Local structural distortions and their role in superconductivity in SmFeAsO_1-xF_x superconductors

EXAFS studies at the As K edge as a function of temperature were carried out in SmFeAsO_1-xF_x (x = 0 and 0.2) compounds to understand the role of local structural distortions in superconductivity observed in F-doped compounds. A significant correlation between the thermal variation of local structural parameters such as anion height and superconducting onset is found in the fluorinated compounds. Such a variation in anion height is absent in the non-superconducting compound. An increase in the Fe-As bond distance just below the superconducting onset temperature indicates a similarity between the distortions observed in the high-T cuprates and these Fe-based superconductors.