New Superconductivity Dome in LaFeAsO_1−xF_x Accompanied by Structural Transition

High-temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO_1−xF_x (x≤0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO_1−xF_x with 0.25≤x≤0.75, where the maximal critical temperature T_c at x_opt=0.5–0.55 is even higher than that at x ≤0.2. By nuclear magnetic resonance and transmission electron microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for x>0.5 above T_c. Our results point to a new paradigm of high temperature superconductivity.