Dispersionless transport in washboard potentials revisited

We reassess the ‘dispersionless transport regime’ of Brownian particles in tilted periodic potentials. We show that the particles exhibit normal diffusive motion right after transitioning into the running state dragged by the constant bias force. No special transient dynamics appears, contrary to conjectures in the previous studies. The observed flat segment in the dispersion evolution curve is solely due to the broad spatial distribution of particles formed in the early superdiffusion stage. We quantitatively describe the whole evolution of the distribution function during superdiffusion and the transition to the normal diffusion that follows, in the framework of the two-well potential in the velocity space model. We show that the superdiffusion exponent is α = 3. The duration of the ostensible ‘dispersionless regime’ is derived analytically. It is shown to diverge exponentially as the temperature decreases to zero.