1. Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06520, USA
2. Center for Research on Interface Structures and Phenomena, Yale University, New Haven, Connecticut 06520, USA
3. Department of Physics and Benjamin Levich Institute, The City College of the City University of New York, New York, New York 10031, USA
4. Department of Physics, Yale University, New Haven, Connecticut 06520, USA
5. Department of Applied Physics, Yale University, New Haven, Connecticut 06520, USA