  We have recently shown that asymmetric lateral spin orbit coupling (LSOC)
resulting from the lateral in-plane electric field of the confining potential
of a side-gated quantum point contact (QPC) can be used to create a strongly
spin- polarized current by purely electrical means1 in the absence of applied
magnetic field. Using the non-equilibrium Green function formalism (NEGF)
analysis of a small model QPC2, three ingredients were found to be essential to
generate the strong spin polarization: an asymmetric lateral confinement, a
LSOC induced by the lateral confining potential of the QPC, and a strong
electron-electron (e-e) interaction. In this paper, NEGF is used to study how
the spin polarization is affected by the presence of impurities in the central
portion of the QPC. It is found that the number, location, and shape of the
conductance anomalies, occurring below the first quantized conductance plateau
(G0=2e2/h), are strongly dependent on the nature (attractive or repulsive) and
the locations of the impurities. We show that the maximum of the conductance
spin polarization is affected by the presence of impurities. For QPCs with
impurities off-center, a conductance anomaly appears below the first integer
step even for the case of symmetric bias on the two side gates. These results
are of practical importance if QPCs in series are to be used to fabricate
all-electrical spin valves with large ON/OFF conductance ratio.
