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

Abstract

The temperature dependences of the conductivity and magnetoconductivity of poly(3,4-ethylenedioxythiophene) doped with PF6 (room-temperature conductivity of 200–300 S/cm) have been studied; σ(T) is weakly temperature dependent with the characteristic resistivity ratio ρr=ρ(1.4 K)/ρ(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 ρr<2.1; the temperature of the resistivity maximum, Tm, decreases with increasing ρr. High magnetic fields induce the transition from positive to negative TCR for all samples with ρr<2.1 and decrease the low-temperature conductivity (negative magnetoconductance) for samples with ρr>2.1. The conductivity at low temperatures is well described by a T1/2 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 H2 dependence at low magnetic fields and an H1/2 dependence at high magnetic fields. These results are successfully explained as resulting from the influence of electron-electron interactions on the low-temperature conductivity.