CHARGE DENSITY WAVE PHASE TRANSITIONS OBSERVED BY X-RAY PHOTOEMISSION

Abstract

X-ray photoemission (XPS) measurements of the binding energies of the Ta 4f core levels in TaS2, TaSe2, and Ta0.97Hf0.03S2 at temperatures between 14 K and 371 K are reported. The presence of charge density waves (CDW's) produces inequivalent potentials at different Ta sites which split the 4f XPS lines ; the splitting is a measure of the CDW amplitude and represents an order parameter for the CDW-lattice system. The form of the components of the split XPS lines allows the CDW-lattice configuration to be deduced, and since the potential gradients at inequivalent Ta sites are different, the unequal widths of the components yields a measure of thefluctuating nature of the CDW-lattice system. The results indicate the existence of CDW fluctuations in commensurate phases, and that in some incommensurate and quasi-commensurate phases the CDW's cannot maintain long range coherence. In 1 T-Ta0.97Hf0.03S2 the Hf impurities pin the incommensurate CDW's into domains.