From quantum point contacts to quantum wires: Density-functional calculations with exchange and correlation effects

We numerically analyze the conductance and spin polarization of realistic quantum point contacts (QPCs) using density-functional theory, including both exchange and correlation effects. The self-consistent calculations are performed as a function of split gate voltage, for different temperatures and QPC lengths.We show that in short enough QPCs (100 nm) there is no spontaneous spin polarization, and the conductance for up-spin and down-spin electrons is the same. As the length of the QPC increases, so does the spin polarization and the difference in conductance between up-spin and down-spin electrons, resulting in an anomalous structure in the total conductance-the 0.7 anomaly. This structure moves from around 0.9 (in units of 2e(2)/h) for a 200 nm QPC to slightly below 0.5 for a 400 nm QPC. Due to the strong ferromagnetic spin polarization in a long QPC, it will effectively work as a spin filter. The temperature dependence of the conductance is discussed in relation to the Reilly model, whose underlying assumption, regarding the shape of the spin gaps, is investigated using the self-consistent results.