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

TL;DR

This study uses neutron spectroscopy to reveal that high-temperature superconductors have a mixed d-wave and s-wave gap symmetry, with the s-wave component being about 20-25%, affecting the understanding of their electronic structure.

Contribution

It provides experimental evidence for an s-wave admixture in the d-wave gap symmetry of high-temperature superconductors, a novel insight into their pairing mechanism.

Findings

The gap function includes a 20-25% s-wave component.

The gap symmetry changes from convex to concave at Tc.

Results suggest a breakup of the Fermi surface into arcs in the pseudogap region.

Abstract

Neutron crystal-field spectroscopy experiments in the Y- and La-type high-temperature superconductors HoBa2Cu3O6.56, HoBa2Cu4O8, and La1.81Sr0.15Ho0.04CuO4 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 Tc and the pseudogap temperature T* which can only be modelled 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 (Tc<T<T*), i.e., a breakup of the Fermi surface into disconnected arcs.