Low temperature conductivity of metallic conducting polymers

In several metallic conducting polymers, both positive and negative temperature coefficient of resistivity (TCR) has been observed at low temperatures; and this can be easily tuned by disorder, pressure and magnetic field. This sign change in TCR is related to the sign change in 'm' [sigma = sigma (0) + mT(1/2)] as a function of the resistivity ratio [rho (r) similar to rho (300 K) / rho (1.4 K)] in both disorder-tuned and pressure-tuned samples of doped polypyrrole and poly(3-methyl thiophene). In both cases, the zero-crossing of 'm' occurs at resistivity ratio around 2. This shows that the TCR, sign of 'm' and the resistivity ratio are consistently related to each other in metallic conducting polymers.