  We study phonon-drag contribution to the thermoelectric power in a
quasi-two-dimensional electron system confined in GaAs/AlGaAs heterostructure
in presence of both Rashba spin-orbit interaction and perpendicular magnetic
field at very low temperature. It is observed that the peaks in the phonon-drag
thermopower split into two when the Rashba spin-orbit coupling constant is
strong. This splitting is a direct consequence of the Rashba spin-orbit
interaction. We show the dependence of phonon-drag thermopower on both magnetic
field and temperature numerically. A power-law dependence of phonon-drag
magnetothermopower on the temperature in the Bloch-Gruneisen regime is found.
We also extract the exponent of the temperature dependence of phonon-drag
thermopower for different parameters like electron density, magnetic field, and
the spin-orbit coupling constant.
