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

Summary form only given. The superconducting phase of (BEDT-TTF)/sub 2/Cu(NCS)/sub 2/ was investigated with use of the scanning tunneling microscope (STM). The differential tunneling conductance was measured at the b-c surface of a single crystal. The conductance curve shows a clear superconducting gap structure below T/sub c/ and the differential conductance value is reduced to almost zero around zero bias voltage at 1.9 K. The gap value /spl Delta/ is estimated as 2/spl Delta/k/sub B/T/sub c//spl tilde/5. However, the total curve is not fitted to the BCS curve with an isotropic gap but it suggests the gap anisotropy. From the analysis of the conductance curve, we can exclude the gap with line nodes expected from the d-wave symmetry, which gives the electronic density of states linear to the energy in the low energy. At present possible models for the superconducting gap are as follows; (1)the anisotropic gap with a minimum value /spl Delta//sub min/ and (2)the gap with point nodes. The obtained conductance curve is consistent with both models within experimental errors. It is understood that the superconducting gap in (BEDT-TTF)/sub 2/Cu(NCS)/sub 2/ is highly anisotropic and the gap value is reduced to 1 meV or less at some direction of the k-space.