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

Abstract

STM spectroscopy on the organic superconductor $\ensuremath{\kappa}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{Cu}(\mathrm{NCS}{)}_{2}$ 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 $b\ensuremath{-}c$ plane, which is parallel to the two-dimensional conducting layers, and on the several surfaces perpendicular to the $b\ensuremath{-}c$ plane. The tunneling spectra observed on the $b\ensuremath{-}c$ 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 $\ensuremath{\pi}/4$ 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 ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-wave symmetry.