Unconventional charge density wave order in ErTe$_{3}$: symmetry breakings and fluctuations probed by Raman scattering
Recent experiments have revealed unexpected mirror-symmetry breakings in the charge density-wave (CDW) state of rare-earth tritellurides, suggesting a putative ferro-axial CDW state. Here we report polarization-resolved Raman scattering measurements tracking the point-group symmetry breakings across the CDW phase transition in the rare-earth tritelluride ErTe$_3$. We show that the mirror-symmetry breakings follow closely the emergence of the CDW order, suggesting a single component CDW order parameter. Furthermore, Raman measurements in the $A_{2g}$ symmetry channel above the CDW transition temperature indicate the absence of significant ferro-axial fluctuations. We discuss the broad implications of these findings with respect to the nature of the CDW state in the tritellurides.