The Nearly Commensurate Phase and Effect of Harmonics on the Successive Phase Transition in 1T–TaS₂

Abstract

The phase transitions of the charge density wave (CDW) state in 1T–TaS₂ are studied both theoretically and experimentally. The positions and intensities of the fundamental and third-order harmonic of the CDW are determined by X-ray diffraction. In order to understand the successive phase transitions, a theory is presented on the basis of McMillan’s free energy. A sufficiently large number of harmonics are taken into account so as to determine the free energy of the incommensurate (I) and the nearly commensurate (NC) phase accurately. The theory explains well not only the magnitude of the observed harmonic but also the first-order transitions between the I–NC and NC commensurate phases. In this connection an explanation is also given for the complicated electron diffraction pattern found by Wilson et al. in the NC phase.