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Magnetic impurity formation in quantum point contacts

Abstract

A quantum point contact (QPC) is a narrow constriction between two wider electron reservoirs, and is the standard building block of sub-micrometre devices such as quantum dots and qubits (the proposed basic elements of quantum computers). The conductance through a QPC changes as a function of its width in integer steps of G0 = 2e2/h (where e is the charge on an electron, and h is Planck's constant), signalling the quantization of its transverse modes1,2. But measurements of these conductance steps also reveal an additional shoulder at a value around 0.7G0 (refs 1–4), an observation that has remained a puzzle for more than a decade. It has recently been suggested5,6 that this phenomenon can be explained by the existence of a magnetic ‘impurity’ in the QPC at low electron densities. Here we present extensive numerical density-functional calculations that reveal the formation of an electronic state with a spin-1/2 magnetic moment in the channel 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, and sometimes even at the opening of the second transverse mode in the QPC. Beyond explaining the source of the ‘0.7 anomaly’, these results may have far-reaching implications for spin-filling of electronic states in quantum dots and for the dephasing of quantum information stored in semiconductor qubits.

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Figure 1: Quantum point contact.
Figure 2: States with spin polarization in a QPC.
Figure 3: Formation of a magnetic moment.
Figure 4: Quasi-bound state in a large magnetic field.

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Acknowledgements

We thank N. Argaman, R. Baer, D. Goldhaber-Gordon, B. I. Halperin, K. Kikoin, W. Kohn and M. Stopa for discussions. This work was supported by the Binational Science Foundation. We also thank the Department of Theoretical Physics at Jožef Stefan Institute for providing access to computer facilities. Author Contributions Y.M. initiated the project. T.R. performed numerical simulations. T.R. and Y.M. analysed results and co-wrote the paper.

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Correspondence to Tomaž Rejec.

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Rejec, T., Meir, Y. Magnetic impurity formation in quantum point contacts. Nature 442, 900–903 (2006). https://doi.org/10.1038/nature05054

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