Measurement of the dynamic charge susceptibility near the charge density wave transition in ErTe₃

A charge density wave (CDW) is a phase of matter characterized by a periodic modulation of valence electron density coupled with lattice distortion. Its formation is closely tied to the dynamical charge susceptibility, [Formula: see text], which reflects the collective electron dynamics of the material. Despite decades of study, [Formula: see text] near a CDW transition has never been measured at nonzero momentum, [Formula: see text], with meV energy resolution. Here, we investigate the canonical CDW transition in ErTe[Formula: see text] using momentum-resolved electron energy loss spectroscopy, a technique uniquely sensitive to valence band charge excitations. Unlike phonons, which soften via the Kohn anomaly, we find the electronic excitations exhibit purely relaxational dynamics well described by a diffusive model, with the diffusivity peaking just below the critical temperature, [Formula: see text]. Additionally, we report for the first time a divergence in the real part of [Formula: see text] in the static limit ([Formula: see text]), a long-predicted hallmark of CDWs. Unexpectedly, this divergence occurs as [Formula: see text], with only a weak thermodynamic signature at [Formula: see text]. Our study necessitates a reexamination of the traditional description of CDW formation in quantum materials.