1. Laboratory for Mesoscopic Systems, Department of Materials, ETH Zürich, 8093 Zürich, Switzerland
2. Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
3. SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom
4. U.S. Naval Research Laboratory, Washington, DC 20375, USA
5. School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom
6. Department of Applied Physics, Yale University, New Haven, Connecticut 06511, USA