Specific heat and resistivity near the charge-density-wave phase transitions in $2H-\mathrm{T}\mathrm{a}{\mathrm{Se}}_2$ and $2H-\mathrm{T}\mathrm{a}{\mathrm{S}}_2$

Detailed measurements of the specific heat and resistivity have been made on samples of $2H-\mathrm{T}\mathrm{a}{\mathrm{Se}}_2$ and $2H-\mathrm{T}\mathrm{a}{\mathrm{S}}_2$ near their charge-density-wave phase transitions. Because the crystal quality of $2H-\mathrm{T}\mathrm{a}{\mathrm{Se}}_2$ is much better than that of $2H-\mathrm{T}\mathrm{a}{\mathrm{S}}_2$, we emphasize the results on $2H-\mathrm{T}\mathrm{a}{\mathrm{Se}}_2$ and make quantitative comparison with the theory of charge-density waves in this material. In $2H-\mathrm{T}\mathrm{a}{\mathrm{Se}}_2$ the normal to incommensurate phase transition was found to be second order; the incommensurate to commensurate transition was first order. Specific-heat measurements on this material indicate that the zero-temperature coherence length is relatively short; $\pi {\xi }_0\simeq 14$ Å. The resistivity of $2H-\mathrm{T}\mathrm{a}{\mathrm{Se}}_2$ immediately above the normal to incommensurate phase transition is dominated by resistive scattering from the periodic structural deformations accompanying the charge-density waves. The specific-heat and resistivity measurements both indicate that the normal to incommensurate phase transition in $2H-\mathrm{T}\mathrm{a}{\mathrm{Se}}_2$ can be analyzed within a nearly-mean-field model.