**Acknowledgements**

The authors acknowledge S. Yazji and G. Abstreiter of Walter Shottky Institute of the Techni‐ sche Universität München, A. Fontcuberta i Morral of the École Polytechnique Fédérale de Lausanne, and P. Postorino of Sapienza University of Rome for their precious contribution to data acquisition and interpretation. We acknowledge financial support from the Swiss National Science Foundation research grant (Project Grant No. 200021\_165784).
