Low temperature conductivity of metallic conducting polymers

In several metallic conducting polymers, both positive and negative tempera ture coefficient of resistivity (TCR) has been observed at low temperatures ; and this can be easily tuned by disorder, pressure and magnetic field. Th is 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 rh o (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 T CR, sign of 'm' and the resistivity ratio are consistently related to each other in metallic conducting polymers.