Contributions of Phasons and of Low-Energy Excitations to the Specific Heat of the Quasi One-Dimensional Compound(TaSe4)2I

Measurements of the specific heat of quasi one-dimensional conductors, ${\left(\mathrm{T}\mathrm{a}{\mathrm{Se}}_4\right)}_2$I and ${\left(\mathrm{N}\mathrm{b}{\mathrm{Se}}_4\right)}_3$I, between 0.1 and 8 K, are reported. Whereas no deviation from the $T^3$ law occurs at low temperature for ${\left(\mathrm{N}\mathrm{b}{\mathrm{Se}}_4\right)}_3$I, we have measured two anomalies in ${\left(\mathrm{T}\mathrm{a}{\mathrm{Se}}_4\right)}_2$I: an excess heat capacity with a peak at $T=1.75\mathrm{}$ K and a specific heat which follows a $T^{0.2}$ law below 0.4 K. We ascribe this excess heat capacity to the charge-density-wave phase excitations above their pinning frequency in the insulating Peierls state ($T_{\mathrm{P}}=263\mathrm{}$ K).