Zero-Gap State in Organic Conductor α-(BEDT-TTF)2NH4Hg(SCN)4

The zero-gap state (ZGS) where the conduction and valence bands coincide only at a contact point corresponding to the chemical potential, is theoretically predicted for organic conductor α-(BEDT-TTF) 2 NH 4 Hg(SCN) 4 using the transfer energies obtained from the X-ray experiment by Kondo et al. The ZGS emerges under uniaxial pressure, P c , along the stacking axis, for P c > 5.0 kbar. Our calculation of the Berry curvature confirms that the phase of wave function exhibits the singularity indicating the Dirac electron. The peaks of Berry curvature are highly anisotropic at the ambient pressure, while those become nearly isotropic at high pressure.