Mechanism of zero gap electronic states in organic conductor

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In this study, we have studied the thermo electric power under hydrostatic pressure up to 1.6 GPa to clarify the electronic state characteristic to the new type zero gap states around the Dirac point, where the conduction band contact with the valence band. We developed the TEP measurements system under hydro static pressure. Under weak pressure region, we observed the rapid reduction of Seebeck coefficient due to the charge ordered gap formation. Above 1.3 GPa, slow reduction of Seebeck coefficient below 50 K represent that reduction of the density of states at the Dirac point caused by the linear dispersion. We have clarified that the density of states (DOS) shows the linear deviates from the linear energy dependence expected in a ZGS system and shows the asymmetry of the electronic structure around the Dirac point. These experimental results provide crucial information on the fundamental nature of the ZGS in this bulk system.