Charge-density waves and many-body effects in x-ray photoelectron spectroscopy of layer-structure chalcogenides

Comparison of x-ray photoemission spectra of semiconducting and metallic layered transition-metal dichalcogenides clearly exhibits the role of many-electron screening in these largely two-dimensional conductors. The effect of the charge-density wave (CDW) has been observed in $1T-\mathrm{Ta}{\mathrm{S}}_2$ and Ta${\mathrm{Se}}_2$ as a perturbation of the initial-state $4f$ electron binding energy. In the commensurate CDW state these levels show a resolved splitting; in the quasicommensurate and incommensurate CDW states they show broadening of magnitude comparable to this splitting. The sign and amplitude of the CDW is determined from a comparison of these observations with a theoretical model.