1. IBM Research–Almaden, San Jose, CA 95120, USA.
2. Department of Materials Science Engineering, Stanford University, Stanford, CA 94305, USA.
3. International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China.
4. Max Planck Institute for Chemical Physics of Solids, Dresden D-01187, Germany.
5. Max Planck Institute for Microstructure Physics, Halle (Saale) D-06120, Germany.