Abstract
Quasiparticle tunneling measurements of the high-temperature superconductors are considered in the context of symmetry of the superconducting order parameter and a two-dimensional (2D) van Hove singularity (vHs) related to saddle points in the electronic band structure. Normal-metal–insulator–superconductor tunneling spectra taken at 4.2 K with a scanning tunneling microscope on Hg-1212 -axis epitaxial films, as well as on Hg-1201 and Hg-1223 polycrystalline samples, show distinct gap characteristics which cannot be easily reconciled with the simple -wave BCS density of states. The data are analyzed with the nodal -wave gap function and the 2D tight-binding electronic dispersion using the quasiparticle tunneling formalism for elastic and specular transmission. The analysis indicates a highly directional and energy-dependent spectral weighting, related to the gap anisotropy and band-structure dependence of the tunneling matrix element and successfully explains the observed gap spectra. Values for the -wave gap maximum are determined to be 50, and 75 meV, respectively, for optimally doped Hg-1201, Hg-1212, and Hg-1223, corresponding to reduced-gap ratios of 9.5, and 13. These ratios are substantially larger than the BCS weak-coupling limit of 3.54. A comparison with data from other high- cuprates indicates an overall trend of rising with in violation of BCS universality.
- Received 31 January 1997
DOI:https://doi.org/10.1103/PhysRevB.57.3650
©1998 American Physical Society