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

Abstract

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 $(100\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ 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 $2{e}^{2}∕h$) for a $200\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ QPC to slightly below 0.5 for a $400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ 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.