Metallic conductivity at low temperatures in poly(3,4-ethylenedioxythiophene) doped with PF6

The temperature dependences of the conductivity and magnetoconductivity pf poly(3,4-ethylenedioxythiophene) doped with PF6 (room-temperature conductivity of 200-300 S/cm) have been studied; sigma(T) is weakly temperature dependent with the characteristic resistivity ratio rho(r)=rho(1.4 K)/rho(291 K) =1.5-2.8. The sign of the temperature coefficient of resistivity (TCR) changes below 10 K from negative to positive for metallic samples with rho(r)<2.1; the temperature of the resistivity maximum, T-m, decreases with increasing rho(r). High magnetic fields induce the transition from positive to negative TCR for all samples with rho(r)<2.1 and decrease the low-temperature conductivity (negative magnetoconductance) for samples with rho(r) > 2.1. The conductivity at low temperatures is well described by a T-1/2 dependence in both cases (negative and positive TCR) in a magnetic field and with the magnetic field equal to zero. The magnetoconductance is negative, isotropic, and exhibits an H-2 dependence at low magnetic fields and an H-1/2 dependence at high magnetic fields. These results are successfully explained as resulting from the influence of electron-electron interactions on the low-temperature conductivity.