  We address theoretically adiabatic regime of charge transport for a model of
two tunnel-coupled quantum dots connected in series. The energy levels of the
two dots are harmonically modulated by an external potential with a constant
phase shift between the two. Motivated by recent experiments with
surface-acoustic-wave excitation, we consider two situations: (a) pure pumping
in the absence of external voltage (also at finite temperature), and (b)
adiabatic modulation of the current driven by large external bias. In both
cases we derive results consistent with published experimental data. For the
case (b) we explicitly derive the adiabatic limit of Tien-Gordon formula for
photon-assisted tunneling and compare it to the outcome of simple conductance
modulation. A tutorial for adiabatic pumping current calculations with the
Green function formalism is included.
