Charge transport in adiabatically driven ratchets - Waveform and phase dependence -

We report on the adiabatic pumping of electrons in a driven lateral superlattice (LSL) with broken symmetry. The device is fabricated from a two-dimensional electron gas (2DEG), located $54.8\:\mathrm{nm}$ below the surface in a GaAs/Al$_x$Ga$_{1-x}$As heterostructure, grown by molecular beam epitaxy. The LSL is realized by two transducer gates, each comprising 75 stripes of 160 nm width and 1 $\mu$m period. The gates are interlaced off center by two thirds of the period and can thus induce a ratchet-like potential modulation in the 2DEG when appropriately biased. When the transducer is driven by two identical but phase-shifted ac signals, a lateral pumping current $I(\phi)$ results, which strongly depends on both the phase shift $\phi$ and the wave form $V(t)$. Surprisingly, we find that for different periodic signals, the phase dependence $I(\phi)$ closely resembles $V(t)$. A simple model of adiabatic pumping in 2DEGs is presented, which can reproduce our experimental findings. Possible applications for waveform sampling are discussed.