Electronic structure of insulating salts of the kappa-(BEDT-TTF)(2)X family studied by low-temperature specific-heat measurements

The low-temperature specific heat of kappa-(BEDT-TTF)(2)X (X=Cu[N(CN)(2)]Cl and Cu[N(CN)(2)]Br) and kappa-(d(8),BEDT-TTF)(2)Cu[N(CN)(2)]Br have been studied down to about 0.85 K and under magnetic fields up to 8 T. The kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Cl and kappa-(d(8),BEDT-TTF)(2)Cu[N(CN)(2)]Br compounds do not have any finite gamma value, which indicates they are of an insulating nature with nonvanishing charge gap at low temperatures. The X=Cu[N(CN)(2)]Cl salt shows no distinct peak around the antiferromagnetic transition at 26-27 K, which suggests a two-dimensional short-range magnetic order far above the three-dimensional ordering temperature typical of a high-T-c cuprate.