First systematic band-filling control in organic conductors

The systematic study of band-filling control for four kinds of organic conductors with various kinds of ground states has succeeded. (1) By partial substitution of (GaCl4)(-) by (MCl4)(2-) [M = Co, Zn] in the anion blocking layer of lambda-ET2(GaCl4)(-) [ET = bis(ethylenedithio)tetrathiafulvalene], single crystals of lambda-ET2(GaCl4)(-) (1-x)(MCl4)(2-) (x)[x = 0.0, 0.05, 0.06] have been obtained. The resistivity at room temperature decreases from 3 Omega cm (x = 0.0) to 0.1 Omega cm (x = 0.06) by doping to the antiferromagnet with an effective half-filled band (x = 0.0). (2) Another 2:1 (donor/anion) salt, delta'-ET2(GaCl4)(-), which is a spin gap material, has been doped as delta'-ET2(GaCl4)(-) (1-x)(MCl4)(2-) (x)[x = 0.05, 0.14]. The resistivity is lowered from 10 Q cm (x = 0.0) to 0.3 Omega cm (x = 0.14). For both 2:1 salts, the semiconducting behaviors have transferred to relatively conductive semiconducting ones by doping. (3) As for a-type 3:1 Salts, the parent material is in a charge-ordering state such as alpha-((ET+ET+ET0))(CoCl4)(2-)(TCE), where the charge-ordered donors are dispersed in the two-dimensional conducting layer. Although the calculation of alpha-ET3(CoCl4)(2-)(TCE) Shows a band-insulating nature, and the crystal structure analysis indicates that this material is in a charge-ordering state, the metallic behavior down to 165 K has been observed. With doping of (GaCl4)(-) to the alpha-system, isostructural alpha-ET3(CoCl4)(2-) (1-x)(GaCl4)(-) (x)(TCE) [x = 0.54, 0.57, 0.62] have been afforded, where the pattern of the horizontal stripe-type charge ordering changes with an increase of x. (4) By doping (GaCl4)(-) to the 3:2 gapless band insulator which is isostructural to beta'-ET3(MCl4)(2)(2-) [M = Zn, Min], the obtained beta'-ET3-(CoCl4)(2-) (2-x)(GaCl4)(-) (x)[x = 0.66, 0.88] shows metallic behavior down to 100 and 140 K, respectively. They are the first metallic states in organic conductors by band-filling control of the gapless band insulator. These systematic studies of band-filling control suggest that the doping to the gapless band insulator with a pseudo-1/2-filled band is most effective.