1. Department of Applied Physics, Aalto University School of Science, P.O. Box 1100, FIN-00076 Aalto, Espoo, Finland
2. COMP CoE, Aalto University School of Science, P.O. Box 1100, FIN-00076 Aalto, Espoo, Finland
3. Faculty of Physical Sciences and Science Institute, University of Iceland, VR-III, 107 Reykjavík, Iceland
4. Department of Physics, Brown University, Providence Rhode Island 02912-1843, USA