Tunneling spectroscopy on the organic superconductorκ−(BEDT−TTF)2Cu(NCS)2using STM

STM spectroscopy on the organic superconductor $\kappa -(\mathrm{BEDT}-\mathrm{TTF}{)}_2\mathrm{Cu}(\mathrm{NCS}{)}_2$ 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 $\pi /4\mathrm{}$ from the $k_b$ and $k_c$ axes considering the $\mathbf{k}$ dependence of the tunneling transition probability. These results strongly indicate that the superconducting pair wave function in this salt has the $d_{x^2-y^2}$-wave symmetry.