A unified phase diagram of F-doped LaFeAsO by means of NMR and NQR parameters
Abstract
We present 75As Nuclear Magnetic and Quadrupole Resonance results (NMR, NQR) on a new set of LaFeAsO1-xFx polycrystalline samples. Improved synthesis conditions led to more homogenized samples with better control of the fluorine content. The structural, magnetic, and superconducting transition temperatures have been determined by NMR spin-lattice relaxation rate and AC susceptibility measurements. The so-determined phase diagram deviates from the published one especially for low F-doping concentrations. However, if the doping level is determined from the NQR spectra, both phase diagrams can be reconciled. The absence of bulk coexistence of magnetism and superconductivity and a nanoscale separation into low-doping-like and high-doping-like regions have been confirmed. Additional frequency dependent intensity, spin-spin, and spin-lattice relaxation rate measurements on underdoped samples at the boundary of magnetism and superconductivity indicate that orthorhombicity and magnetism originate from the low-doping-like regions, and superconductivity develops at first in the high-doping-like regions.