75As NMR study of the growth of paramagnetic-metal domains due to electron doping near the superconducting phase in LaFeAsO _1-xF _x

We studied the electric and magnetic behavior near the phase boundary between antiferromagnetic (AF) and superconducting phases for a prototype of high-T _c pnictides LaFeAsO _1-xF _x by using nuclear magnetic resonance, and found that paramagnetic-metal (PM) domains segregate from AF domains. PM domains grow in size with increasing electron doping level and are accompanied by the onset of superconductivity, and thus the application of pressure or increasing the doping level causes superconductivity. The existence of PM domains cannot be explained by the existing paradigm that focuses only on the relationship between superconductivity and antiferromagnetism. Based on orbital fluctuation theory, the existence of PM domains is evidence of the ferroquadrupole state.