1. Department of Chemistry, University of Zürich, CH-8057 Zürich, Switzerland.
2. Physik-Institut der Universität Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland.
3. Laboratory for Muon Spin Spectroscopy, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland.
4. Department of Physics, Columbia University, New York, NY 10027, USA.
5. Laboratory for Topological Quantum Matter and Spectroscopy, Department of Physics, Princeton University, Princeton, NJ 08544, USA.