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

TL;DR

This study reports a new superconductivity dome in LaFeAsO$_{1-x}$F$_{x}$ with higher $T_c$ values, occurring without magnetic fluctuations but associated with a structural transition, suggesting a new mechanism for high-temperature superconductivity.

Contribution

The paper discovers a novel superconductivity dome in LaFeAsO$_{1-x}$F$_{x}$ that occurs independently of magnetic fluctuations, linked instead to a structural transition, challenging existing theories.

Findings

Superconductivity dome at $x$=0.5-0.55 with higher $T_c$

Absence of low-energy magnetic fluctuations in the new dome

Structural transition breaking C4 symmetry above $T_c$

Abstract

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