Ratchet behaviour of polarons in molecular chains

We study the ratchet behaviour of polarons in diatomic molecular chains under the influence of an external electromagnetic field which is periodic in time. We show that in asymmetric chains a harmonic unbiased field causes a drift of polarons. This phenomenon has a threshold with respect to the intensity and the frequency of the field. In spatially symmetric chains a harmonic periodic electric field generates oscillations of polarons but does not result in their movement. The polaron drift current can be induced in symmetric chains by a time periodic asymmetric external field. This complex dynamics of polarons is generated by the interplay between the Peierls-Nabarro barrier and dissipative effects in the chains.