1. Laboratory for Solid State Physics, ETH Zürich, Otto-Stern-Weg 1, CH-8093 Zürich, Switzerland
2. L-NESS, Department of Physics, Politecnico di Milano, via Anzani 42, I-22100 Como, Italy
3. Scientific Center for Optical and Electron Microscopy (ScopeM), ETH Zürich, Auguste-Piccard-Hof 1, CH-8093 Zürich, Switzerland