Conduction-band structure and charge-density waves in 1T-TaS2.

Abstract

Angle-resolved inverse photoemission spectroscopy (ARIPES) has been utilized to study the conduction-band structure of 1T-${\mathrm{TaS}}_{2}$ in the presence of charge-density waves (CDW's). In room-temperature spectra taken along the principal symmetry directions \ensuremath{\Gamma}M, \ensuremath{\Gamma}M', and \ensuremath{\Gamma}K we can clearly identify Ta 5d--derived ${\mathit{t}}_{2\mathit{g}}$ and ${\mathit{e}}_{\mathit{g}}$ manifolds, which reveal a substructure of almost dispersionless peaks attributable to CDW effects. The intensity modulation of the substructure appears to reflect the band dispersion of the undistorted 1T phase. Distinct differences in intensity are observed between the \ensuremath{\Gamma}M and \ensuremath{\Gamma}M' spectra as a consequence of the threefold symmetry of the unit cell. In temperature-dependent ARIPES between 120 and 400 K we do not, within the experimental accuracy, observe changes of the electronic structure other than a linear shift due to the electron-lattice interaction. In particular, there is no evidence of an opening of a localization gap in the unoccupied band structure as has been observed in the occupied states. From combined photoemission and inverse photoemission results the Fermi level is rather found to be pinned close to the bottom of the lowest unoccupied band.