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

The family of rare-earth (R) tritellurides RTe3 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 RTe3 series apart is the observation of a second CDW transition at lower temperatures having an in-plane ordering wave vector q(CDW,2) parallel to [100] of almost the same magnitude but orthogonal to the ordering wave vector g o:A y, parallel to [001] observed at higher temperatures [N. Ru et al., Phys. Rev. B 77, 0351 (2008)]. Here, we report an inelastic x-ray scattering investigation of the lattice dynamics of DyTe3 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-CDW(,1) = 308 K. Surprisingly, these soft modes close to q(CDW,2) = (0.68, 0, 0) show strong softening near T-CDW(,1) but do not exhibit any response to the lower-temperature transition at T-CDW,T-2 = 68 K. Our results indicate that the low-temperature CDW order is not just the 90 degrees rotated analogue of the one appearing at high temperatures.