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 HoBa₂Cu₃O_6.56, HoBa₂Cu₄O₈, and La_1.81Sr_0.15Ho_0.04- 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.