First-principles calculation of the electronic and optical properties of the organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$

The electronic structure and the optical properties of the organic superconductor κ-(BEDT-TTF${)}_2$Cu(NCS${)}_2$ are fully elucidated by means of first-principles self-consistent local-density calculation. The band effective masses on the Fermi surfaces are found to be ${\mathit{m}}_{\mathrm{\alpha }}^{\mathit{b}\mathrm{*}}$=1.77 m and ${\mathit{m}}_{\mathrm{\beta }}^{\mathit{b}\mathrm{*}}$=3.50 m, and the density of state at the Fermi level is 12.8/eV cell. These results are quite different from the existing calculations based on semiempirical methods, and are in much better agreement with experiments. Remarkable similarities in the electronic structure between the organic superconductors and the alkali-doped fullerene superconductors are pointed out and the implications discussed.