Orthorhombic distortion at the superconducting transition in YBa2Cu3O7: Evidence for anisotropic pairing

We report the results of a high-resolution x-ray scattering study of the structure of YBa2Cu3O7 as a function of temperature. We find that there is an anomaly in the orthorhombic splitting, b-a, near the superconducting transition, whereas there is little or no anomaly in the unit-cell volume. Our results suggest that the superconducting wave function has a uniaxial anisotropy within the a-b plane. © 1987 The American Physical Society.