Competing soft phonon modes at the charge-density-wave transitions in DyTe3

Abstract

The family of rare-earth ($R$) tritellurides $R\mathrm{T}{\mathrm{e}}_3$ features charge-density-wave (CDW) order related to strongly momentum-dependent electron-phonon coupling. Similar to other CDW compounds, superconductivity is observed when the CDW order is suppressed via hydrostatic pressure [J. J. Hamlin et al., Phys. Rev. Lett. 102, 177002 (2009)]. What sets the heavier members of the $R\mathrm{T}{\mathrm{e}}_3$ series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wave vector $q_{\mathrm{CDW},2}\parallel \left[100\right]$ of almost the same magnitude but orthogonal to the ordering wave vector $q_{\mathrm{CDW},1}\parallel \left[001\right]$ observed at higher temperatures [N. Ru et al., Phys. Rev. B 77, 035114 (2008)]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of $\mathrm{DyT}{\mathrm{e}}_3$ In particular, we show that there are several phonon modes along both in-plane directions, which respond to the onset of the CDW transition at $T_{\mathrm{CDW},1}=308\mathrm{K}$. Surprisingly, these soft modes close to $q_{\mathrm{CDW},2}=\left(0.68,0,0\right)$ show strong softening near $T_{\mathrm{CDW},1}$ but do not exhibit any response to the lower-temperature transition at $T_{\mathrm{CDW},2}=68\mathrm{K}$. Our results indicate that the low-temperature CDW order is not just the 90° rotated analogue of the one appearing at high temperatures.