  By measuring the thermoelectric effect in high-mobility quantum wells with
two occupied subbands in perpendicular magnetic field, we detect magnetophonon
oscillations due to interaction of electrons with acoustic phonons. These
oscillations contain specific features identified as combined resonances caused
by intersubband phonon-assisted transitions of electrons in the presence of
Landau quantization. The quantum theory of phonon-drag magnetothermoelectric
effect, generalized to the case of multi-subband occupation, describes our
experimental findings.
