Journal ArticleMeasurement of the dynamic charge susceptibility near the charge density wave transition in ErTe ₃Chaudhuri, Dipanjan (ORCID:0000000260324192); Jiang, Qianni; Guo, Xuefei; Chen, Jin; Kengle, Caitlin S (ORCID:0000000215259483); Hoveyda-Marashi, Farzaneh; Bernal-Choban, Camille (ORCID:0000000175503153); de_Vries, Niels; Chiang, Tai-Chang (ORCID:0000000171188299); Fradkin, Eduardo (ORCID:000000016837463X); Fisher, Ian R (ORCID:0000000212787862); Abbamonte, Peter (ORCID:0000000311091510)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,<inline-formula><math display='inline' id='i5' overflow='scroll'><mrow><mi>χ</mi><mo stretchy='false'>(</mo><mi>q</mi><mo>,</mo><mi>ω</mi><mo stretchy='false'>)</mo></mrow></math></inline-formula>, which reflects the collective electron dynamics of the material. Despite decades of study,<inline-formula><math display='inline' id='i6' overflow='scroll'><mrow><mi>χ</mi><mo stretchy='false'>(</mo><mi>q</mi><mo>,</mo><mi>ω</mi><mo stretchy='false'>)</mo></mrow></math></inline-formula>near a CDW transition has never been measured at nonzero momentum,<inline-formula><math display='inline' id='i7' overflow='scroll'><mi>q</mi></math></inline-formula>, with meV energy resolution. Here, we investigate the canonical CDW transition in ErTe<inline-formula><math display='inline' id='i8' overflow='scroll'><msub><mrow/><mn>3</mn></msub></math></inline-formula>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,<inline-formula><math display='inline' id='i9' overflow='scroll'><msub><mi>T</mi><mrow><mi>C</mi><mn>1</mn></mrow></msub></math></inline-formula>. Additionally, we report for the first time a divergence in the real part of<inline-formula><math display='inline' id='i10' overflow='scroll'><mrow><mi>χ</mi><mo stretchy='false'>(</mo><mi>q</mi><mo>,</mo><mi>ω</mi><mo stretchy='false'>)</mo></mrow></math></inline-formula>in the static limit (<inline-formula><math display='inline' id='i11' overflow='scroll'><mrow><mi>ω</mi><mo stretchy='false'>→</mo><mn>0</mn></mrow></math></inline-formula>), a long-predicted hallmark of CDWs. Unexpectedly, this divergence occurs as<inline-formula><math display='inline' id='i12' overflow='scroll'><mrow><mi>T</mi><mo stretchy='false'>→</mo><mn>0</mn></mrow></math></inline-formula>, with only a weak thermodynamic signature at<inline-formula><math display='inline' id='i13' overflow='scroll'><mrow><mi>T</mi><mo>=</mo><msub><mi>T</mi><mrow><mi>C</mi><mn>1</mn></mrow></msub></mrow></math></inline-formula>. Our study necessitates a reexamination of the traditional description of CDW formation in quantum materials.Proceedings of the national Academy of Science (PNAS)2025-06-2410674107Proceedings of the National Academy of Sciences12225e24244301220027-8424https://doi.org/10.1073/pnas.24244301222225920National Science Foundation