Spectroscopic signatures of phase transitions in a charge-density-wave system: 1T-TaS2

Photoelectron spectroscopy with high-energy resolution has been utilized to investigate the charge-density wave (CDW) in 1T-${\mathrm{TaS}}_2$ between 20 and 360 K. Constant binding-energy curves reveal discontinuities in the temperature dependence of the photoelectron spectral function ρ(E), and demonstrate that sudden modifications of the electronic structure, namely, in the vicinity of the Fermi level ${\mathit{E}}_{\mathit{F}}$, mark the first-order CDW transitions. In the commensurate phase, below 180 K, ρ(${\mathit{E}}_{\mathit{F}}$) reflects the formation of a correlation pseudogap. On disordered surfaces, however, the long-range coherence typical of the commensurate phase is lost, and a normal metallic behavior is recovered.