Stripe structure in the CuO2 plane of perovskite superconductors

The Cu-O pair-distribution function in a Bi2Sr2CaCu2O8+y superconductor has been measured by polarized Cu K-edge extended x-ray-absorption fine structure at T<Tc. The results show an anomalous long Cu-O (planar) distance, 1.96 Å, assigned to distorted CuO2 stripes of width W intercalated with undistorted stripes of width L. From the measurement of L=15±0.5 Å 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 ωD is the Debye frequency, giving the Tc amplification.