Nonlinear transport in pinned one-dimensional charge-density wave systems

The results of an experimental study of nonlinear electrical transport at low temperatures are described for the one-dimensional organic conductors TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane) and TSeF-TCNQ (tetraselenafulvalene-tetracyanoquinodimethane). The $b$-axis conductivity is nonlinear in both compounds with the onset of nonlinearity at applied electric fields of a few V/cm. Measurements along different crystallographic directions show that the nonlinear transport is directly related to the one-dimensional nature of the electronic system. Traditional single-particle mechanisms of nonlinearity in semiconductors are ruled out through direct experimental study. The experimental results are discussed in terms of nonlinear phase-kink solitons of the pinned charge-density wave condensate at low fields, and field-induced depinning at high fields.