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DANES STUDY OF YBa2Cu3O6.6

The x-ray absorption fine structure in the near-edge region is called the x-ray absorption near-edge structure (XANES), and, by analogy, the same structure in the elastic scattering channel is called the diffraction anomalous near-edge structure (DANES). Just as EDAFS combines diffraction and EXAFS, DANES combines the wavevector and site selectivity of diffraction with the valence, empty orbital, and structural sensitivity of XANES. Consequently, for crystals with inequivalent sites, DANES can assign near-edge features to specific sites. However, in this region (i.e., within about 30 eV of the edge energy) the low energy photoelectrons are very strongly scattered by the surrounding atoms, producing large multiple scattering effects, and making the data analysis considerably more demanding than it is in the extended fine structure region.

 figure526

The recent advances in MS-XAFS and MS-DAFS theory and analysis now allow the full curved-wave multiple-scattering effects to be calculated almost down to the continuum threshold [4]. There are also separate codes for calculating the DANES and XANES signals which involve transitions to the bound states below the continuum threshold [30]. DANES and XANES will become much more useful, however, when convenient analysis methods that allow accurate predictions of the near-edge structure, including both the multiple-scattering and bound state effects, are developed.

An example of the ability of DANES to assign features in the near-edge region to specific sites is provided by the application of DANES techniques to separate the near-edge signals for the two inequivalent Cu sites in the YBa2Cu3O6.6 superconductor film described in the previous section. In the case of tex2html_wrap_inline1946, the original assignment of near-edge features to particular Cu sites was difficult and controversial [31]. While site specific assignments were eventually made using polarized XANES measurements on oriented grains [32], and on single crystals [33], the site selectivity of the DANES measurements presented here provide much cleaner and much easier assignments.




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Next: Rapid site assignments of Up: Diffraction anomalous fine structure: Previous: Kramers-Kronig and FEFF analysis