Magnetic impurity formation in quantum point contacts

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.

preprint version of a paper published in Nature

Cited by in corpus (25)

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• Probing the microscopic structure of bound states in quantum point contacts
• Spin coupling in zigzag Wigner crystals
• Detection of spin polarized currents in quantum point contacts via transverse electron focusing
• Electron-electron interaction effects in quantum point contacts
• Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy
• Conductance of a quantum point contact based on spin-density-functional theory
• 0.7 Structure and Zero Bias Anomaly in Ballistic Hole Quantum Wires
• Non-Kondo zero-bias anomaly in quantum wires
• The influence of device geometry on many-body effects in quantum point contacts: Signatures of the 0.7 anomaly, exchange and Kondo
• Realistic modelling of quantum point contacts subject to high magnetic fields and with current bias at out of linear response regime
• Ferromagnetic Spin Coupling as the Origin of 0.7 Anomaly in Quantum Point Contacts
• Detector Backaction on the Self-Consistent Bound State in Quantum Point Contacts
• Conductance Anomaly and Fano Factor Reduction in Quantum Point Contacts
• Suppression of Shot Noise in Quantum Point Contacts in the "0.7" Regime
• Length-Dependent Conductance of a Spin-Incoherent Hubbard chain; Monte Carlo Calculations
• The theory of the "0.7 anomaly" in quantum point contacts
• Electronic transport in inhomogeneous quantum wires
• Fractional quantization of ballistic conductance in 1D hole systems
• Theory of NMR in semiconductor quantum point contact devices
• Dephasing in a quantum dot coupled to a quantum point contact
• Nuclear magnetic resonance probes for the Kondo scenario for the 0.7 feature in semiconductor quantum point contact devices
• Where are the edge-states near the quantum point contacts? A self-consistent approach
• Entanglement and transport anomalies in nanowires
• Mechanism of electron localization in a quantum wire