Again to the question of d- or s-symmetry

At present, it can be considered practically established that the complex order parameter in high-temperature superconductors has d-wave symmetry. In traditional superconductors, the phase of the order parameter does not depend on the coordinates, and its amplitude can vary with direction. A similar calculation for lead in the framework of the theory of a strong electron-phonon coupling (to which we will return below) was carried out almost 35 years ago by Bennett [1]. He found that the main source of anisotropy of superconducting properties is the angular dependence of the phonon spectrum, although a certain contribution is made by the anisotropy of the Fermi surface. As a result, it turned out that in the equatorial plane the maximum spread of the gap values for lead is about 10%.