Edge states in the three-quarter filled system, $\alpha$-(BEDT-TTF)${}_2$I${}_3$

We study the edge states in the two-dimensional conductor $\alpha$-(BEDT-TTF)${}_2$I${}_3$ theoretically. We show that the Dirac points and the edge states appear at the 3/4 and 1/4 filling as well as the half filling, due to four sites in the unit cell. This situation is in contract with the graphene, where the Dirac points and the edge states appear only at the half filling case. It is shown that the localization length of the edge states can become very large.