Low-temperature metallic conductance in PF6-doped polypyrrole

The transport and thermodynamical properties of highly conducting PF6-doped polypyrrole exhibiting resistivity maximum near 20 K are presented. The measurements were also carried out on elongated PF6-doped, nonmetallic PF6-doped and p-toluensulfonate-doped samples. The thermoelectric power, heat capacity and spin susceptibility all show a finite density of states at Fermi level. The Hall cofficient is found to be negative in all the samples, contradictory to the sign of thermoelectric power. The data including the magnetoresistance suggests the inhomogeneous structure of polypyrrole with metallic grain or domain separated by nonmetallic regions. The metallic conductance appearing below 20 K is explained by a percolation path model, but a logarithmic temperature dependence appearing below 0.5 K is remained unexplained.