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 λ-ET2(GaCl4)- [ET = bis(ethylenedithio)tetrathiafulvalene], single crystals of λ-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 Ω cm (x = 0.0) to 0.1 Ω 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, δ‘-ET2(GaCl4)-, which is a spin gap material, has been doped as δ‘-ET2(GaCl4)- 1 - x (MCl4)2- x [x = 0.05, 0.14]. The resistivity is lowered from 10 Ω cm (x = 0.0) to 0.3 Ω cm (x = 0.14). For both 2:1 salts, the semiconducting behaviors have transferred to relatively conductive semiconducting ones by doping. (3) As for α-type 3:1 salts, the parent material is in a charge-ordering state such as α-(ET+ET+ET0)(CoCl4)2-(TCE), where the charge-ordered donors are dispersed in the two-dimensional conducting layer. Although the calculation of α-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 α-system, isostructural α-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 β‘-ET3(MCl4)2 2- [M = Zn, Mn], the obtained β‘-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.