ELECTRON SPIN MOTION IN TRANS-(CH)x

One of the most important questions about the conducting polymer trans-(CH)x is what
is the motion of the electrons? This question is especially intriguing because of
the expected one-dimensional motion of solitons, which have been used to explain
many of the physical properties of trans-(CH)x 11,21. Since the uncharged
soliton has the spin of one electron, both nuclear magnetic resonance (NMR) and
electron spin resonance (ESR) experiments have played a key role in studies
related to the existence of solitons and their motion /3-9/.

In this paper we report several magnetic resonance experiments which probe the
motion of the unpaired electronic spins in undoped trans-(CH)x over an unusually
wide range of temperature, .035 - 300 K. Within the framework of these and related
experiments, we can not tell unequivocally if the entities under study are actually
solitons. For this reason, we shall refer to them as 'the unpaired spins' or 'the
electrons'. Our interpretation of the experiments includes the following points:
(a) the spins become progressively immobilized as the temperature is lowered, (b)
the average time scale for unpaired spin translational motion by an intercarbon
distance is 10^-8 s near 50 K and longer than 10^-4 s below 0.5 K, and (c) at all
temperatures nuclear spin diffusion plays an important role in the proton spin lattice relaxation time (T]_). The last point means that the model of nuclear
relaxation based solely upon diffusion of solitons is incorrect, and casts doubt on
the high values of the soliton diffusion constant obtained using it /3-5/.

Three samples were used in this work. All of them were fabricated using the
Shirakawa method /15/ and isomerized under typical conditions. Care was taken to
avoid exposing them to oxygen. The sample used for the experiments reported in
Figs.1,2, and 4 was not sealed, but was handled in a helium atmosphere when being
placed in the apparatus, which also provided a helium atmosphere. It probably
received a small exposure to oxygen, whose effect was minimized by flushing with He
gas and pumping after exposure 151. The other experiments were performed with samples
which were prepared under oxygen-free conditions , sealed into glass ampoules with a
He atmosphere (for thermal contact), and stored at 77 K when not being used for
experiments. Nuclear spin relaxation measurements at 4.2 K and above at fields below
3.5 kOe were performed using rapid (1.5 ms) field cycling. Nuclear and electron spin relaxation measurements below 1 K were measured using two different field cycling setups.