Hirohito Aizawa, Kazuhiko Kuroki, Syuma Yasuzuka, Jun-ichi Yamada
We perform a first principles band calculation for a group of two-dimensional organic conductors \beta-(BDA-TTP)2MF6 (M=P, As, Sb, Ta). The ab-initio calculation shows that the density of states (DOS) is correlated with the band width of semi-occupied molecular orbital (SOMO), while it is not necessarily correlated with the unit cell volume. The direction of the major axis of the cross section of the Fermi surface lies in the \Gamma-B direction, which differs from that obtained by the extended Huckel calculation. Then, we construct a tight-binding model which accurately reproduces the ab-initio band structure. The obtained transfer energies give smaller dimerization than in the extended Huckel band. As for the difference of the anisotropy of the Fermi surface, the transfer energies along the inter-stacking direction are smaller than those obtained in the extended Huckel calculation. Assuming spin-fluctuation-mediated superconductivity, we apply random phase approximation (RPA) to a two-band Hubbard model obtained by adding an on-site (intra-molecule) repulsive interaction to the tight-binding model derived from the first principles band structure. The obtained superconducting gap changes sign four times along the Fermi surface like in a d-wave gap, and the nodal direction is different from that obtained in the extended Huckel model. Anion dependence of Tc is qualitatively consistent with the experimental observation.
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http://arxiv.org/abs/1205.0396
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