Electronic structure of (TaSe₄)₂I and (NbSe₄)₃I from high-resolution photoemission

Abstract
We used photoelectron spectroscopy (ARPES) with high energy and momentum resolution to investigate the electronic structure of the quasi-1D materials (TaSe4)2I and (NbSe4)3I. The ARPES spectra of (TaSe4)2I bear clear traces of the charge-density-wave (CDW) transition below Tc=263 K, with the opening of a Peierls gap of 130 meV. The spectra of the normal state, however, are puzzling. We observe a clear quasiparticle state dispersing with the periodicity of the lattice, but no sign of the expected Fermi surface crossing. The spectral intensity is strongly suppressed within 0.4 eV of the chemical potential, indicative of a broad pseudogap. Critical fluctuations, which are already very large at room temperature, cannot account for this large energy scale. Together with the unusual spectral lineshape, they suggest that electronic correlations must also be important in this system. In (NbSe4)3I we observe a similar lineshape and dispersion, but a broader energy gap at room temperature, consistent with the semiconducting character of this material. A strong shadow band near the zone center, not expected from band structure results, indicates that the conduction electrons are subject to a non negligible superlattice potential with a periodicity twice that of the underlying structural distortion.