Crystal and electronic structures and physical properties of Tc=10.4 K superconductor, (BEDT-TTF)2Cu(NCS)2

(BEDT-TTF)2Cu(NCS)2 was found to be the first ambient pressure organic superconductor with the Tc higher than 10 K. The salt has a layered structure which is composed of donor and anion sheets. The packing pattern of donors of the Cu(NCS)2 salt is nearly analogous to κ-(BEDT-TTF)2I3. In anion layers CuI is trigonal coordinated to two N atoms and one S atom to construct a one-dimensional planar polymer. The valence state of CuI was observed by ESCA at 298 K and ESR at 298-4 K. One broad Lorentzian ESR signal ascribed to a BEDT-TTF cation radical was obtained. The g-values are independent of temperature, while the linewidth shows broadening with lowering temperature even though the resistivity decreases below 90 K. The ESR magnetic susceptibility is constant down to 90 K, which is suggesting Pauli-like paramagnetism. The thermopower is anisotropic in the two-dimensional bc plane, which is originated from the anisotropy of the band structure.