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STM spectroscopy on the organic superconductor-(BEDT-TTF) 2 Cu(NCS) 2 has been performed in the superconducting state with the use of single crystals. The anisotropy of the superconducting gap has been investigated both on the b-c plane, which is parallel to the two-dimensional conducting layers, and on the several surfaces perpendicular to the b-c plane. The tunneling spectra observed on the b-c plane are well explained by the anisotropic gap with the d-wave symmetry. The spectra on the lateral surfaces are also consistent with the d-wave gap with line nodes along the direction /4 from the k b and k c axes considering the k dependence of the tunneling transition probability. These results strongly indicate that the superconducting pair wave function in this salt has the d x 2 Ϫy 2-wave symmetry.

A series of bis(p-dimethylaminophenyl) (p-substituted phenylethynyl) carbonium perchlorates has been prepared. The electronic spectral properties are affected by both coplanarity of the cation parts and the effect of substituent on phenyl ring. A linear relationship was obtained between the wave numbers (v) of the longest wavelength absorption maxima and op.

The synthesis, electrochemical properties, and molecular structure of a new pi-electron donor, 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP), is described. In contrast to the hitherto-known tetrachalcogenafulvalene pi-donors providing organic superconductors, this donor contains only the bis-fused 1,3-dithiole-2-ylidene unit as a pi-electron system, yet produces a series of ambient-pressure superconductors beta-(BDA-TTP)2X [X = SbF6 (magnetic T(c) = 6.9 K, resistive T(c) = 7.5 K), AsF6 (magnetic T(c) = 5.9 K, resistive T(c) = 5.8 K), and PF6 (magnetic T(c) = 5.9 K)], which are isostructural. The values of the intermolecular overlap integrals calculated on the donor layers of these superconductors suggest a two-dimensional (2D) electronic structure with loose donor packing. Tight-binding band calculations also indicate that these superconductors have the 2D band dispersion relations and closed Fermi surfaces.