Tunneling of solitons and charge-density waves through impurities

In the presence of impurity potentials which couple directly to the phase of the charge-density waves the ground state as well as soliton excitations are pinned. The quantum-mechanical tunneling of a sine-Gordon soliton through an impurity potential is calculated. The small tunneling probability for even a moderate impurity potential means that the conductivity due to solitons will be greatly reduced. We also study the electric field and temperature dependence of a charge-density wave pinned by impurities when quasiclassical tunneling is taken into account. The temperature dependence is found to be dominated by simple thermal excitation and the conductivity is activated in both the strong- and weak-pinning limits.