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.
We have investigated the magnetism and the superconductivity of the electron-doped Pr 1Ϫx LaCe x CuO 4 by means of zero-field muon spin rotation/relaxation and magnetic susceptibility measurements. At low temperatures, a well-defined muon spin rotation free from the effect of rare-earth moments was observed for samples with xр0.08 corresponding to the antiferromagnetic ͑AF͒ order of Cu spins. Bulk superconductivity was identified in a wide Ce concentration range of 0.09рxр0.20 with a maximum transition temperature of 26 K. Abrupt appearance of the superconducting ͑SC͒ phase at xϳ0.09 is concomitant with a destroy of the AF ordered phase, indicating the competitive relation between two phases. Possible relation between the wide SC phase and the lattice spacing is discussed.
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