Direct observation of in-plane gap anisotropy in (BEDT-TTF)2Cu(NCS)2

The superconducting phase of kappa-(BEDT-TTF)(2)Cu(NCS)(2) was investigated by the electron tunneling spectroscopy using low temperature STM. The tunneling differential conductance obtained at the b-c surface shows V-shaped gap structure consistent with the d-wave symmetry. The lateral surface of single crystals was also investigated. We found that the tunneling spectrum varies its shape depending on the tip direction. This indicates the gap anisotropy. Taking into account the k-dependence of the tunneling transition probability, the in-plane anisotropy of the conductance is well explained by the d-wave symmetry with line nodes along the direction 45 degrees from the c-axis. The d(x)(2)-(2)(y)-wave pairing is strongly suggested in this material.