Excitation spectrum in Ni- and Cu-dopedZrTe3

Abstract

ZeTe${}_{3}$ belongs to the ample class of chainlike chalcogenide charge-density-wave (CDW) materials. Its peculiarity consists in the formation of the CDW condensate at ${T}_{\mathrm{CDW}}$ along the crystallographic directions perpendicular to the $b$-axis, Zr-Zr chains. Ni and Cu intercalation of ZrTe${}_{3}$ leads to the onset of bulk superconductivity at ${T}_{c}l{T}_{\mathrm{CDW}}$, raising questions about the possible coexistence as well as competition between both broken-symmetry ground states. Here, we report on the optical investigation of Ni${}_{0.05}$ZrTe${}_{3}$ and Cu${}_{0.05}$ZrTe${}_{3}$ single crystals performed with electromagnetic radiation polarized along the in-plane crystallographic $a$ and $b$ axes, over a broad spectral range and as a function of temperature. The charge dynamics of the CDW state displays a polarization dependence within the $ab$ plane and gives evidence for a partial gapping of the Fermi surface, which affects almost exclusively the crystallographic direction parallel to the $a$ axis. We provide a complementary analysis of the spectral weight distribution, disentangling its reshuffling in the excitation spectrum across the symmetry-breaking CDW transition. While the superconducting energy gap lies outside our measurable spectral range, we observe nonetheless that the CDW-gap feature persists at $Tl{T}_{c}$. We propose that CDW and superconductivity coexist along the $a$ axis.