Emergence of Dirac electron pair in the charge-ordered state of the organic conductor α-(BEDT-TTF)2I3

We reexamine the band structure of the stripe charge ordered state of alpha-(BEDT-TTF)(2)I-3 under pressure by using an extended Hubbard model within the Hartree mean-field theory. By increasing pressure, we find a topological transition from a conventional insulator with a single-minimum in the dispersion relation at the M point in the Brillouin zone, toward a new phase which exhibits a double minimum. This transition is characterized by the appearance of a pair of Dirac electrons with a finite mass. Using the Luttinger-Kohn representation at the M point, it is shown that such a variation of the band structure can be described by an effective 2 x 2 low-energy Hamiltonian with a single driving parameter. The topological nature of this transition is confirmed by the calculation of the Berry curvature which vanishes in the conventional phase and has a double peak structure with opposite signs in the new phase. We compare the structure of this transition with a simpler situation which occurs in two-component systems, like boron-nitride.