Neutron diffraction has been used to study the lattice and magnetic structures of the insulating and superconducting RbyFe1.6+xSe2. For the insulating RbyFe1.6+xSe2, neutron polarization analysis and single crystal neutron diffraction unambiguously confirm the earlier proposed √ 5 × √ 5 block antiferromagnetic structure. For superconducting samples (Tc = 30 K), we find that in addition to the tetragonal √ 5 × √ 5 superlattice structure transition at 513 K, the material develops a separate √ 2 × √ 2 superlattice structure at a lower temperature of 480 K. These results suggest that superconducting RbyFe1.6+xSe2 is phase separated with coexisting √ 2 × √ 2 and √ 5 × √ 5 superlattice structures.

The coexisting regime of spin-density wave (SDW) and superconductivity in iron pnictides represents a novel ground state. We have performed high-resolution angle-resolved photoemission measurements on NaFe 1Àx Co x As (x ¼ 0:0175) in this regime and revealed its distinctive electronic structure, which provides some microscopic understandings of its behavior. The SDW signature and the superconducting gap are observed on the same bands, illustrating the intrinsic nature of the coexistence. However, because the SDW and superconductivity are manifested in different parts of the band structure, their competition is nonexclusive. Particularly, we find that the gap distribution is anisotropic and nodeless, in contrast to the isotropic superconducting gap observed in a SDW-free NaFe 1Àx Co x As (x ¼ 0:045), which puts strong constraints on theory. For iron-pnictide superconductors, a spin-density wave (SDW) phase appears next to the superconducting (SC) phase [5][6][7], and, in some cases, they even coexist [8][9][10][11][12][13], which gives a unique SC ground state. While the coexisting SDW and SC phases may have a significant impact on the SC mechanism [9], much is not clear about the subtle interacting nature between magnetism and superconductivity [14]. In fact, theories based on s þþ pairing symmetry suggest that there must be nodes in the SC gap in this regime [15], and the coexisting SDW and SC phases cannot be microscopic [9]. On the other hand, theories based on s þÀ pairing symmetry suggest nodeless SC gaps in the presence of weak magnetic order; moreover, the coexistence may cause angular variation of the SC gap and even give rise to nodes in the limit of strong antiferromagnetic (AFM) ordering [15,16], as indicated in a thermal conductivity study on Ba 1Àx K x Fe 2 As 2 [17].The coexistence of SDW and superconductivity in various iron pnictides has been illustrated by neutron scattering [8][9][10][11][12], nuclear magnetic resonance [18,19], and angleresolved photoemission spectroscopy (ARPES) experiments [13]. Recent scanning tunneling microscope (STM) studies show the real-space coexistence and competition of SDW and superconductivity in NaFe 1Àx Co x As [20,21]. However, so far, little is known regarding the electronic structure of the coexisting phase in the momentum space, such as its SC gap distribution, and how the two orders coexist and compete in the same electronic structure. In this paper, we report ARPES studies on NaFe 0:9825 Co 0:0175 As in this coexisting regime. The bandstructure reconstruction corresponding to the SDW formation and the SC gap could be observed on the same bands, which provides direct evidence for the intrinsic coexistence of the two orders. We find that SDW formation does not cause much depletion of the states near the Fermi energy (E F ); therefore, this formation allows the superconductivity to occur. Moreover, the SC gap distribution is found to be nodeless on all Fermi surface sheets: It is isotropic on the hole pocket, but it is highly anisotropic on the electron pocke...

We have performed 75 As nuclear magnetic resonance (NMR) Knight shift measurements on single crystals of NaFe0.975Co0.025As to show that its superconductivity is a spin-paired, singlet state consistent with predictions of the weak-coupling BCS theory. We use a spectator nucleus, 23 Na, uncoupled from the superconducting condensate, to determine the diamagnetic magnetization and to correct for its effect on the 75 As NMR spectra. The resulting temperature dependence of the spin susceptibility follows the Yosida function as predicted by BCS for an isotropic, single-valued energy gap. Additionally, we have analyzed the 23 Na spectra that become significantly broadened by vortices to obtain the superconducting penetration depth as a function of temperature with λ ab (0) = 5, 327± 78Å.

We have performed 75 As and 23 Na nuclear magnetic resonance (NMR) measurements on a single crystal of NaFe0.9835Co0.0165As and found microscopic coexistence of superconductivity with a twocomponent spin density wave (SDW). Using 23 Na NMR we measured the spatial distribution of local magnetic fields. The SDW was found to be incommensurate with a major component having magnetic moment (∼ 0.2 µB/Fe) and a smaller component with magnetic moment (∼ 0.02 µB/Fe). Spin lattice relaxation experiments reveal that this coexistence occurs at a microscopic level. PACS numbers:The relationship between antiferromagnetism (AFM) and superconductivity in unconventional superconductors, such as cuprates and heavy fermion superconductors, is an intriguing problem of substantial current interest [1,2]. In particular, demonstrating coexistence of these two condensations on a microscopic scale is of special importance given the antithetical nature of magnetism and superconductivity [3][4][5]. Although it has been shown that iron-based superconductivity [6] can coexist with a spin density wave (SDW) [7][8][9][10][11][12][13][14][15][16], in order to better understand this phenomenon it would be helpful to have a clear determination of the spatial distribution of local fields using a high resolution probe. In this Letter we identify an incommensurate SDW that coexists with superconductivity in underdoped NaFe 0.983 Co 0.017 As (NaCo17) taking advantage of very narrow NMR spectra that provide a faithful visualization of the local field distribution. We find that the SDW has unusual character appearing with two components, one with an amplitude an order of magnitude larger than the other.Theory predicts that coexistence of an SDW and superconductivity is possible where phases of the superconducting wave functions on different portions of the Fermi surface with s ± gap symmetry are different by a factor of π for either isotropic or anisotropic superconducting gaps [17]. It has been argued that an incommensurate SDW is more likely to coexist with superconductivity than for a commensurate SDW [18,19]. In contrast, for s ++ gap symmetry where the phase of the superconductor is a constant, coexistence is only possible when the superconducting gap has nodes [20]. Nuclear magnetic resonance (NMR) gives a direct measure of distributions of local magnetic fields, utilized previously to study SDW's in pnictides [9,12,13]. Here we investigate a high quality pnictide single crystal, free of paramagnetic impurities, using two resonances, 75 As and 23 Na. The two nuclei are located on opposite sides of the Fe-layer, Fig.1(b), providing complementary views of the local fields as they develop in the SDW state. The As nucleus has a strong hyperfine coupling to the conduction electrons, in contrast to the Na nucleus, which is much more weakly coupled by a factor of ∼ 20 [21]. Owing to the unusually narrow linewidth of 23 Na NMR in NaCo17, ∼4 kHz at T = 30 K for H 0 = 16.36 T, we have determined the spatial distribution of the incommensurate SDW in the ...