Stripe structure in the ${\mathrm{CuO}}_2$ plane of perovskite superconductors

The Cu-O pair-distribution function in a ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathit{y}}$ superconductor has been measured by polarized Cu K-edge extended x-ray-absorption fine structure at T<${\mathit{T}}_{\mathit{c}}$. The results show an anomalous long Cu-O (planar) distance, 1.96 Å, assigned to distorted ${\mathrm{CuO}}_2$ 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 ${\mathit{E}}_{\mathit{n}}$ of the bottom of the one-dimensional subbands of the superlattice and found that the Fermi level ${\mathit{E}}_{\mathit{F}}$ is tuned to a ‘‘shape resonance’’ ${\mathit{E}}_{\mathit{F}}$-${\mathit{E}}_{\mathit{n}}$<ħ${\mathrm{\omega }}_{\mathit{D}}$, where ${\mathrm{\omega }}_{\mathit{D}}$ is the Debye frequency, giving the ${\mathit{T}}_{\mathit{c}}$ amplification. © 1996 The American Physical Society.