  A new nonlinear magnetothermal effect, namely unidirectional Seebeck
effect(USE), has been recently reported in magnetic/nonmagnetic topological
insulator (TI) heterostructure and is ascribed to the asymmetry magnon
scattering. Here, we show a new mechanism to generate USE in the Rashba two
dimensional electron gas (2DEG), in which the magnetism or magnetic order is
completely absent. It's found that the USE has a quantum origin from the
spin-momentum locking generated from Rashba spin-orbit coupling. We find that
the USE exhibits a sine dependence on the orientation of the magnetic field
with respect to the direction of the temperature gradient and is dominant from
the states above the Lifshitz point. The USE in the semiconductor Ge(111)
subsurface states has been theoretically and systematically investigated.
