A. S. Katz

Analysis of the interlayer infrared conductivity of cuprate high-transition temperature superconductors reveals an anomalously large energy scale extending up to midinfrared frequencies that can be attributed to formation of the superconducting condensate. This unusual effect is observed in a va- riety of materials, including Tl2Ba2CuO6+x, La2-xSrxCuO4, and YBa2Cu3O6.6, which show an incoherent interlayer response in the normal state. Midinfrared range condensation was examined in the context of sum rules that can be formulated for the complex conductivity. One possible interpretation of these experiments is in terms of a kinetic energy change associated with the superconducting transition.

We report a new class of $c$-axis Josephson tunneling experiments in which a conventional superconductor (Pb) is deposited across a single twin boundary of a $\mathrm{YBa}{}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ (YBCO) crystal. We measure the critical current as a function of the magnitude and angle of a magnetic field applied in the plane of the junction. In all samples, we observe a clear experimental signature of an order parameter phase shift across the twin boundary. These results provide strong evidence for mixed $d$- and $s$-wave pairing in YBCO, with a reversal in the sign of the $s$-wave component across the twin boundary.

This paper reports neutron-diffraction results on a single icosahedral quasicrystal of Al-Cu-Fe. The basic properties of the structure have been extracted using six-dimensional (6D) Patterson analysis from 219 independent orbits of reflections. Described in 6D space, the structure has hyperspace group F\ensuremath{\bigotimes}m35 and is defined by the three atomic surfaces located at special points with full icosahedral symmetry of the F lattice. These points are the two inequivalent nodes of the underlying primitive lattice plus one of the two inequivalent body centers, the remaining one being empty. The atomic surfaces are embedded in perpendicular space and are well approximated by polyhedra bounded by two-fold planes. These are a large triacontahedron located at the origin, a triacontahedron of the same size truncated along the five-fold directions at the other node, and a small polyhedron bounded by twofold planes at the occupied body center. Although no speculation has been made for distributing the atomic species within these atomic surfaces, the raw reliability factor between experimental and calculated diffraction intensities is already 0.20 with no fitting parameters and the density is found only 2.9% lower than the experimental one. The model presented here can be considered as a zero-order structure to be used for subsequent modeling. The atomic surfaces generate no unacceptably short distances between atoms. Both interatomic distances and coordination numbers of the three first shells are in good agreement with the most recent extended x-ray-absorption fine-structure results. The atomic surfaces are connected together by 3D pieces embedded in the parallel space. They define a partition of the 6D space in hyperprisms, which can be decomposed in direct products of 3D facets located in perpendicular and parallel spaces similar to the oblique cell decomposition of the 3D Penrose tiling. Phasons can propagate along the five-fold and two-fold directions by atomic jumps of 0.1705 and 0.179 nm, respectively.

In heavily twinned crystals or films of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ (YBCO), ${d}_{{x}^{2}{\ensuremath{-}y}^{2}}$ pairing symmetry is expected to cause the cancellation of first-order Josephson tunneling through a YBCO-Pb tunnel junction grown on an a-b face; any residual tunneling is thus second order. As a result, microwaves at frequency f are predicted to induce steps on the current-voltage characteristic at voltages that are multiples of $\frac{1}{2}(hf/2e)$. Experimentally, steps are observed only at multiples of $hf/2e$, suggesting that s-wave pairing is present in YBCO; however, the simultaneous presence of d-wave pairing is by no means ruled out.