Conduction properties of the organic superconductor $\kappa -{\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2{\mathrm{Cu}\left(\mathrm{NCS}\right)\mathrm{}}_2$ based on Hubbard–unrestricted-Hartree-Fock band calculations

The organic superconductor $\kappa -{\mathrm{}(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}-\mathrm{T}\mathrm{T}\mathrm{F})\mathrm{}}_2{\mathrm{Cu}\left(\mathrm{NCS}\right)\mathrm{}}_2$ (with $T_c=10.4\mathrm{}\mathrm{K}$) 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 $U_{\mathrm{Coul}}\mathrm{\sim }0.6775\mathrm{}\mathrm{eV},$ 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.