XAFS measurements of negatively correlated atomic displacements in HgBa2CuO4+delta

We have investigated the local structure about the Cu, Ba, and Hg atoms in HgBa2CuO4+delta using the x-ray absorption fine structure (XAFS) technique. We find that most of the local structure results are consistent with the diffraction measurements except for two peaks in the XAFS spectra corresponding to the Cu-O(2) and Ba-O(2) atom pairs. These peaks are exceptionally broad, much broader than expected from the diffraction results. Our analysis, which includes neutron-diffraction results, indicates that the atomic displacements for the Cu and O(2) atoms are negatively correlated. The Ba and O(2) atoms may also be negatively correlated. Although this anticorrelation could be static or dynamic, we consider that it may be the result of the rapid hopping of charge between the apical O(2) ion and the CuO2 plane; this hopping may also provide a coupling mechanism for the formation of Cooper pairs.