Probing physics in local lattice displacements: the case of inhomogeneous state and superconductivity in the copper oxides

Local lattice displacements in the copper oxide superconductors are determined by polarized Cu K-edge extended x-ray absorption fine structure (EXAFS) measurements. Temperature dependent local atomic displacements show anomalies, at the Tc and a temperature Ts where the charge inhomogeneous state appears, as revealed by a change in the correlated Debye-Waller factor (DWF) of the Cu-O bonds. While the DWF shows a clear drop at the Tc, an order parameter like up-turn appears at the Ts. The anomalies shows-up with different amplitude, depending on the superconducting transition temperature of the system and the micro-strain in the electronically active CuO2 plane. The measured Cu-O displacements appear to be closely related to the kink structure in the angle resolved photoemission experiments. The results are discussed to find some correlation between electron-lattice coupling, inhomogeneous charge state and superconductivity in the copper oxides