2 Boeing Phantom Works, Seattle, Washington 98124 the generalized BTK theory, with b as the tunnel-cone width, Z the barrier strength, D 0 the d-wave gap maximum, and 2D 0 ͞k B T c the reduced-gap ratio.

Quasiparticle tunneling measurements of the high-temperature superconductors HgBa 2 Ca nϪ1 Cu n O 2nϩ2ϩ␦ (Hg-12(nϪ1)n,nϭ1,2,3) are considered in the context of d x 2 Ϫy 2 symmetry of the superconducting order parameter and a two-dimensional ͑2D͒ van Hove singularity ͑vHs͒ 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 c-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 s-wave BCS density of states. The data are analyzed with the nodal d-wave gap function ⌬ k ϭ⌬ 0 (cos k x Ϫcos k y )/2 and the 2D tight-binding electronic dispersion k ϭϪ2t(cos k x ϩcos k y )ϩ4tЈ(cos k x cos k y )Ϫ, 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 ͉T͉ 2 , and successfully explains the observed gap spectra. Values for the d-wave gap maximum are determined to be ⌬ 0 Ϸ33, 50, and 75 meV, respectively, for optimally doped Hg-1201, Hg-1212, and Hg-1223, corresponding to reduced-gap ratios of 2⌬ 0 /k B T c Ϸ7.9, 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-T c cuprates indicates an overall trend of 2⌬ 0 /k B T c rising with T c , in violation of BCS universality.

Point-contact spectroscopy was performed on single crystals of the heavy-fermion superconductor CeCoIn(5) between 150 mK and 2.5 K. A pulsed measurement technique ensured minimal Joule heating over a wide voltage range. The spectra show Andreev-reflection characteristics with multiple structures which depend on junction impedance. Spectral analysis using the generalized Blonder-Tinkham-Klapwijk formalism for d-wave pairing revealed two coexisting order parameter components with amplitudes Delta(1) = 0.95 +/- 0.15 meV and Delta(2) = 2.4 +/- 0.3 meV, which evolve differently with temperature. Our observations indicate a highly unconventional pairing mechanism, possibly involving multiple bands.

The Fulde-Ferrell-Larkin-Ovchinnikov ͑FFLO͒ state has received renewed interest recently due to the experimental indication of its presence in CeCoIn 5 , a quasi-two-dimensional ͑2D͒ d-wave superconductor. However direct evidence of the spatial variation of the superconducting order parameter, which is the hallmark of the FFLO state, does not yet exist. In this work we explore the possibility of detecting the phase structure of the order parameter directly using conductance spectroscopy through microconstrictions, which probes the phase sensitive surface Andreev bound states of d-wave superconductors. We employ the Blonder-Tinkham-Klapwijk formalism to calculate the conductance characteristics between a normal metal ͑N͒ and a 2D s-or d x 2 −y 2-wave superconductor in the Fulde-Ferrell state, for all barrier parameter z from the point contact limit ͑z =0͒ to the tunneling limit ͑z 1͒. We find that the zero-bias conductance peak due to these surface Andreev bound states observed in the uniform d-wave superconductor is split and shifted in the Fulde-Ferrell state. We also clarify what weighted bulk density of states is measured by the conductance in the limit of large z.

No abstract