Studies of one-dimensional metals became of interest in the 1950s after Rudolf Peierls found that it was energetically favorable for such a chain to distort spontaneously, creating a gap between the filled valence band and the unfilled conductance band and converting the chain from a conductor to a semiconductor. In polyacetylene, this distortion causes alternating pairs of carbon atoms to move closer to one another. The negatively charged iodine ion remains associated with the polymer, but is not as mobile as the hole. The hole, together with the local distortion it produces in the carbon chain, is called a polaron, as illustrated in panel b of the figure. Bound pairs of polarons are called bipolarons.
Solitons can carry spin but no charge, or charge but no spin.
Although ideas of an excitation associated with a bond-alternation defect had previously been floated in the chemistry literature, in 1979, Heeger, with Robert Schrieffer and Wu Pei Su, then his colleagues at Penn, made specific predictions of solitons in polyacetylene. 2 Similar models were independently put forward by Michael Rice at Xerox Corp 3 and by Sergey A. Brazovski and Natasha Kirova in Moscow. 4 Experiments in the 1980s showed that solitons can be excited by photons and current injection as well as by doping. The details of how polymers carry current are still not fully understood. As Arthur Epstein of Ohio State University explained it, most models were initially based on long, isolated chains.