Admixture of an s-Wave Component to the d-Wave Gap Symmetry in High-Temperature Superconductors

摘要

Neutron crystal-field spectroscopy experiments in the Y- and La-type high-temperature superconductors HoBa{sub}2Cu{sub}3O{sub}6.56, HoBa{sub}2Cu{sub}4O{sub}8, and La{sub}1.81Sr{sub}0.15Ho{sub}0.04-CuO{sub}4 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 Ho{sup}(3+) 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{sub}c and the pseudogap temperature T{sup}* 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{sub}c ≤ T ≤ T{sup}*), i.e., a break up of the Fermi surface into disconnected arcs.