  We have performed conductivity measurements on a Si-MOSFET sample with a slot
in the upper gate, allowing for different electron densities n_1 and n_2 across
the slot. Dynamic longitudinal resistance was measured by a standard lock-in
technique, while maintaining a large DC current through the source-drain
channel. We find that in a parallel magnetic field, the resistance of the
sample, R(I_DC), is asymmetric with respect to the direction of the DC current.
The asymmetry becomes stronger with an increase of either the magnetic field or
the difference between n_1 and n_2. These observations are interpreted in terms
of the effective spin injection: the degree of spin polarisation is different
in the two parts of the sample, implying different magnitudes of spin current
away from the slot. The carriers thus leave the excess spin (of the appropriate
sign) in the region around the slot, leading to spin accumulation (or
depletion) and to the spin drift-diffusion phenomena. Due to the positive
magnetoresistance of the two-dimensional electron gas, this change in a local
magnetisation affects the resistivity near the slot and the measured net
resistance, giving rise to an asymmetric contribution. We further observe that
the value of R(I_DC) saturates at large I_DC; we suggest that this is due to
electron tunnelling from the two-dimensional n-type layer into the p-type
silicon (or into another "spin reservoir") at the slot.
