Giant enhancement of cubic nonlinearity in a polycrystalline quasi-one-dimensional conductor

We show that the cubic nonlinear response of a polycrystalline quasi-one-dimensional conductor, such as a conducting polymer, can be expressed exactly in terms of the single-crystal cubic susceptibility and the electric fields in the analogous linear polymer. We also propose a simple nonlinear decoupling approximation which allows the polycrystalline nonlinear susceptibility to be simply estimated. Using this method, we show that local field effects may hugely enhance the nonlinear susceptibility of the polycrystal above its single-crystal value. A comparable enhancement is shown to exist in the conductivity noise.