Thermal properties of layered transition-metal dichalcogenides at charge-density-wave transitions

The heat capacity and resistivity at the charge-density-wave (CDW) transitions of several layered transition-metal dichalcogenides are presented. The magnitude of the thermal and resistive anomalies in the $2H$ polytypes indicates only a small fraction of the Fermi surface is involved in the CDW distortion. The transition from the normal to the incommensurate CDW state at $T_0$ is second order, while the transition to the commensurate CDW state at $T_d$ is first order. However, the heat capacity anomaly at $T_0$, especially in $2H-\mathrm{Ta}{\mathrm{Se}}_2$, does not show the simple BCS-like jump expected from Landau models, rather a peak in the heat capacity occurs at $T_0$.