Subtle effect of doping on the charge density wave in TaTe2−δ ( δ=0.028–0.123 ) crystals revealed by anisotropic transport measurements and Raman spectroscopy

Abstract

The formation of charge density wave (CDW) is attributed to Fermi-surface nesting in textbooks, but this common paradigm is challenged by recent studies that reveal the pronounced effects of electron-phonon interaction and defects. Transition-metal dichalcogenide $\mathrm{Ta}{\mathrm{Te}}_{2}$, being a prototype of charge density wave at around 170 K, is an ideal playground for exploring the mechanism of CDW. In this work, we reveal the subtle effect of defects on the CDW through studying a series of $\mathrm{Ta}{\mathrm{Te}}_{2\ensuremath{-}\ensuremath{\delta}}$ ($\ensuremath{\delta}=0.028\ensuremath{-}0.123$) crystals. Transport measurements show that $\mathrm{Ta}{\mathrm{Te}}_{1.972}$, being close to the stoichiometric compound, have a resistivity-kink feature (fingerprint of CDW) around 160 K along both $a$- and $b$ axes; $\mathrm{Ta}{\mathrm{Te}}_{1.933}$ only has a resistivity-kink feature along the $b$ axis. Remarkably, the resistivity kink completely disappears in $\mathrm{Ta}{\mathrm{Ta}}_{1.877}$. Raman spectroscopy not only confirms the corresponding structural phase transition, but also verifies decreased electron-phonon interaction at the \ensuremath{\Gamma} point of the Brillouin zone from $\mathrm{Ta}{\mathrm{Te}}_{1.972}$ to $\mathrm{Ta}{\mathrm{Te}}_{1.933}$ based on analysis of theoretical Fermi surface of 1 ${T}^{\ensuremath{'}}$ phase of $\mathrm{Ta}{\mathrm{Te}}_{2}$ before CDW phase transition. We propose that the CDW of $\mathrm{Ta}{\mathrm{Te}}_{2\ensuremath{-}\ensuremath{\delta}}$ ($\ensuremath{\delta}=0.028\ensuremath{-}0.067$) along the $a$ axis is mainly due to electron-phonon interaction, while CDW along the $b$ axis is the synergetic effect of Fermi-surface nesting and electron-phonon interaction; if dopant concentration is high enough $(\ensuremath{\delta}=0.123)$, CDW is completely quenched by disorder induced by doping. Our study reveals the subtle effects of doping on CDW, and may give more hints on the interplay between CDW and high-temperature superconductor in the doped Mott insulators.