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After the discovery of superconductivity in a quasi-two-dimensional LaFeAs(OF) [1], the oxygen-free K- doped BaFe2As2 with the well-known quasi-two-dimensional ThCr2Si2-type (I4/mmm) structure becomes superconducting at 38 K [2]. The parent compound, AFe2As2 (A = Ca, Eu, Sr, and Ba), undergoes a commensurate spin density wave (SDW) antiferromagnetic transition. This SDW transition in AFe2As2 is suppressed by applied pressure, giving rise to superconductivity at low temperature. Since pressure tuning of a stoichiometric compound is a powerful technique for understanding phase transitions, AFe2As2 is promising candidate for studying the mechanism of superconductivity in these iron arsenide compounds. The 57Fe and 151Eu nuclear forward scattering (NFS) experiments under high pressure at low temperatures were carried out using the EuFe2As2 single-crystal sample on the NE1 beamline at the accumulation ring of the High Energy Accelerator Research Organization and on the BL09XU beamline at SPring-8 to investigate the correlation between magnetism and superconductivity in the pressure-induced EuFe2As2 superconductor. We find a microscopic coexistence of magnetism at the Fe and Eu sublattices and bulk superconductivity under pressure and at low temperature. The magnetic structure of the Fe sublattice changes to a modified antiferromagnetic structure in the superconducting state from the commensurate stripe-type antiferromagnetic one in the normal conducting state below 3 GPa. This modified antiferromagnetic structure in the superconducting state has never been observed in the magnetic ordered states in AFe2As2 under pressure and in substituted AFe2As2. The modified antiferromagnetic state with superconductivity in EuFe2As2 under pressure most likely is caused by the change of the electronic state of the Fe atom due to the effective As-As hybridization induced by pressure in the tetragonal symmetry of the crystal structure [3]. Furthermore, the order parameter of the modified antiferromagnetic state does not compete with that of superconductivity in the pressure-induced EuFe2As2 superconductor. References [1] Y. Kamihara, et al., J. Am. Chem. Soc. 130, 3296 (2008). [2] M. Rotter, et al., Phys. Rev. Lett. 101, 107006 (2008). [3] H. Kobayashi, et al., J. Phys.: Condens. Matter 25, 022201 (2013).

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Hisao Kobayashi, University of Hyogo
Seminar Room of the Institute of Physics II
Contact: Mohsen Abd-Elmeguid