Local spin polarization in ballistic quantum point contacts

Self-consistent calculations of the electronic structure and conductance of a ballistic quantum point contact (QPC) in the one-subband limit are reported. The spin-polarized density-functional theory of Kohn-Sham is used. The self-consistent results show that spontaneous spin polarization occurs locally in the region of the saddle point as the electron density is lowered. As a consequence, the effective potential barriers become different for spin up and spin down electrons. Transport associated with spin up electrons, let us say, then suddenly takes place via tunneling, while spin down electrons still carry the current via propagating states in a normal way. The onset of spontaneous spin polarization induces anomalies in the conductance. Our results support the recent interpretations of experimentally observed conductance anomalies in QPCs near pinch off. The agreement with measurements is qualitative, however, rather than quantitative. Reasons for the discrepancy are proposed.