1. National Laboratory of Solid State Microstructures, College of Engineering and Applied Sciences, and School of Physics Nanjing University Nanjing 210093 China
2. Department of Physics Kennesaw State University Marietta GA 30060 USA
3. Pritzker School of Molecular Engineering University of Chicago Chicago IL 60637 USA
4. Department of Physics University of Arkansas Fayetteville AR 72701 USA