Density functional study on quantum point contacts
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The transport of electrons through a quantum point contact (QPC) has been intensively studied in the past decade both from experimental and theoretical points of view. In addition to the stepwise nature (at multiples of G=2e^2/h),the conductance G of a QPC system also exhibits a small plateau around G=0.7, varying at different temperatures and magnetic fields. This so-called "0.7 anomaly" remains an unsettled issue to date. In this work, we use density functional theory (DFT) based on the local density approximation (LDA) to study the spatial distribution of charge density of a QPC system. Periodic boundary condition is imposed in our calculations. Despite the neglect of spin effect, we show similar features of charge density distribution in comparison with other works. We also study the asymmetric potential modification of a QPC when a source-drain bias V sd is applied across the QPC. It is found that β, the ratio of potential drop between source and unbiased QPC to V sd , depends on the magnitude of the confinement potential V sg produced by the split gates. Different chemical potentials are set at source and drain for some numerical reason to keep equal bulk density at both sides. Our focus on the biased calculations is in the pinch-off regime, and β is about 0.45 from our results. However, experimental results show that β is about 0.25 when pinched-off (higher V sg ) and about 0.5 at conducting state (lower V sg ). Discussion and comment on this inconsistency will be made at the end of this article.