1. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA
2. Department of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853, USA
3. International Center for Quantum Materials, Peking University, Beijing 100871, China
4. Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA
5. Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, USA