Local Lattice Anomalies in Epitaxial YBa₂Cu₃O_7-δ Thin Film Studied by X-Ray Absorption Fine Structure

Abstract

The temperature dependence of the local structure in epitaxial YBa₂Cu₃O_7-δ thin film has been studied by polarized extended X-ray absorption fine structure (EXAFS) on the Cu K-edge. The results suggest that the local lattice anomalies occur not only at T_c but also well above T_c. The mean-square relative displacements for the two out-of-plane Cu--O bonds (Cu1--O4, Cu2--O4) simultaneously increased at T_c. The in-plane Cu--O bonds showed the anomalous decrease of dynamical disorder below 120, K-- 1.5T_c followed by a sharp increase at T_c which saturates at lower temperature, consistent with other lattice anomalies or dynamical spin susceptibilities observed at -- 1.5T_c. These suggest that the anomaly in in-plane oxygen atom vibrations below 1.5T_c is caused by the coupling between lattice distortions and magnetic excitations. The correlated displacement of apical oxygen atoms may induce a bilayer coupling associated with a charge transfer.