Electrical readout of the local nuclear polarization in the quantum Hall effect: a hyperfine battery.
It is demonstrated that the now well-established "flip-flop" mechanism of spin exchange between electrons and nuclei in the quantum Hall effect can be reversed. We use a sample geometry which utilizes separately contacted edge states to establish a local nuclear spin polarization--close to the maximum value achievable--by driving a current between electron states of different spin orientation. When the externally applied current is switched off, the sample exhibits an output voltage of up to a few tenths of a mV, which decays with a time constant typical for the nuclear spin relaxation. The surprising fact that a sample with a local nuclear spin polarization can act as a source of energy and that this energy is well above the nuclear Zeeman splitting is explained by a simple model which takes into account the effect of a local Overhauser shift on the edge state reconstruction.
Würtz
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Laboratorium für Festkörperphysik, Universität Duisburg-Essen, Lotharstr. 1, D-47048 Duisburg, Germany. lorke@uni-duisburg.de
Müller
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Lorke
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Reuter
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Wieck
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