the Hund's rule coupling J_Hund. On the basis of the normal stateproperties, we estimate J_Hund to be 0.35-0.4 eV. In the atomic limit, thisvalue of J_Hund leads to the critical ratio of the exchange constantsJ_1/J_2~2. While normal state incoherence is in common to all stronglycorrelated superconductors, the mechanism for emergence of the incoherent statein iron-oxypnictides, is unique due to its multiorbital electronic structure.

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