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We report $^{75}$As nuclear magnetic resonance (NMR) / nuclear quadrupoleresonance (NQR) and transmission electron microscopy (TEM) studies onLaFeAsO$_{1-x}$F$_{x}$. There are two superconducting domes in this material.The first one appears at 0.03 $\leq$ $x$ $\leq$ 0.2 with $T_{\rm c}$$^{max}$ =27 K, and the second one at 0.25 $\leq$ $x$ $\leq$ 0.75 with $T_{\rmc}$$^{max}$ = 30 K. By NMR and TEM, we demonstrate that a $C4$-to-$C2$structural phase transition (SPT) takes place above both domes, with thetransition temperature $T_{\rm s}$ varying strongly with $x$. In the firstdome, the SPT is followed by an antiferromagnetic (AF) transition, but neitherAF order nor low-energy spin fluctuations are found in the second dome. InLaFeAsO$_{0.97}$F$_{0.03}$, we find that AF order and superconductivity coexistmicroscopically via $^{75}$As nuclear spin-lattice relaxation rate (1/$T_1$)measurements. In the coexisting region, 1/$T_1$ decreases at $T_{\rm c}$ butbecomes to be proportional to $T$ below 0.6$T_{\rm c}$, indicating gaplessexcitations. Therefore, in contrast to the early reports, the obtained phasediagram for $x \leq$ 0.2 is quite similar to the doped BaFe$_{2}$As$_{2}$system. The electrical resistivity in the second dome can be fitted by $\rho ={{\rho }_{0}}+A{{T}^{n}}$ with $n$ = 1 and a maximal coefficient $A$ at around$x_{opt}$ = 0.5$\sim$0.55 where $T_{\rm s}$ extrapolates to zero and $T_{\rmc}$ is the maximal, which suggest the importance of quantum criticalfluctuations associated with the SPT. We have constructed a complete phasediagram of LaFeAsO$_{1-x}$F$_{x}$, which provides insight into the relationshipbetween SPT, antiferromagnetism and superconductivity.

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