1. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, U.K.
2. Department of Mechanical Engineering & Materials Science, Yale University, New Haven, Connecticut 06520, United States
3. Department of Physics, Yale University, New Haven, Connecticut 06520, United States
4. Department of Applied Physics, Yale University, New Haven, Connecticut 06520, United States
5. Benjamin Levich Institute and Physics Department, The City College of the City University of New York, New York, New York 10031, United States
6. WPI-Advanced Institute for Materials Research (WPI-AIMR), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan