Iron Isotope Effect and Local Lattice Dynamics in the (Ba, K)Fe2As2 Superconductor Studied by Temperature-Dependent EXAFS

Currently available investigations of the isotope effect in Fe-based superconductors are all based on macroscopic resistivity and susceptibility measurements and results are highly controversial. Here we present an investigation of the Fe isotope effect in the (Ba, K)Fe2As2 superconductor by temperature-dependent EXAFS in the framework of Einstein model. This method separates the static structural isotope contribution and the local lattice isotope contribution. Results point out the isotope substitution has a negligible effect on inter-atomic distances. On the contrary, both the local vibrations of the Fe-As and the Fe-Fe bonds show strong iron isotope dependence, which is a marker of an "intrinsic'' isotope effect. This bulk iron isotope contribution to the local lattice dynamics suggests the local phonons play an important role in the (Ba, K)Fe2As2 superconductor and a three-dimensional electron-phonon interaction should be considered to understand the superconducting mechanism of iron-based superconductors.