Effect of anharmonicity on charge transport in hydrogen-bonded systems

We study solitonic charge transport in a hydrogen-bonded model system representing a one-dimensional polypeptide chain. Supersonic solitons are constructed for zero temperature in the frame of a Morse lattice model for which an (nonlinear) electronic system is coupled in a tight-binding approximation to H-bond vibrations of the molecular chain. The latter are of anharmonic nature. Charge transport is realized via the coupling between the electron and the local lattice deformations. This electron-lattice coupling is described by the soliton solutions, assigned to states of a localized charge in association with its local chain deformation. By retaining the discrete nature of the underlying lattice system it is shown that even strongly localized states are mobile. In fact, we illustrate that for nonlinear electron-vibration interaction supersonic solitonic carriers in the lattice assist the transport of narrow electron and lattice solitons. Moreover, by using realistic values from polypeptides for the system parameters we demonstrate that the interaction between the H-bond vibrations and the electron is strong enough to sustain thermal perturbations up to T=300 K. Most importantly localization is maintained over extended periods of time during which the electron travels directionally over such long distances along the chain exceeding by far those achievable with single-step tunneling. Furthermore, we discuss the role of an applied electric field. It is demonstrated that in a wide range of its values the velocity of the soliton motion and hence the electric current remains unaffected by the electric field. Above this range the velocity of the solitons is proportional to the field strength so that the corresponding current follows Ohm's law. Then for still higher field strengths above a critical value the coupling between electron and soliton dynamics breaks down.