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Coupling of spin and charge in polyaniline pellets doped with HCl or DBSA

We have carried out a comparative experimental study of the electrical conductivity \sigma and magnetic susceptibility \chi of own-made polyaniline (PANI) pellets doped either with HCl or with dodecybenzenesulfonic acid (DBSA), the latter being a long molecule with surfactant properties. While the doping mechanism (protonation) is the same for the two dopants, there are both differences and similarities in properties of the corresponding doped PANI materials. X-ray diffraction patterns for PANI-HCl exhibit signatures of partial crystallinity whereas PANI-DBSA is completely amorphous. Furthermore, the dopant content in PANI-HCl is limited by the number of available protonation sites, whereas a stable two-phase system consisting of fully doped PANI-DBSA and excess DBSA molecules can form during the doping process [1]. Nevertheless, these two quite opposite types of doped PANI show remarkable similarities in temperature (T) dependences of \sigma and \chi, and there are strong arguments in favour of the underlying physics being the same. For all samples, we find that \sigma at low T is governed by the variable-range hopping (VRH) in a homogeneously disordered three-dimensional system of coupled one-dimensional chains. Depending on the doping and corresponding disorder level, the VRH exponents are either 1/2, 2/5 or 1/4, and at higher T in many of the samples we find the exponent 1 that signifies the nearest-neighbour hopping [2, 3]. All these exponents are predicted in a theory of Fogler, Teber and Shklovskii [4] for the charge transport in coupled chain-like conductors, and conditions for their appearance depend on disorder and T. Changes from one exponent into another appear at crossover temperatures T*, where there are also noticeable features in \chi(T). This coupling of charge and spin is discussed in the spirit of kBT* being the thermal energy which causes an enhancement of the density of delocalised (Pauli) spins at the expense of localised (Curie) spins as T rises above T* [3]. [1] M. Baćani, D. Babić, M. Novak, I. Kokanović, S. Fazinić, Synth. Met. 159, 2584 (2009) [2] M. Novak, I. Kokanović, D. Babić, M. Baćani, A. Tonejc, Synth. Met. 159, 649 (2009) [3] M. Novak, I. Kokanović, D.Babić, M. Baćani, J. Non-Cryst. Solids 356, 1725 (2010) [4] M. M. Fogler, S. Teber, B. I. Shklovskii, Phys. Rev. B 69, 035413 (2004)

Conducting polymers; Polyaniline; Static magnetisation; Magnetic susceptibility

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