Hitoshi Seo, Shoji Ishibashi, Yuichi Otsuka, Hidetoshi Fukuyama, Kiyoyuki Terakura
Electronic states of isostructural single-component molecular conductors [M(tmdt)2] (M = Ni, Au, and Cu) are theoretically studied. By considering fragments of the molecular orbitals as the basis functions, we construct a multi-orbital model common for the three materials. The tight-binding parameters are estimated from first-principles band calculations, leading to a systematic view of their electronic structures. We find that the interplay between a p\pi-type orbital (L) on each of the two ligands and a pd\sigma-type orbital (M\sigma) centered on the metal site plays a crucial role; their energy difference controls the electronic states near the Fermi energy. For the magnetic materials (M = Au and Cu), we take into account Coulomb interactions on different orbitals, i.e., consider the multi-orbital Hubbard model. Its ground-state properties are calculated within mean-field approximation where various types of magnetic structures with different orbital nature are found. An explanation for the experimental facts in [Cu(tmdt)2] is provided: The quasi-degeneracy of the two types of orbitals leads to a dual state where localized M\sigma spins appear while the L sites show a non-magnetic state due to dimerization. On the other hand, [Au(tmdt)2] locates in the subtle region of orbital mixing. We propose possible scenarios for its puzzling antiferromagnetic phase transition, involving the M\sigma orbital in contrast to previous discussions mostly concentrating on the L sector.
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http://arxiv.org/abs/1301.1116
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