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

The dynamical charge susceptibility, χ(q,ω), reveals the collective behavior of charge carriers in a system and is crucial for understanding charge density wave (CDW) phases, characterized by a periodic modulation of the valence electron density and lattice structure. However, χ(q,ω) near a CDW transition was never experimentally measured with high energy and momentum resolution, despite decades of extensive studies. Here, using momentum-resolved electron energy loss spectroscopy (m-EELS), we investigate the χ(q,ω) in the vicinity of the canonical CDW transition in ErTe₃ and observe a purely relaxational charge response that is well-described by a diffusive model. This observation is in sharp contrast to the lattice dynamics which exhibit a conventional phonon softening due to a Kohn anomaly. The charge diffusivity peaks just below the critical temperature, and the real part of χ(q,ω) exhibits a divergence in the static limit (ω→0). These results highlight the importance of energy- and momentum-resolved measurements of χ(q,ω) and demonstrate the power of m-EELS as a versatile probe of charge dynamics in materials.