New Superconductivity Dome in LaFeAsO${}_{1-x}$F${}_x$ Accompanied by Structural Transition

Abstract

High temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO${}_{1-x}$F${}_x$ ($x\le$ 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${}_{1-x}$F${}_x$ with 0.25$\le x\le$0.75, where the maximal critical temperature $T_c$ at $x_{opt}$ = 0.5$\sim$0.55 is even higher than that at $x\le$ 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.

6 pages, 8 figures