Massless fermions in organic conductor

The electronic states in two-dimensional organic conductor alpha-(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 fennions in a-(BEDT-TTF)(2)I-3 contains intrinsically new terms of Pauli matrices alpha(x) and alpha(0) in addition to the Weyl equation which consists of alpha(z) and alpha(0). The new massless fertnions are robust against the charge disproportionation, and induce the anomalous momentumdependence in the charge density.