Correlation between axial-oxygen anharmonicity and ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and related compounds

Cu-O radial distribution functions in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$, ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.5}$, and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_{2.8}$${\mathrm{Co}}_{0.2}$${\mathrm{O}}_{7+\mathrm{\delta }}$ at 10 K have been determined from Cu K-edge polarized extended x-ray-absorption fine structure of these materials, using model potentials to describe the effect of the Cu-O relative motion. The analysis shows that the axial oxygen atom moves in a double-well potential. A correlation between interwell tunneling and ${\mathit{T}}_{\mathit{c}}$ is observed, and interpreted as a correlation between anharmonicity and ${\mathit{T}}_{\mathit{c}}$.