STM spectroscopy in superconducting phase of (BEDT-TTF)2Cu(NCS)2

The superconducting phase of (BEDT-TTF)2Cu(NCS)2 was investigated with use of the scanning tunneling microscope (STM). The tunneling conductance was measured at the b-c surface of a single crystal. The differential conductance shows a clear superconducting gap structure below the transition temperature and its value is reduced to almost zero around zero bias voltage at 1.9 K. The conductance curve is not well fitted to the BCS one but suggests the gap anisotropy. However, the gap with line nodes, expected from the simple d-wave symmetry, is excluded. Possible models for the superconducting gap are as follows; (1) the gap with point nodes or (2) the anisotropic gap with a minimum value Δmin∼1 mev. It is understood that the superconducting gap is highly anisotropic in k-space.