EXAFS and diffraction studies on high

Abstract

The modulated structure of the CuO2 plane in Bi2Sr2CaCu2O8+y (Bi2212) superconductor has been solved by joint polarized Cu K-edge extended x-ray absorption fine structure (EXAFS) and Cu K-edge anomalous x-ray diffraction. The statistical distribution of the Cu-O pairs has been measured by EXAFS and the spatial distribution of the lattice distortions of the CuO2 plane by anomalous diffraction. The incommensurate superstructure involves a displacement of Cu ions along the c-axis with a large anharmonic character giving a split distribution of the Cu-O(apical) bonds in agreement with the statistical distribution obtained by EXAFS. The EXAFS results show an anomalously large Cu-O(planar) distance, 1.96 A, assigned to distorted CuO2 stripes of width W in between undistorted stripes of width L. From the measurement of L=15±0.5 A we have calculated the energies En of the bottom of the one-dimensional subbands of the superlattice and found that the Fermi level EF is tuned to a “shape resonance” EF−En<ℏωD, where ω...