Theoretical Study of Lattice Instability and Lattice Dynamics of 2H-TaSe₂ and 2H-NbSe₂ II. Lattice Dynamics and Phonon Anomalies

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Phonon anomalies of the 2H-polytype TaSe₂ and NbSe₂, which reveal a structural phase transition at T₀=122.3 K and 33.5 K, respectively, have been studied theoretically. First, the phonon dispersion curves are calculated using the rigid ion model, and are in good agreement with the observation except for the Σ₁ mode along the ΓM line. Then by taking into account the effective ion-ion interaction caused by the electron-lattice interaction, the observed anomalous behavior of the acoustic Σ₁ phonon branch is explained. It is shown that the temperature dependence of the effective interaction leads to a softening of the Σ₁ mode of the wave vector q=(2⁄3)ΓM which characterizes the superlattice below T₀.