Fig. 1

Crystal structure of κ-(ET)₄Hg_2.89Br₈ (denoted by κ-HgBr). a ET denotes bis(ethylenedithio)-tetrathiafulvalene. b Shows the conducting ET layer, which is parallel to the b-c plane. The electronic bands of κ-ET compounds are well described by the tight binding of antibonding molecular orbitals in the ET dimers (indicated by broken circles). The lattice of dimers is modeled as a nearly isotropic triangular lattice with a t′/t value close to unity, where t and t′ are the transfer integrals between the antibonding orbitals of dimers forming a conduction band¹¹. The non-stoichiometric composition of Hg, 2.89, originates from the incommensurability of the Hg sublattice with the Br and ET sublattices, and is precisely known by the incommensurability determined by X-ray diffraction¹³