AC field induced quantum rectification effect in tunnel junctions

We study the appearance of directed current in tunnel junctions, quantum ratchet effect, in the presence of an external ac field f(t). The current is established in a one-dimensional discrete inhomogeneous "tight-binding model". By making use of a symmetry analysis we predict the right choice of f(t) and obtain the directed current as a difference between electron transmission coefficients in opposite directions, $\Delta T = T^{LR}-T^{RL}$. Numerical simulations confirm the predictions of the symmetry analysis and moreover, show that the directed current can be drastically increased by a proper choice of frequency and amplitudes of the ac field f(t).