THE MEASUREMENT OF THE POLARON SIZE IN THE METALLIC PHASE OF CUPRATE SUPERCONDUCTORS

The statistical distribution of the Cu site conformations in the CuO2 plane of Bi2Sr2CaCu2O8+y (Bi2212) has been measured by a fast and local probe Cu K-edge extended X-ray absorption fine structure (EXAFS) as a function of temperature. The Cu site conformations are frozen at T < T* = 120 K similar to 1.4 T-e in two well-defined conformations characterized by two Cu-O(apical) bond lengths separated by 0.17 +/- 0.02 Angstrom. The Cu site conformations with short bond 2.38 +/- 0.02 Angstrom have a probability of 41 +/- 2% at T < T*. The diffraction experiments show that the Cu site conformations exhibit a one-dimensional anharmonic modulation, with period lambda(p) = 25 Angstrom, that has been measured by electron diffraction. These data show that the modulation is anharmonic and the plane is decorated with stripes of distorted lattice characterized by the short Cu-O(apical) bonds of width W = 11 +/- 1 Angstrom at T < T* that form domain walls for the stripes of width L similar to 14 +/- 1 Angstrom, characterized by the long Cu-O(apical) bond. The stripes of distorted lattice run below the lines of interstitial oxygen ions in the BiO plane, playing the role of acceptors. The lattice modulation is therefore assigned to polaronic impurity states ordered in linear arrays. The measure of the width W of the stripes provides the measure of the area of the polarons S-p similar to 4a(2) (a = 5.4 Angstrom) where each hole dressed by the lattice distortion spans a domain of 8 Cu sites.