  Experimental results obtained by using x-ray absorption spectroscopy show
that the configurations of Cu sites in the CuO2 plane of Bi 2:2:1:2 high Tc
superconductors are not homogeneous. Different Cu sites are characterized by
short 0.23 nm and long 0.245 nm Cu-O(apical) distances. The linear arrays of
different Cu sites forming domains with a corrugated-iron like shape is
proposed to be a key characteristic of superconducting domains in the CuO2
plane. The wavelength of the modulation is close to the superconducting
coherence length. The ordering of the distorted Cu sites is suggested to be
evidence for ordering of polarons driven by the pseudo Jahn Teller electron
lattice interaction. The Cu L3 XAS experiments on Bi 2:2:1:2 system indicate
that the electronic states added by doping, 4% have the a1 symmetry (i.e. with
Cu 3d(3z2-r2), the combination of O (planar) 2px,y orbital with a1 symmetry
L(a1), and O(apical) 2pz orbital character) and 15% have the b1 symmetry
(3d(x2-y2) and the combination of O (planar) 2p(x,y) orbital with b1 symmetry
L(b1)). This new scenario supports the pairing mechanisms for high Tc
superconductivity in the presence of two components: 1) the more delocalized
component with b1 symmetry and 2) the more localized component, with partially
a1 symmetry associated with different parts of the Fermi surface.
