  We show that spin-dependent electron-phonon interaction in the energy
relaxation of a two-dimensional electron gas results in equal and oppositely
directed currents in the spin-up and spin-down subbands yielding a pure spin
current. In our experiments on SiGe heterostructures the pure spin current is
converted into an electric current applying a magnetic field that lifts the
cancellation of the two partial charge flows. A microscopic theory of this
effect, taking account of the asymmetry of the relaxation process, is developed
being in a good agreement with the experimental data.
