Massless Fermions in Organic Conductor

The electronic states in two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 have been investigated to show the noticeable property of the massless fermions, i.e., the linear dispersion which exists on the contact point between the conduction band and the valence band. These fermions are well known in bismuth and graphite, where the former are described by the Dirac equation and the latter obeys the Weyl equation corresponding to the massless fermion. In the present study, we show that the effective Hamiltonian describing the massless fermions in α-(BEDT-TTF) 2 I 3 contains intrinsically new terms of Pauli matrices σ z and σ 0 in addition to the Weyl equation which consists of σ x and σ y . The new massless fermions are robust against the charge disproportionation, and induce the anomalous momentum-dependence in the charge density.