Temperature dependence of the charge-density-wave gap in the rare-earth tritelluride compounds

The layered rare-earth tritellurides $R$Te$_3$ ($R$= Er and Ho) host an unidirectional, incommensurate charge-density-wave (CDW) transition at $T_{CDW1}\sim$ 265 and 280 K and a further transition to a bidirectional CDW state at $T_{CDW2}\sim$ 160 and 120 K for the Er and Ho compound, respectively. We present optical reflectivity data collected as a function of temperature over a very broad energy interval, ranging from the far-infrared up to the ultraviolet. We extract the temperature dependence of the CDW gap and compare it with our previous results on the whole rare-earth series ($R$=La, Ce, Pr, Nd, Sm, Gd, Tb and Dy) as a function of chemical and externally applied pressure. We provide clear-cut evidence that upon destroying the CDW state with increasing temperature and pressure there is a progressive closing of the CDW gap excitation.