Metal-insulator transition in PF6 doped polypyrrole: Failure of disorder-only models

The free-carrier scattering time and plasma frequency in PF6 doped polypyrrole near the metal-insulator transition (MIT) is obtained by dielectric spectroscopy (8-700 GHz) down to 2 K. The specific T-dependence allows us to quantify the distance to the MIT for both metallic and insulating samples. The evolution of the free-carrier parameters when going deeper into the metallic regime conflicts with both the homogeneous and heterogeneous disorder models. We suggest this failure to stem from the interplay between electronic correlations and interchain charge transfer, which is not explicitly considered in these theories.