Doping dependence of pseudogap in LaFeAsO_1-xF_x

We have performed systematic ultrahigh-resolution photoemission spectroscopy on the iron-based superconductor LaFeAsO_1-xF_x (x = 0-14%) to elucidate the evolution of electronic states with doping. We found that the density of states (DOS) around the Fermi level of the superconducting sample is markedly smaller than that of the undoped sample in contrast to conventional phonon-mediated superconductors. We observed a pseudogap in the DOS above the superconducting transition temperature (T _c) which opens/closes at a temperature (T*) well above T _c. The energy scale of pseudogap and the T* value are gradually reduced upon fluorine substitution. The origin of pseudogap is discussed in terms of the partial nesting of Fermi surfaces due to the spin-density-wave formation or the short-range spin correlations.