Extended x-ray-absorption fine-structure study ofLa2−xSrxCuO4−ysuperconductors

From measurements of extended x-ray-absorption fine structure at the Cu $K$ edge between 10 and 300 K, we have determined the temperature dependence of the mean-square relative displacements ${\sigma }^2$ for Cu-O, Cu-La, and Cu-Cu near neighbors in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$ with $x=0, 0.15, \mathrm{and} 0.3$. It is found that ${\sigma }^2$ for Cu-O and Cu-La neighbors shows little variation with $x$. Model calculations are used to demonstrate that ${\sigma }_{\mathrm{Cu}-\mathrm{O}}^2$ is sensitive to softening of the Cu-O breathing mode which has been predicted to be important for superconductivity in these materials. The Cu-O bonding is observed to be quite strong and no evidence is found for any significant change in the breathing-mode frequency with doping.