We present57 Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba 0:6 K 0:4 Fe 2 As 2 (T c ¼ 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1=T 1 ) demonstrate the development of wave-number (q)-dependent spin fluctuations, except at q ¼ 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57 Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The 57 Fe-1=T 1 results are totally consistent with a s AE -wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1=T 1 data for Ba 0:6 K 0:4 Fe 2 As 2 and LaFeAsO 0:7 , which seemingly follow a T 5 -and a T 3 -like behaviors below T c , are consistently explained in terms of this model only by changing the size of the superconducting gap. The recent discovery of superconductivity in the iron (Fe)-based oxypnictide LaFeAsO 1Àx F x at the superconducting (SC) transition temperature T c ¼ 26 K has provided a new route toward the realization of high-T c superconductivity.1) The mother material, LaFeAsO, exhibits a structural phase transition from tetragonal (P4=nmm) to orthorhombic (Cmma) form at T $ 155 K and then exhibits a striped antiferromagnetic (AFM) order with Q ¼ ð0; Þ or ð; 0Þ and T N $ 140 K.2) The calculated Fermi surfaces (FSs) for undoped LaFeAsO consist of two small electron cylinders around the tetragonal M point and two hole cylinders, plus a heavy 3D hole pocket, around the À point.3) Measurements of the nuclear spin-lattice relaxation rate (1=T 1 ) for the LaFeAsO system in the SC state revealed the lack of a coherence peak below T c and the presence of T 3 -like behavior, suggesting an unconventional SC nature. [4][5][6]

We report 57 Fe-NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO0.7 (Tc = 28 K) enriched by 57 Fe isotope. In the superconducting state, the spin component of 57 Fe-Knight shift 57 K decreases almost to zero at low temperatures and the nuclear spin-lattice relaxation rate 57 (1/T1) exhibits a T 3 -like dependence without the coherence peak just below Tc, which give firm evidence of the unconventional superconducting state formed by spin-singlet Cooper pairing. All these events below Tc are consistently argued in terms of the extended s±-wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, we found the remarkable decrease of 1/T1T upon cooling for both the Fe and As sites, which originates from the decrease of low-energy spectral weight of spin fluctuations over whole q space upon cooling below room temperature. Such behavior has never been observed for other strongly correlated superconductors where an antiferromagnetic interaction plays a vital role in mediating the Cooper pairing.

No abstract

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...

No abstract