Calculation and XPS measurements of the Ta4f CDW splitting in Cu, Cs and Li intercalation phases of 1T-TaX2 (X=S, Se)

Metallic quasi-two-dimensional transition metal dichalcogenides (TMDCs) 1T-TaS2 and 1T-TaSe2 show charge density wave (CDW) at room temperature. These show up as a superstructure in low energy electron diffraction (LEED) and as a distinct splitting of the Ta4f core level and a part of the Ta dz2 derived valence band. The changes in this Ta4f CDW splitting are investigated for in situ Cu, Li and Cs intercalated crystals by photoelectron spectroscopy (PES, XPS, SXPS) and LEED. The intercalation effects a certain number of CDW phase transitions, where each CDW phase is related to a characteristic CDW related splitting of the Ta4f core level. We extend the simple electrostatic model of Hughes and Pollak [1] to calculate the form of the splitting for different CDW phases and find a good correlation to the XPS measurements. This agreement confirms that the Ta4f CDW splitting is brought about by an electrostatic interaction between the Ta atoms and the (free) metallic charge density. The comparison of the phase transition sequence for three different intercalants indicates that a three-dimensional rather than a two-dimensional Fermi surface nesting mechanism is responsible for the phase transitions, as originally proposed by Wooley and Wexler [2].