Temperature-Dependent As K-Edge EXAFS Studies of LaFe 1-xCoxAsO (x = 0.0 and 0.11) Single Crystals

We report the experimental results of temperature-dependent polarized As K-edge extended X-ray absorption fine structure (EXAFS) of LaFe1-x CoxAsO(x=0.0 and 0.11) single crystals. By aligning the Fe-As bond direction in the direction of the X-ray beam polar- ization, we have been able to identify an anomaly in the Fe-As bond correlations at the tetragonal to orthorhom- bic transition at 150 K, while previous investigations with standard unpolarized EXAFS of undoped LaFeAsO powder samples were not able to detect any such anomaly. Using our approach, we have been able to identify in the superconducting doped sample, LaFe0.89Co0.11AsO, a broad anomaly around 60 K. The low-temperature anomaly has good correlations with the temperature dependence of several properties like resistivity, magnetic susceptibility and linear thermal expansion, indicating the emergence of the dynamical oscillations of the Fe-As pairs.