\section*{Acknowledgements}
We thank the Cleanroom Operations Team of the Binnig and Rohrer Nanotechnology Center (BRNC) for their help and support.
We are grateful to C.~Bruder for useful comments on the manuscript.
W.W.~acknowledges support from the Swiss National Science Foundation (grant number 200020\_207538).
J.C.C.~thanks the Spanish Ministry of Science and Innovation (Grant No. PID2020-114880GB-I00) for financial support and the Deutsche Forschungsgemeinschaft (DFG; German Research Foundation) via SFB 1432 for sponsoring his stay at the University of Konstanz as a Mercator Fellow. 
W.B.~acknowledges support from the DFG via SFB 1432 (ID 425217212) and BE 3803/14-1 (ID 467596333).
F.N.~acknowledges support from the European Research Council (grant number 804273) and the Swiss National Science Foundation (grant number 200021\_201082).


\section*{Author contributions}
F.N.~conceived the experiment.
E.C., F.K., R.S.~and W.W.~developed and provided the heterostructure material.
M.C.~designed and fabricated the devices.
M.C.~and F.N.~performed the electrical measurements with contributions from D.Z.H.~and D.S.
M.C., D.Z.H., D.S.~and F.N.~interpreted the data with inputs from M.H.~and S.C.t.K. and theoretical support from H.W., A.E.S., J.C.C.~and W.B.
H.W.~performed the numerical simulations.
A.E.S.~carried out the analytical studies.
H.W.~and A.E.S.~wrote the Theory section.
M.C.~and F.N.~prepared the manuscript with feedback from all the authors.



\section*{Data availability}
Data presented in this work will be available on Zenodo. The data that support the findings of this study are available upon reasonable request from the corresponding author.


