Effect of interactions between electrons of like spin in conducting polymers

We consider the effects of repulsive electron-electron interactions between electrons of like spin in conducting polymers. Because the ratio W/2m of the physical bandwidth W to the band gap 2m is large in these systems, they may be represented by a field-theory model with a cutoff. When only interactions between like spins are included, the resulting model is equivalent to the massive Thirring model which has been exactly solved by Bethe's ansatz. From the exact solution we are able to conclude that perturbation theory in the interaction strength g is quite accurate so long as (g/π)ln(W/2m)<<1, while renormalization-group-improved perturbation theory is valid for (g/π)<<1. For (g/π)> or approx. =1, results for the ground-state energy and mass gap are sensitive to the band structure far from the Fermi surface, and no simple field-theory model is adequate