Quasiparticle tunneling into high-T-c Mercury-cuprate superconductors: Effects of d(x2-y2) order parameter symmetry and a two-dimensional van Hove singularity

Quasiparticle tunneling measurements of the high-T-c superconductors Hg1Ba2Can-1CunO2n+2+delta (n = 1,2,3) are considered in the context of d(x)2(-y)2 symmetry of the superconducting orderparameter and a 2-D van Hove singularity related to saddle-points in the bandstructure. We present tunneling spectra taken with a low-temperature scanning tunneling microscope on Hg-1212 epitaxial films and on polycrystalline Hg-1201 and Hg-1223. The data is analyzed with elastic tunneling formalism, using the nodal gap-function Delta(k) = Delta(0)(cos kx - cos ky)/2 and the 2D electronic dispersion E(k) = -2t(cos kx + cos ky) + 4t'(cos kx cos ky). Our analysis accounts for all the generic spectral features: Gap-anisotropy introduces spectral smearing. Breakdown of the WKB-approximation near band extrema gives rise to a quasi-linear conductance background. Doping-dependence and next-nearest-neighbor interaction produce spectral asymmetry. Low c-axis dispersion enables enhancement of the spectral peaks by the van Hove singularity.