1. Department of Physics, Department of Applied Physics and Stanford Synchrotron Radiation Laboratory, Stanford University, Stanford, California 94305, USA
2. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
3. Geballe Laboratory for Advanced Materials and Department of Applied Physics, Stanford University, Stanford, California 94305-4045, USA
4. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6114, USA

Correspondence to: D. H. Lu¹Z.-X. Shen¹ Correspondence and requests for materials should be addressed to Z.-X.S. (Email: zxshen@stanford.edu) or D.H.L. (Email: dhlu@slac.stanford.edu).

Abstract

The recent discovery of superconductivity in the iron oxypnictide family of compounds^{1, 2, 3, 4, 5, 6, 7, 8, 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 approach^{12, 13, 14, 15, 16, 17}, and an itinerant ground state in the weak-coupling approach^{18, 19, 20, 21, 22}. The first approach stresses on-site 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, T_c = 5.9 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.

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