New Superconductivity Dome in LaFeAsO1−xFx Accompanied by Structural Transition**Supported by the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant No XDB07020200, the National Basic Research Program of China under Grant Nos 2012CB821402, 2011CBA00109 and 2011CBA00101, and the National Natural Science Foundation of China under Grant No 11204362.

Abstract

High-temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO1−xFx (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 Tc at xopt = 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 Tc. Our results point to a new paradigm of high temperature superconductivity.