Conduction properties of the organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) based on Hubbard-unrestricted-Hartree-Fock band calculations

The organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2) (with T-c = 10.4 K) possesses a number of puzzling electronic properties, including the temperature dependence of resistivity, magnetic susceptibility, and Hall coefficient. To provide a basis for understanding these properties, we carried out band calculations using the two-dimensional Hubbard model with the unrestricted-Hartree-Fock theory. The electron transfer hopping interactions are from ab initio calculations. For a Hubbard parameter of U(Coul)similar to 0.6775 eV, the calculated results give good agreement with Shubnikov-de Haas and magnetic breakdown experiments. The calculations lead to a two-band semimetal with a momentum gap separating the electron and hole bands. The anomalous experimental observations are explained in terms of BEDT-TTF related phonons coupling these two bands. These results also provide a framework for describing the conduction properties of similar BEDT-TTF crystals. This approach should also be useful for other organic conductors. [S0163-1829(97)03442-5].