1. Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA
2. Department of Physics, National University of Singapore, Singapore 117551
3. Applied Physics Program, Rice Quantum Institute, Rice University, Houston, Texas 77005, USA
4. Department of Chemistry, Rice University, Houston, Texas 77005, USA
5. Department of Materials Science and Nanoengineering, Rice University, Houston, Texas 77005, USA
6. Department of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA