STRIPE STRUCTURE IN THE CUO2 PLANE OF PEROVSKITE SUPERCONDUCTORS

The Cu-O pair-distribution function in a Bi2Sr2CaCu2O8-y superconducto r has been measured by polarized Cu K-edge extended X-ray-absorption f ine structure at T<T-c. The results show an anomalous long Cu-O (plana r) distance, 1.96 Angstrom, assigned to distorted CuO2 stripes of widt h W intercalated with undistorted stripes of width L. From the measure ment of L=15+/-0.5 Angstrom we have calculated the energies E(n) of th e bottom of the one-dimensional subbands of the superlattice and found that the Fermi level E(F) is tuned to a ''shape resonance'' E(F)-E(n) <h omega(D), where omega(D) is the Debye frequency, giving the T-c amp lification.