1. The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
2. CAPES Foundation, Ministry of Education of Brazil, Brasilia, DF 70040-020, Brazil
3. School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, United Kingdom
4. U.S. Naval Research Laboratory, Washington, D.C., United States
5. Instituto de Estructura de la Materia (IEM-CSIC), Consejo Superior de Investigaciones Científicas, Serrano 121, 28006 Madrid, Spain
6. Nanoinstitut München, Fakultät für Physik, Ludwig-Maximilians-Universität München, 80799 München, Germany
7. Department of Mechanical Engineering, Vanderbilt University, Nashville, Tennessee 37205, United States