Towards a limited XANES refinement in strongly correlated systems

Three components of O1 atom displacements have been derived from the direct fit of the polarized E||ab Cu K-edge XANES spectrum of Nd_1.85Ce_0.15CuO_4-δ superconductor. The best fit is achieved for a 60 to 40% mixture of 2x2 undistorted antiferromagnetic and modified charge density wave (MCDW) domains. The local structure of the MSDW is characterized by correlated displacements of ΔO1_||=0.12±0.04, ΔO1_⊥=0.25±0.08, and ΔO1_z=0.12±0.1 Å. An agreement factor over the range of 8980-9040 eV is 2-3 times smaller than it may be obtained from single-electron MS calculations. This improvement comes from accounting for many-body excitations and the relationship between electronic and local structure in the CuO₂ planes.