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 LaFeAsO1-xFx with 0.25 ≤ x ≤ 0.75, where the maximal critical temperature T_c at x_opt= 0.50.55 is even higher than that at x ≤ 0.2. By nuclear magnetic resonance and transmission electron microscopy, we show that a C 4 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.