Screened dielectric response of sliding charge-density waves

The linear response of a model of deformable charge-density waves pinned by weak disorder is calculated, including the screening effects of free carriers and using a simple model for the distribution of pinned modes. Owing to the screening of localized pinned collective oscillations, the dielectric function develops a low-frequency loss peak in addition to the expected absorption close to the pinning frequency. These results provide a quantitative explanation of the apparent discrepancies between low-frequency ({}10 MHz) electrical measurements and higher-frequency (microwave or infrared) measurements of the dielectric response. Implications for measurements of the nonlinear dc conductivity are discussed.