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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.

We demonstrate the relation between critical temperature T c and transport properties in RFeP 1Àx As x O 0:9 F 0:1 (R ¼ La, Pr, and Nd). T c and resistivity power-law exponent n form a universal line on the T c vs n plane for all the R-systems with x < 0:6{0:8, indicating that T c increases with bosonic fluctuation. Transport properties show anomalies suggesting a change of Fermi surfaces around x ¼ 0:6{0:8. Above x ¼ 0:6{0:8, T c and n approach the second T c -n line for the higher T c systems. A further increase of T c above x ¼ 0:6{0:8 indicates the presence of an additional T c -rising mechanism in this system.

We studied the anisotropy and superconducting properties of BaFe2(As1-xPx)2 films with various phosphorus contents x (=0.19–0.45) fabricated on MgO by pulsed laser deposition. We demonstrated that the x=0.28 film with optimal critical temperature (Tczero=26.5 K) has the lowest anisotropy of upper critical field (γH=1.54), as well as the highest irreversibility field. This is different from the case of cuprate superconductors, and preferable for application. Moreover, the minimum angular dependence of the critical current density for the x=0.28 film at 1 T and 10 K is over 1.0 MA/cm2, indicating that this film could be a promising candidate for in-field application.