Recently, ammonia-thermal reaction has been used for molecular intercalation in layered FeSe, resulting a new Lix(NH3)yFe2Se2 superconductor with Tc ~ 45 K. Here, we have used temperature dependent extended x-ray absorption fine structure (EXAFS) to investigate local atomic displacements in single crystals of this new superconductor. Using polarized EXAFS at Fe K-edge we have obtained direct information on the local Fe-Se and Fe-Fe bondlengths and corresponding mean square relative displacements (MSRD). We find that the Se-height in the intercalated system is lower than the one in the binary FeSe, suggesting compressed FeSe4 tetrahedron in the title system. Incidentally, there is hardly any effect of the intercalation on the bondlengths characteristics, revealed by the Einstein temperatures, that are similar to those found in the binary FeSe. Therefore, the molecular intercalation induces an effective compression and decouples the FeSe slabs. Furthermore, the results reveal an anomalous change in the atomic correlations across Tc, appearing as a clear decrease in the MSRD, indicating hardening of the local lattice mode. Similar response of the local lattice has been found in other families of superconductors, e.g., A15-type and cuprates superconductors. This observation suggests that local atomic correlations should have some direct correlation with the superconductivity.

Alkali superoxide CsO 2 is one of candidates for the spin-1 2 one-dimensional (1D) antiferromagnet, which may be sequentially caused by an ordering of the π-orbital of O − 2 molecule below T S ∼ 70 K. Here, we report on the magnetism in powder CsO 2 and high-magnetic field magnetization measurements in pulsed-magnetic fields of up to 60 T. We obtained the low temperature phase diagram around the antiferromagnetic ordering temperature T N = 9.6 K under the magnetic field. At 1.3 K, remarkable up-turn curvature in the magnetization around a saturation field of ∼ 60 T is found, indicating the low-dimensional nature of the spin system. The saturated magnetization is also estimated to be ∼ 1µ B , which corresponds to the spin-1 2 . We will compare it with the theoretical calculation.

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Metal intercalation using a solvent has produced significant advances in the development of Fe-chalcogenide superconducting materials. Recently, the superconducting transition temperature (TC) of metal-intercalated FeSe has been raised to 46 K using ammonia as the solvent for the alkali-metal atom. However, multiple superconducting phases have been found, which may arise from different concentrations of intercalants, further complicating the situation. Here, we report the synthesis of single-crystals of metal-intercalated FeSe superconductors using liquid ammonia, and their physical properties. Particularly, utilization of single-crystals allows us to investigate the resistivity in these ammoniated metal-intercalated FeSe superconductors, Ax(NH3)yFe2-δSe2, for the first time. Firstly, we compared with their TC s and the interlayer distance between the FeSe layers (dI) as a function of ionic radius (r) of the exchangeable intercalant. We found that both TC and dI show weak dependence on the r, if the r is smaller than the effective size of another intercalant, i.e., NH3 molecule. Besides the enhancement of the dI by the insertion of NH3 molecule, one expects that the charge-transfer due to the intercalation of cation would leads to the shift of the Fermi energy. The intercalants (Ba and K) have similar rs, but different valences (Ba2+ and K+), which may throw light on the significance of charge transfer. When the metal concentrations were investigated on the cleaved surfaces of these single crystals, clear differences were found between the two compounds, with the K-concentration about double the Ba-concentration, which may imply the identical charge-transfer. This was also supported for the investigation of the Li-concentration. These results suggest not only the local environment within the conductive FeSe layers but also the significance of the number of electronic charges supplied to the FeSe layers by the intercalated metal and/or ammonia molecules.

We report NMR experiments on heavily electron-doped Fe-based superconductor in comparison with the results on the parent Fe-based compounds. The typical parent Fe-based compound LaFe(As1-x′P x′)O exhibits the re-emergent antiferromagnetic (AFM) order at x′ ~ 0.6 (AFM2) separated from the parent AFM order at x′ =0 (AFM1). Systematic 31P-NMR study on [Sr4Sc2O6]Fe2(As1−x P x )2 (SrSc42622), which has local lattice parameters of iron-pnictogen (FePn) layer similar to the series of LaFe(As1−x′/P x′)O, also revealed that the presence of AFM1 order is universal for most of parent Fe-based compounds. In contrast, the static AFM2 order was absent in this series, however, the dynamical low-energy AFM spin fluctuations are enhanced at around x ~ 0.8, indicating that the onset of the static AFM2 is quite sensitive to the local lattice parameters of FePn layer. In order to elucidate the further universality and diversity, we have carried out 77Se-NMR measurement on Li x (NH3)yFe2−δ Se2 (Tc = 44 K) in heavily electron-doped regime. Although the spin fluctuations at low energies does not significantly develops upon cooling, the moderate spin fluctuations were extracted at high temperatures from comparison of the temperature (T) dependences of Knight shift and nuclear relaxation rate (1/T 1 T). We discuss the universality and diversity of the relationship between the Tc and the characteristics of the spin fluctuations in the Fe-based compounds from a microscopic point of the NMR measurements.