"0.7 anomaly" and magnetic impurity formation in quantum point contacts

TL;DR

This study investigates the 0.7 conductance anomaly in quantum point contacts using spin DFT transport calculations, finding that the anomaly may not be directly related to magnetic impurity formation as previously suggested.

Contribution

It performs transport calculations to test the magnetic impurity hypothesis for the 0.7 anomaly, highlighting limitations of DFT in accurately modeling charge localization.

Findings

Transport calculations do not reproduce the 0.7 anomaly.

Localized spin states are observed, but not the conductance anomaly.

Self-interaction errors in DFT may cause spurious magnetic impurity artifacts.

Abstract

The origin of the 0.7-conductance anomaly in quantum point contacts (QPCs) has been a subject of lively discussions since its discovery more than 10 years ago. In a recent letter, based on spin density functional theory (DFT), Rejec and Meir explained the origin of the 0.7-anomaly as being due to the formation of a magnetic impurity in the QPCs [T. Rejec and Y. Meir, Nature 442, 900 (2006)]. They did not, however, perform transport conductance calculations so the central question whether the 0.7-anomaly is indeed related to the formation of a magnetic impurity has remained unanswered. In this communication, we perform spin DFT transport calculations for the structures considered by Rejec and Meir. While we recover the findings reported by Rejec and Meir concerning the formation of localized spin-degenerate quasi-bound states, our transport calculations do not contain the 0.7 anomaly in the conductance. We suggest that the inability of the DFT to reproduce quantitatively the 0.7 anomaly may be due to the uncorrected self-interaction errors in the DFT calculation of electronic transport for the case when localization of charge is expected to occur, so that the magnetic impurity formation may be an artefact of DFT due to the spurious self-interaction.