Effect of substitutional impurities on the low-temperature specific heat of (TaSe4)2I

The large anomaly centered near 1.75 K in the specific heat of the charge-density-wave material (${\mathrm{TaSe}}_4$ ${)}_2$I is found to be strongly suppressed with the introduction of small amounts of niobium impurities. By comparing the thermal experiments with conductivity measurements performed in the millimeter-wave regime, we find that the effects of impurities in the two are correlated, but with slightly different energy scales. Although the observations are suggestive of thermal excitations of the collective mode, simple analyses fail to account for the impurity dependence.