1. Department of Physics, Yale University, New Haven, Connecticut 06511, USA
2. Center for Research on Interface Structures and Phenomena, Yale University, New Haven, CT 06511, USA
3. Department of Physics & Astronomy and Quantum Matter Institute, University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada
4. Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, USA
5. Department of Applied Physics, Yale University, New Haven, CT 06511, USA
6. Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511, USA