Magnetic impurity formation in quantum point contacts

TL;DR

This paper uses density-functional calculations to show that magnetic impurities can form in quantum point contacts, explaining the 0.7 conductance anomaly and impacting quantum dot spin states and qubit coherence.

Contribution

It provides the first detailed numerical evidence of magnetic moment formation in QPCs under various conditions, linking it to the 0.7 anomaly.

Findings

Magnetic moments form in QPC channels as density increases.

Impurity formation occurs at specific magnetic fields and mode openings.

Results suggest implications for quantum dot spin filling and qubit dephasing.

Abstract

A quantum point contact (QPC), a narrow region separating two wider electron reservoirs, is the standard building block of sub-micron devices, such as quantum dots - small boxes of electrons, and qubits - the proposed basic elements of quantum computers. As a function of its width, the conductance through a QPC changes in integer steps of G0 = $2e^2/h$, signalling the quantization of its tranverse modes.1,2 Such measurements also reveal an additional shoulder at a value around 0.7, an observation which remains a puzzle even after more than a decade. Recently it has been suggested5,6 that this phenomenon can be explained if one invokes the existence of a magnetic impurity in the QPC at low densities. Here we present extensive numerical density-functional calculations that reveal the formation of a magnetic moment in the channel as the density increases above pinch-off, under very general conditions. In addition we show that such an impurity will also form at large magnetic fields, for a specific value of the field (corresponding to a degeneracy point between the upper spin state in the first mode and the lower spin state in the second mode), and sometimes even at the opening of the second mode in the QPC. Beyond explaining the source of the "0.7 anomaly", these results may have far reaching implications on spin filling of electronic states in quantum dots and on dephasing of quantum information stored in semiconductor qubits.