Abstract
Temperature-dependent local lattice structures of 1111-type ReFeAsO1−x F x (Re=La, Sm) are studied by extended X-ray absorption fine structure (EXAFS) and compared with the results of impurity-doped La1.85Sr0.15CuO4. As electron carriers are introduced by doping with F ions, pnictgen coordination around Fe atom shows unusual local lattice response below T ∗, described as an anomalous temperature dependence of mean-squared relative displacement. The observed local lattice distortion (leading to two distinct bond lengths R 1 and R 2 where ΔR=R 1−R 2∼0.1 A) is a signature of localized polaron formation. The effect of magnetic impurities in La1.85Sr0.15CuO4 indicates (bond stretching-type) lattice distortion favours polaron resonance as a common prerequisite for high-temperature superconductivity.
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