Pressure-induced zero-gap semiconducting state in organic conductor α-(BEDT-TTF)2I3 salt

We show a zero-gap semiconducting (ZGS) state in the quasi-two-dimensional organic conductor alpha-(BEDT-TTF)(2)I-3 salt, which emerges under uniaxial pressure along the alpha-axis (the stacking axis of the BEDT-TTF molecule). The ZGS state is the state in which a Dirac cone with the band spectrum of a linear dispersion exists around the Fermi point connecting an unoccupied (electron) band with an occupied (hole) band. The spectrum exhibits a large anisotropy in velocity, which depends on the direction from the Fermi point. By varying the magnitude of several transfer energies of a tight-binding model with four sites per unit cell, it is shown that the ZGS state exists in a wide pressure range, and is attributable to the large anisotropy of the transfer energies along the stacking axis.