Impact of the charge-density-wave state in the electrodynamic response of ZrTe_3-xSe_x: optical evidence for a pseudogap phase

The emergence of superconductivity upon progressively suppressing the long-range, charge-density-wave (CDW) order characterises the phase diagram of several materials of interest in the on-going solid-state physics research. Se-doped ZrTe_3-xSe_x compounds provide the most recent, suitable arena in order to investigate the interplay of otherwise competing orders in layered-like two-dimensional systems. We present an optical study of the CDW state in ZrTe_3-xSe_x at selected Se-doping, based on the measurement of the reflectivity from the far-infrared up to the ultraviolet, as a function of temperature. We particularly focus our attention to the redistribution of the spectral weight, which images the impact of the CDW state within the optical conductivity across the phase diagram of the title compounds. The electrodynamic response is consistent with a scenario based on a long-range CDW condensate at low Se-doping. Upon increasing the Se content, this then gives way to precursor effects as consequence of local, short range order CDW segments. Our spectral weight analysis reveals the presence of a pseudogap phase, as fingerprint of the CDW precursor effects and thus shaping the charge dynamics of the title compounds in their normal state, preceding the onset of superconductivity.