Microscopic model of charge-density waves in2H−TaSe2

A microscopic model of charge-density waves in $2H-\mathrm{Ta}{\mathrm{Se}}_2$ at zero temperature is developed and applied to calculate the lattice dynamics in the commensurate phase. The results are compared with the Raman data of Holy et al. At finite temperatures the conventional microscopic approach is to assume that the important excitons are electrons excited across the Peierls gap in the band structure. This conventional theory leads to gross inconsistencies in attempting to fit the experimental data for $2H-\mathrm{Ta}{\mathrm{Se}}_2$. The theory is reformulated, assuming that the coherence length is very short and that the dominant entropy is the lattice entropy. The lattice-entropy model is in good agreement with experiment at the semiquantitative level; the quantitative discrepancies appear to be due in part to the neglect of critical fluctuation effects.