Realization of superconductivity at ambient pressure by band-filling control in kappa-(BEDT-TTF)(2)Cu-2(CN)(3)

The resistivity, thermoelectric power (TEP), ESR and static susceptibility were measured on kappa- and and kappa'-(BEDT-TTF)(2)Cu-2(CN)(3); also, a band calculation based on an extended Huckel method nas re-examined and the results were compared with those of related BEDT-TTF (ET) salts. Although the crystal- and calculated band structures of these salts are identical within experimental error, the former is a semiconductor; on the contrary, the latter is a metal at ambient pressure (AP). The band calculation and the spin susceptibility (chi(spin)) measurement suggest that the former salt is a Mott-insulator with an exactly half-filled band. Although kappa-(ET)(2)Cu-2(CN)(3) shows only the ESR signal of ET cation radicals, the kappa'-salt shows an additional ESR signal of Cu2+ below about 20 K. The content of Cu2+ is 88-1200 ppm of total copper atoms. It is found that the metallic state and the superconductivity at AP of kappa'-(ET)(2)Cu-2(CN)(3) is realized by a band-filling control by substituting Cu+ with Cu2+.