Phonon-drag effect in GaAs-AlxGa1−xAs heterostructures at very low temperatures

The thermoelectric power of high-mobility GaAs-${\mathrm{Al}}_{\mathrm{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathrm{x}}$As heterojunctions has been investigated in the temperature range from 0.1 to 10 K in order to study the phonon-drag effect. The phonon-drag contribution to the Seebeck coefficient can be well described by the classical $T^3$ dependence below 2.5 K. At very low temperatures (T<0.6 K), the phonon drag becomes vanishingly small and a linear temperature dependence of the diffusion thermoelectric power was resolved. In addition, the thermoelectric power was measured in magnetic fields up to 7 T and different temperature dependences were found for the maxima of the thermoelectric power oscillations at different Landau levels.