Electronic structure of alpha t-(BEDT-TTF)2I3: A photoemission study.

Abstract

Photoemission experiments have been performed on in situ cleaved crystals of the organic superconductor ${\mathrm{\ensuremath{\alpha}}}_{\mathit{t}}$-(BEDT-TTF${)}_{2}$${\mathrm{I}}_{3}$. Nine distinct features are observed in the valence-band regime with binding energies between 1.4 and 11.2 eV. None of the valence-band structures shows dispersion along the \ensuremath{\Gamma}-Z (${\mathit{k}}_{\mathit{z}}$) direction, i.e., perpendicular to the conducting BEDT-TTF layers. The spectral intensity is zero at, and to within about 0.5 eV below the Fermi level. This is ascribed to the presence of a fairly large band gap in this direction, in agreement with band-structure calculations. The photoemission spectrum of the valence band shows remarkable changes when the photon energy is changed. These changes are qualitatively understood as a cross-section dependence. The observed photon energy dependence suggests that both p orbitals and the lower-lying s orbitals of the atoms constituting the BEDT-TTF molecule contribute significantly to the molecular orbitals forming the valence band. Core-level spectroscopy on the I 4d and S 2p levels revealed no surface or chemically shifted components. The large width of these core levels is ascribed primarily to phonon broadening, although a contribution from disorder cannot be completely ruled out.