Structural Phase Transition, Antiferromagnetism and Two Superconducting Domes in LaFeAsO${}_{1-x}$F${}_x$ (0 $<x\le$ 0.75)

Abstract

We report ${}^{75}$As nuclear magnetic resonance (NMR) / nuclear quadrupole resonance (NQR) and transmission electron microscopy (TEM) studies on LaFeAsO${}_{1-x}$F${}_x$. There are two superconducting domes in this material. The first one appears at 0.03 $\le$ $x$ $\le$ 0.2 with $T_{\mathrm{c}}$${}^{max}$ = 27 K, and the second one at 0.25 $\le$ $x$ $\le$ 0.75 with $T_{\mathrm{c}}$${}^{max}$ = 30 K. By NMR and TEM, we demonstrate that a $C4$-to-$C2$ structural phase transition (SPT) takes place above both domes, with the transition temperature $T_{\mathrm{s}}$ varying strongly with $x$. In the first dome, the SPT is followed by an antiferromagnetic (AF) transition, but neither AF order nor low-energy spin fluctuations are found in the second dome. In LaFeAsO${}_{0.97}$F${}_{0.03}$, we find that AF order and superconductivity coexist microscopically via ${}^{75}$As nuclear spin-lattice relaxation rate (1/$T_1$) measurements. In the coexisting region, 1/$T_1$ decreases at $T_{\mathrm{c}}$ but becomes to be proportional to $T$ below 0.6$T_{\mathrm{c}}$, indicating gapless excitations. Therefore, in contrast to the early reports, the obtained phase diagram for $x\le$ 0.2 is quite similar to the doped BaFe${}_2$As${}_2$ system. The electrical resistivity in the second dome can be fitted by $\ddot{I}={\ddot{I}}_0+A{T}^n$ with $n$ = 1 and a maximal coefficient $A$ at around $x_{opt}$ = 0.5$\sim$0.55 where $T_{\mathrm{s}}$ extrapolates to zero and $T_{\mathrm{c}}$ is the maximal, which suggest the importance of quantum critical fluctuations associated with the SPT. We have constructed a complete phase diagram of LaFeAsO${}_{1-x}$F${}_x$, which provides insight into the relationship between SPT, antiferromagnetism and superconductivity.

23 pages, 15 figures