Crystal and Electronic Structures of the Quasi-Two-Dimensional Organic Conductor $\alpha$-(BEDT-TTF)2I3 and Its Selenium Analogue $\alpha$-(BEDT-TSeF)2I3 under Hydrostatic Pressure at Room Temperature

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We perform crystal structure analyses of the organic conductor $\alpha$-(BEDT-TTF)2I3 [where BEDT-TTF denotes bis(ethylene)dithiotetrathiafulvalene] and its selenium analogue $\alpha$-(BEDT-TSeF)2I3 [where BEDT-TSeF denotes bis(ethylene)dithiotetraselenafulvalene] at room temperature under hydrostatic pressure, where they are expected to have zero-gap points at the Fermi level with a Dirac-cone-type dispersion for a massless particle. We calculate the electronic band structures using a combination of the extended Hückel molecular orbital calculation based on the obtained crystal structures and the tight-binding band structure calculation. We find that I) the obtained Fermi surfaces of both salts under hydrostatic pressure are simply larger than those at ambient pressure owing to an increase in overlap integrals caused by the pressure, and II) electronic structures having zero-gap points with a Dirac-cone-type dispersion at the Fermi level are realized only in $\alpha$-(BEDT-TTF)2I3 by the introduction of suitable site potentials in wide parameter areas. These results suggest that, to realize such a unique electronic structure in this class of materials, some mechanisms are needed to effectively give the site potentials employed in this study. Examples are electronic correlation effects and I3 anion potentials.
2009-11-15