1. Department of Applied Physics Aalto University School of Science Aalto 00076 Finland
2. Department of Physics Dalian Maritime University Dalian 116026 China
3. Department of Electrical and Computer Engineering Rice University Houston TX 77005 USA
4. School of Physics Academy for Advanced Interdisciplinary Studies Collaborative Innovation Center of Quantum Matter Peking University Beijing 100871 China
5. Department of Physics and Astronomy Rice University Houston TX 77005 USA
6. Department of Materials Science and NanoEngineering Rice University Houston TX 77005 USA