Ratchet Currents in Driven Nanoscale Conductors

Electromagnetic ac fields can alter significantly the
transport properties of mesoscopic conductors such as
molecular wires and coherently coupled quantum dots.
Resonant excitations of electrons e.g. enhance drastically
the time-averaged currents. These systems may also be used
to study the so-called ratchet or pump effect: in spatially
asymmetric setups, an ac field induces a dc current even in
the absence of any bias voltage. Of particular interest is
that the ratchet current as a function of the conductor
length converges to a non-zero value. The opposite
phenomenon also exists: a proper off-resonant driving field
reduces the coherent transport resulting in a strong current
suppression. Most of these effects require a treatment
beyond linear response theory.

The corresponding transport mechanisms leave their
fingerprints also in the noise whose relative strength is
measured by the so-called Fano factor. In general, we find
that resonant excitations reduce the noise level while
current suppressions are accompanied by a noise reduction.

In our studies, we model the external field by a periodic
time-dependence of the wire Hamiltonian. This requires a
generalization of established transport theories like, e.g.,
the Landauer formula. Such a generalization, that is based
on the Floquet theorem and includes the full nonlinear
response to the driving, will be presented and the main
differences to the static situation will be discussed.
Finally, we discuss how Coulomb repulsion modifies
the results.