On the Possible Zero-Gap State in Organic Conductor α-(BEDT-TSF)2I3 under Pressure

We study the electronic state of the organic conductor alpha-(BEDT-TSeF)(2)I-3 (BETS) in which a zero-gap state (ZGS) of massless Dirac fermions is expected under pressure. A transition from an insulating state to a ZGS is investigated using an extended Hubbard model with seven kinds of transfer energy between the four molecules A, A ' (= A), B, and C per unit cell and with the interaction terms, namely, on-site and nearest-neighbor Coulomb interactions, and an anion potential. From the analysis of the change in the transfer energies, we find that the transfer energies along the stacking axis play an important role in stabilizing the ZGS. The inclusion of the renormalization effect of the transfer energies, which arises from the Fock mean fields, is crucial to understanding the appearance of the ZGS under high pressure. We present the phase diagram that is obtained by including this renormalization effect.