STM spectroscopy of (BEDT-TTF)2Cu(NCS)2

The superconducting phase of (BEDT-TTF)2Cu(NCS)2 was investigated by the electron tunneling spectroscopy using low temperature STM. The tunneling differential conductance was obtained at the b-c plane of single crystal varying the tip position. The tunneling conductance is reduced to almost zero and flat near zero bias voltage while it is finite inside the gap edge suggesting the gap anisotropy. The obtained curve is not fitted to the BCS density of states nor the simple d-wave. The anisotropic model with finite gap in which Δ(k) varies depending on the direction in k-space is examined. It is suggested that the superconducting gap is finite and highly anisotropic.