Impact of local atomic fluctuations on superconductivity of Pr-substituted CaFe₂As₂ studied by X-ray fluorescence holography

The local structures of Ca₁−_xPr_xFe₂As₂ (x ¼ 0; 0:1) were studied by X-ray fluorescence holography. Significant fluctuations were observed in As positions for CaFe₂As₂. The fluctuations increased strongly around Ca atoms by Pr substitution, while those around the substituted Pr atoms were largely unchanged. The inherent positional fluctuations of As in CaFe₂As₂ can be related to the various bonding forms of As and the complex structure with many energy minima suggested from the single-crystal and powder X-ray diffraction experiments. In the Pr-doped CaFe₂As₂, the increased and unchanged fluctuations around Ca and Pr atoms, respectively, could explain the local superconductivity around Pr atoms and its absence around Ca atoms suggested from scanning tunneling microscopy=spectroscopy.