Electronic structure of the organic conductor α-(BEDT-TTF)2I3sstudied by angle-resolvedand core-level photoelectron spectroscopy

The electronic structure of the organic conductor α-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ has been studied by photoelectron spectroscopy on in situ cleaved crystals. The valence-band regime consisted of eight distinct features with binding energies between 1.2 and 10.9 eV. Angle-resolved measurements showed no dispersion along the Γ-Z (${\mathrm{k}}_{\mathrm{z}}$ ) direction, i.e., perpendicular to the conducting BEDT-TTF layers, for the structures in the valence-band regime. Angle-resolved measurements along the a and b axis, i.e., approximately along a and b, gave no evidence that the structures closest to the Fermi level show any dispersion in the highly conducting plane. The present experiment sets an upper limit for the dispersion of about 0.25 eV. The lack of any (observable) dispersion and the lack of a sharp Fermi edge in spite of α-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ showing metallic transport properties are thoroughly discussed. It is argued that correlation effects or the calculated narrow-gap semiconductor band structure of α-(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ are the most probable explanations for the latter phenomena. From the photon energy dependence, the atomic origin of several structures in the valence-band regime is suggested. Core-level spectroscopy on the I 4d level revealed the presence of one bulk component and a surface shifted component. S 2p core-level spectra revealed no surface or chemically shifted components. The large width of the bulk components of these core levels is primarily ascribed to phonon broadening.