Infrared signature of the charge-density-wave gap in ZrTe₃

The chain-like $ZrTe_3$ compound undergoes a charge-density-wave (CDW) transition at $T_{CDW}=63$ $K$, most strongly affecting the conductivity perpendicular to the chains. We measure the temperature ($T$) dependence of the optical reflectivity from the far infrared up to the ultraviolet with polarized light. The CDW gap $\Delta(T)$ along the direction perpendicular to the chains is compatible for $T<T_{CDW}$ with the behavior of an order parameter within the mean-field Bardeen-Cooper-Schrieffer (BCS) theory. $\Delta(T)$ also persists well above $T_{CDW}$, which emphasizes the role played by fluctuation effects.