The crystal structure of LnFeAsO 1Ày (Ln = La, Nd) has been studied by the powder neutron diffraction technique. The superconducting phase diagram of NdFeAsO 1Ày is established as a function of oxygen content which is determined by Rietveld refinement. The small As-Fe bond length suggests that As and Fe atoms are connected covalently. FeAs 4 -tetrahedrons transform toward a regular shape with increasing oxygen deficiency. Superconducting transition temperatures seem to attain maximum values for regular FeAs 4 -tetrahedrons.
Fe-based superconductors have attracted research interest because of their rich structural variety, which is due to their layered crystal structures. Here we report the new-structure-type Fe-based superconductors CaAFe4As4 (A = K, Rb, Cs) and SrAFe4As4 (A = Rb, Cs), which can be regarded as hybrid phases between AeFe2As2 (Ae = Ca, Sr) and AFe2As2. Unlike solid solutions such as (Ba(1-x)K(x))Fe2As2 and (Sr(1-x)Na(x))Fe2As2, Ae and A do not occupy crystallographically equivalent sites because of the large differences between their ionic radii. Rather, the Ae and A layers are inserted alternately between the Fe2As2 layers in the c-axis direction in AeAFe4As4 (AeA1144). The ordering of the Ae and A layers causes a change in the space group from I4/mmm to P4/mmm, which is clearly apparent in powder X-ray diffraction patterns. AeA1144 is the first known structure of this type among not only Fe-based superconductors but also other materials. AeA1144 is formed as a line compound, and therefore, each AeA1144 has its own superconducting transition temperature of approximately 31-36 K.
In iron-pnictide superconductivity, the interband interaction between the hole and electron Fermi surfaces (FSs) is believed to play an important role. However, KFe(2)As(2) has three zone-centered hole FSs and no electron FS but still exhibits superconductivity. Our ultrahigh-resolution laser angle-resolved photoemission spectroscopy unveils that KFe(2)As(2) is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with "octet-line nodes" on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A(1g) superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.
We have synthesized a series of oxygen-deficient, fluorine-free polycrystalline samples with nominal chemical formula NdFeAsO 1Ày using high-pressure technique. The ZrCuSiAs type crystal structure, as expected for NdFeAsO, is formed as the main phase for the composition 0 y 0:6. Sharp superconducting (SC) transition with their transition temperature (T c ) as high as 54 K is observed for 0:3 y 0:8 samples, which is higher than that of F-doped NdFeAsO. While T c does not depend on y, SC volume fraction shows maximum at y ¼ 0:4. Around the same y, the lattice parameters show discontinuous shrinkage. These experimental results suggest the existence of a stable superconducting phase with a fixed atomic ratio close to Nd : Fe : As : O ¼ 1 : 1 : 1 : 0:6, presumably associated with a regular array of oxygen deficiency.
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