\section*{Acknowledgement}

This work was supported by the Austrian Science Fund (FWF):
P29603, the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme (SPQRel, Grant Agreement 679183), the AWS Austria Wirtschaftsservice, PRIZE  Programme, under
Grant No. P1308457. We thank G. Weihs, B. Pressl, and R. Keil (Univ. Innsbruck) and V. Volobuev, Y. Huo for fruitful discussions and U. Kainz for help during the device fabrication.

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