Admixture of an s-wave component to the d-wave gap symmetry in high-temperature superconductors

Neutron crystal-field spectroscopy experiments in the Y- and La-type high-temperature superconductors HoBa2Cu3O6.56, HoBa2Cu4O8, and La1.81Sr0.15Ho0.04- CuO4 are reviewed. By this bulk-sensitive technique, information on the gap function is obtained from the relaxation behavior of crystal-field transitions associated with the Ho3+ ions which sit as local probes close to the superconducting copper-oxide planes. The relaxation data exhibit a peculiar change from a convex to a concave shape between the superconducting transition temperature T-c and the pseudogap temperature T* which can only be modeled satisfactorily if the gap function of predominantly d-wave symmetry includes an s-wave component of the order of 20-25%, independent of the doping level. Moreover, our results are compatible with an unusual temperature dependence of the gap function in the pseudogap region (T (c) <= T <= T*), i.e., a break up of the Fermi surface into disconnected arcs.