Stripe structure in the CuO 2 plane of perovskite superconductors

The Cu-O pair-distribution function in a ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathit{y}}$ superconductor has been measured by polarized Cu K-edge extended x-ray-absorption fine structure at T${\mathit{T}}_{\mathit{c}}$. The results show an anomalous long Cu-O (planar) distance, 1.96 \AA{}, assigned to distorted ${\mathrm{CuO}}_{2}$ stripes of width W intercalated with undistorted stripes of width L. From the measurement of L=15\ifmmode\pm\else\textpm\fi{}0.5 \AA{} we have calculated the energies ${\mathit{E}}_{\mathit{n}}$ of the bottom of the one-dimensional subbands of the superlattice and found that the Fermi level ${\mathit{E}}_{\mathit{F}}$ is tuned to a ``shape resonance'' ${\mathit{E}}_{\mathit{F}}$-${\mathit{E}}_{\mathit{n}}$${\mathrm{\ensuremath{\omega}}}_{\mathit{D}}$, where ${\mathrm{\ensuremath{\omega}}}_{\mathit{D}}$ is the Debye frequency, giving the ${\mathit{T}}_{\mathit{c}}$ amplification. \textcopyright{} 1996 The American Physical Society.