Lawrence Berkeley National Laboratory

The recent discovery of superconductivity in the iron oxypnictide family of compounds1?9 has generated intense interest. The layered crystal structure with transition-metal ions in planar square-lattice form and the discovery of spin-density-wave order near 130 K (refs 10, 11) seem to hint at a strong similarity with the copper oxide superconductors. An important current issue is the nature of the ground state of the parent compounds. Two distinct classes of theories, distinguished by the underlying band structure, have been put forward: a local-moment antiferromagnetic ground state in the strong-coupling approach12?17, and an itinerant ground state in the weak-coupling approach18?22. The first approach stresses onsite correlations, proximity to a Mott-insulating state and, thus, the resemblance to the high-transition-temperature copper oxides, whereas the second approach emphasizes the itinerant-electron physics and the interplay between the competing ferromagnetic and antiferromagnetic fluctuations. The debate over the two approaches is partly due to the lack of conclusive experimental information on the electronic structures. Here we report angle-resolved photoemission spectroscopy (ARPES) of LaOFeP (superconducting transition temperature, Tc55.9 K), the first-reported iron-based superconductor2. Our results favour the itinerant ground state, albeit with band renormalization. In addition, our data reveal important differences between these and copper-based superconductors.