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.

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Metallic behavior down to low temperature is shown by charge transfer salts of BDH-TTP (1), a structural isomer of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF). In the case of (BDH-TTP) PF this behavior is attributed to the structure, which is made up of κ-type sheets of BDH-TTP donor molecules and sheets of PF anions.

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