Optical Excitation Spectrum in Ni- and Cu-doped ZrTe$_3$

We report on an optical study performed on Cu$_x$ZrTe$_3$ and Ni$_x$ZrTe$_3$ single crystals. ZrTe$_3$ was previously found to display a BCS-like CDW-gap opening in the optical spectra along the direction orthogonal to the Zr-chains and to undergo a filamentary superconducting transition below a T$_c$ of about 2 K. The intercalation by Cu and Ni between the ZrTe$_3$ layers partially fills the CDW gap and induces bulk superconductivity coexisting with the CDW state below T$_c$. Here we show the effect of Ni and Cu intercalation on the reflectivity and optical conductivity above and below the CDW phase-transition temperature. Furthermore, we analyze the optical spectral weight, providing equivalent information in both compounds about the partial gapping of the Fermi surface and the overall redistribution of spectral weight across the CDW phase transition.