Systematic31 P-NMR studies of LaFe(As1−xPx)O compounds have revealed the emergence of a novel antiferromagnetic ordered phase (AFM-2) at 0.4≤ x ≤0.7 that intervenes between two superconductivity (SC) phases. This AFM-2 phase with Néel temperature TN= 35 K for x=0.6 is in strong contrast to the AFM order (AFM-1) at x=0 exhibiting TN of 140 K. Previous 31 P-NMR studies of LaFe(As1−xPx)(O1−yFy) have revealed that Tc reaches a maximum of 24 K for x=0.6 as a result of the marked enhancement of AFM spin fluctuations at low energies due to electron doping by the flourine substitution of y=0.05 for oxygen. The reason for this unexpected result has been found in the present work, that is, the emergence of AFM-2 at 0.4 ≤ x ≤0.7 without electron doping. We note that AFM spin fluctuations arising from interband nesting on the dXZ/dY Z orbits must be a key factor for the occurrence of SC around AFM-2.Iron (Fe) oxypnictide LaFeAsO with an orthorhombic structure exhibits antiferromagnetic (AFM) order, and the substitution of F − for O 2− induces superconductivity (SC) with a maximum transition temperature of T c =26 K in LaFeAsO 1−y F y .1 The isostructural compound LaFeP(O 1−y F y ) with P substituted for As also reveals the SC transition at T c =4 -7 K, which is lower than that in the case of LaFeAs(O,F).2 In Fepnictide superconductors, T c reaches a maximum of 55 K 3, 4 when a FeAs 4 block forms a nearly regular tetrahedral structure:5 The optimal values of the As-Fe-As bonding angle (α), the height of pnictogen (h Pn ) from the Fe plane, and the a-axis length (a) are 109.5• , 5∼1.38Å, 6 and ∼3.9Å, 4 respectively. In this context, since the substitution of P for As makes the a-axis length smaller, α wider, and h Pn smaller than the optimal values for high-T c Fe pnictides, it is anticipated that T c might decrease monotonically as x increases in solid solution compounds LaFe(As 1−x P x )(O 1−y F y ). Unexpectedly, T c exhibits a nonmonotonic variation with x in LaFe(As 1−x P x )(O 1−y F y ) compounds.7-9 Previous 31 P-NMR studies of these compounds have revealed that T c reaches its respective maxima of 27 and 24 K for x=0.4 with y=0.1 and for x=0.6 with y=0.05, as a result of the marked enhancement of AFM spin fluctuations(AFMSFs) at low energies.10 The result provides clear evidence that T c is enhanced by AFMSFs at low energies even though the lattice parameters deviate from their optimum values. However, another question should be addressed: Why are AFMSFs enhanced despite the fact that the lattice parameters of the compounds are far from those of the AFM mother compound LaFeAsO.In this Letter, we report the results of our 31 P-NMR studies that a novel AFM ordered phase (AFM-2) emerges at 0.4 ≤ x ≤0.7, intervening between two SC phases (SC-1 and SC-2) in LaFe(As 1−x P x )O. The 31 P- * E-mail: mukuda@mp.es.osaka-u.ac.jp † NMR Knight shift indicates the appearance of a sharp density of states (DOS) at the Fermi level derived from a d 3Z 2 −r 2 orbit, which is less relevant with the onset of SC-2. On the other hand, AFMSF...

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Systematic31 P-NMR studies on LaFe(As1−xPx)(O1−yFy) with y=0.05 and 0.1 have revealed that the antiferromagnetic spin fluctuations (AFMSFs) at low energies are markedly enhanced around x=0.6 and 0.4, respectively, and as a result, Tc exhibits respective peaks at 24 and 27 K against the P substitution for As. This result demonstrates that the AFMSFs are responsible for the increase in Tc for LaFe(As1−xPx)(O1−yFy) as a primary mediator of the Cooper pairing. From a systematic comparison of AFMSFs with a series of (La1−zYz)FeAsO δ compounds in which Tc reaches 50 K for z=0.95, we remark that a moderate development of AFMSFs causes Tc to increase up to 50 K under the condition that the local lattice parameters of the FeAs tetrahedron approach those of the regular tetrahedron. We propose that Tc of Fe-pnictides exceeding 50 K is maximized under an intimate collaboration of the AFMSFs and other factors originating from the optimization of the local structure.

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