Evidence for an axial oxygen-centered lattice fluctuation associated with the superconducting transition in YBa2Cu3O7

The axial oxygen contributions to the Cu K-edge, polarized, extended x-ray-absorption fine structure of YBa₂Cu₃O₇ are analyzed from 10<=T<=105 K, using a novel approach in which the radial distribution function is directly calculated from model potentials. The best fit is obtained with a double-well potential with two nearly equally populated O(4) sites 0.13 Å apart. The site separation decreases 0.02 Å in a region near T_c. This perturbation in the structure is accompanied by a large increase of the tunneling frequency of ~80 K between the two wells, suggesting a coupling between superconducting fluctuations and nonlinear phonons.