Relationship between Superconductivity and Antiferromagnetism in LaFe(As${}_{1-x}$P${}_x$)O Revealed by ${}^{31}$P-NMR

We performed ${}^{31}$P-NMR measurements on LaFe(As${}_{1-x}$P${}_x$)O to investigate the relationship between antiferromagnetism and superconductivity. The antiferromagnetic (AFM) ordering temperature $T_{\mathrm{N}}$ and the moment ${\mu }_{\mathrm{o}\mathrm{r}\mathrm{d}}$ are continuously suppressed with increasing P content $x$ and disappear at $x=0.3$ where bulk superconductivity appears. At this superconducting $x=0.3$, quantum critical AFM fluctuations are observed, indicative of the intimate relationship between superconductivity and low-energy AFM fluctuations associated with the quantum-critical point in LaFe(As${}_{1-x}$P${}_x$)O. The relationship is similar to those observed in other isovalent-substitution systems, e.g., BaFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$ and SrFe${}_2$(As${}_{1-x}$P${}_x$)${}_2$, with the “122” structure. Moreover, the AFM order reappears with further P substitution ($x>0.4$). The variation of the ground state with respect to the P substitution is considered to be linked to the change in the band character of Fe-3$d$ orbitals around the Fermi level.