Abstract
We report a new sample synthesis technique for polycrystalline (Ca1−xNax)Fe2As2 (0<x≤0.6) and measurements of an iron isotope effect in optimally doped (Ca0.4Na0.6)Fe2As2 with three types of iron isotopes (54Fe, natural Fe, and 57Fe). We synthesized isotope samples carefully not to give rise to a difference in the Na content x between different isotope samples, which becomes potentially a factor for an extrinsic difference in the superconducting transition temperature Tc between those samples. No significant difference in lattice parameters between those samples is shown by measurements of powder x-ray diffraction (XRD), implying that the Na content in samples is well-controlled. Our estimate of the iron isotope coefficient αFe defined by -d InTc/d lnMFe, where MFe is the iron isotope mass, is -0.19. These indicate that in (Ca0.4Na0.6)Fe2As2, the iron isotope coefficient becomes definitely negative. We discuss the implications of this fact, considering previous measurements of an iron isotope effect in different iron-based superconductors.
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