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 the 75 As nuclear quadrupole resonance (NQR) and specific heat measurements of the heavily hole-doped superconductor KFe 2 As 2 (superconducting transition temperature T c ' 3:5 K). The spin-lattice relaxation rate 1=T 1 in the superconducting state exhibits a gradual temperature dependence with no coherence peak below T c . The quasiparticle specific heat C QP =T shows a small jump, which is about 30% of the electronic specific heat coefficient just below T c . The C QP =T suggests the existence of low-energy quasiparticle excitation at the lowest measurement temperature T ¼ 0:4 K ' T c =10. The T dependences of 1=T 1 and C QP =T can be explained by a multiple nodal superconducting gap scenario rather than by a multiple fully gapped s AE -wave scenario determined using simple gap analysis.

We report a genuine phase diagram for a disorder-free CuO 2 plane based on the precise evaluation of the local hole density (N h ) by site-selective Cu-NMR studies on five-layered high-T c cuprates. It has been unraveled that (1) the antiferromagnetic metallic state (AFMM) is robust up to N h % 0:17, (2) the uniformly mixed phase of superconductivity (SC) and AFMM is realized at N h 0:17, (3) the tetracritical point for the AFMM/(AFMM+SC)/SC/PM (paramagnetism) phases may be present at N h % 0:15 and T % 75 K, (4) T c is maximum close to a quantum critical point (QCP) at which the AFM order collapses, suggesting the intimate relationship between the high-T c SC and the AFM order. The results presented here strongly suggest that the AFM interaction plays the vital role as the glue for the Cooper pairs, which will lead us to a genuine understanding of why the T c of cuprate superconductors is so high.

We report the 75 As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor Ba 1Àx K x Fe 2 As 2 with different lattice parameters and different superconducting volume fractions (T c ' 38 K).75 As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. The spin-lattice relaxation rate 1=T 1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic spin fluctuation. The 1=T 1 in the superconducting state down to the lowest measurement temperature T varies close to T 3 . In addition, it exhibits no coherence peak just below T c . This shows a T dependence similar to those of other iron pnictides.KEYWORDS: Ba 1Àx K x Fe 2 As 2 , superconductivity, magnetic order, phase separation, nuclear magnetic resonance DOI: 10.1143/JPSJ.78.033704The discovery of superconductivity in F-doped LaFeAsO with a superconducting transition temperature T c ¼ 26has accelerated further investigations of related superconductors.2-9) The 3d electrons originating from an FeAs layer form multiple bands at the Fermi level and play an important role in superconductivity. 10,11) In particular, nondoped materials commonly exhibit an antiferromagnetic (AF) order with an adjacent structural phase transition, which resembles the parent materials of high-T c cuprates. Hence, the relation between magnetic order and superconductivity is one of the vital issues in the investigations of such compounds. K-doped BaFe 2 As 2 is the firstly reported oxygen-free iron-pnictide superconductor with T c ¼ 38 K.6) The crystal structure of Ba 1Àx K x Fe 2 As 2 is of the ThCr 2 Si 2 -type. This structure possesses an FeAs layer similar to that realized in LaFeAsO. The parent material BaFe 2 As 2 exhibits AF anomaly at T N ¼ 140 K.12) Neutron diffraction measurements revealed an ordered moment of 0.87 B (Bohr magneton) at the Fe site with a q vector of ð1; 0; 1Þ for the orthorhombic structure.13) It is important to note that this compound exhibits structural phase transition as well as AF anomaly.12,13) The zero-field 75 As-NMR spectrum also revealed that the magnetically ordered state of this compound is commensurate.14,15) The evaluated H int at 4.2 K decreases gradually with increasing pressure.16) The results are consistent with the other group's NMR results obtained using single crystals. 17)Several groups reported the phase diagram of Ba 1Àx K xFe 2 As 2 . 18,19) For the compound, the AF anomaly disappears and superconductivity is induced by hole doping, K substitution for Ba. The superconducting (SC) state is confirmed to be the bulk from the specific heat jump at T c . 20)The most striking feature of the phase diagram is that the SC region widely spreads for 0:2 x 1 and thus the possibility of the coexistence of the AF and SC states exists for 0:2 x 0:4. Bulk measurements cannot determine whether or not this coexistence is macroscopic. Hence, measurements with a local probe are strongly required. Recent SR measurements show the existence of the phase-sepa...

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe 2−x Co x As 2 (Ba122) (x=0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electrondoped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating the three-dimensional (3D) electronic structure. This observation suggests that the antiferromagnetism and superconductivity in the pnictides may have to be considered including the orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.The discovery of superconductivity (SC) in layered iron pnictides 1 with the critical temperature T c reaching ∼56 K 2 has opened a new route for the high-T c research in addition to that of the cuprates, bringing new challenges to the materials science community on both experimental and theoretical sides.This new class of iron-based systems share some common properties with the cuprates such as the layered crystal structures 1 and antiferromagnetic (AFM) ordering in the parent compounds. 3, 4 However, many differences exist between the two families especially in their electronic structures. These differences started to appear from the early stage when local-density-approximation (LDA) band-structure calculations predicted that all the Fe 3d-derived bands exist near the Fermi level (E F ), resulting in complicated hole-and electron-like Fermi surface (FS) sheets, 5-7 whereas only a single band with one FS