1. Department of Physics, Yale University, New Haven, Connecticut 06511, USA
2. Center for Research on Interface Structures and Phenomena, Yale University, New Haven, Connecticut 06511, USA
3. Department of Physics and Astronomy and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
4. Department of Applied Physics, Yale University, New Haven, Connecticut 06511, USA
5. Department of Mechanical Engineering and Materials Science, Yale University, New Haven, Connecticut 06511, USA