The effect of high magnetic field on thermoelectric power of doped polyacetylene

Abstract

We have measured the temperature dependent thermoelectric power (TEP) of doped polyacetylene (PA) under the high magnetic field up to 20 tesla. The dopants are I 2, FeCl 3, AuCl 3and NbCl 5. The overall temperature dependence is characteristic to that of the diffusive metals, i.e. the TEP is positive and it decreases quasi-linearly upon cooling. However, the zero field TEP, S(T, H=0T), shows significant differences between the iodine doped PA and the other metal-halide doped ones at low temperature. While the S(T, H=0T) for iodine doped PA is quasi-linear in temperature down to 4.2 K, the S(T, H=0T) of metal-halide doped samples become temperature independent below T≅20 K and some of them show even negative TEP with a broad minimum peak at low temperature. At H=20T, both iodine and metal halide doped PA shows an abrupt decrease of 1~2 μV/K in TEP at T~70K, except the most heavily doped [CH(AuCl 4) y] x where no such drop appears. In the low temperature region, the ΔS(T)=S(T,H=20T)-S(T,H=0T) of metal-halide doped PA becomes more negative and the minimum peak of TEP shifts slightly to higher temperature. Similar tendency is observed at zero field TEP of [CH(AuCl 4) y] x as we increase the doping concentration y. For the iodine doped PA, the low temperature AS(T)=S(T,H=20T)-S(T,H=0T) is more or less the same as that of T~70K where the abrupt jump occurred, i.e., the S(T,H=20T) of iodine doped PA remains quasi-linear as a function of temperature down to 4.2 K. The magneto resistance ρ(T,H) of these samples is also measured and the results are compared with the above S(T,H) data. Since no appreciable amount of anisotropy of the stretch oriented PA with respect to the applied magnetic field direction is observed in S(T,H) and ρ(T,H) and furthermore, because of the significant difference of S(T,H) between the iodine doped PA and the metal-halide doped ones, the spin-spin interaction between the conduction electron spin in the metallic PA chain and the localized spin in the metal-halide dopant located nearby the polymer chain could be the prime origin of the observed magneto TEP phenomena.