Electronic transport properties and structural transformations of κ-(BEDT-TTF)2Cu[N(CN)2]I

Formation of an insulating ground state in the salt of κ-(BEDT-TTF)2Cu[N(CN)2]I was studied by measuring the temperature dependence of the electronic transport properties (the anisotropy of the resistivity, thermopower along the principal in-plane directions, and the Hall effect) and the x-ray diffraction patterns. Qualitative agreement of all the transport properties with the metallic state predicted by the band structure calculation was found above 230 K. Transformation to the insulating state proceeds via an intermediate stage between 230 and ∼100 K, in which the transport properties are affected by localization phenomena in the disordered metallic state. The state is induced by the formation of a commensurate superstructure in the anion layer, leading to a unit cell doubling in the c-direction. The electronic spectrum starts to form a real gap below 100 K, as indicated by the sharp thermopower change, which is mainly related to the destruction of the hole pocket. The gap formation is accompanied by an in-plane resistivity anisotropy increase. This transformation is due to a short-range ordering, with a wave vector close to c*/3, strongly interacting with the electronic system