1. Department of Materials Science and Engineering Southern University of Science and Technology Shenzhen 518055 China
2. Department of Materials Science and Engineering University of Science and Technology of China Hefei 230026 P. R. China
3. Department of Physics Southern University of Science and Technology Shenzhen 518055 China
4. Department of Physics and Astronomy and Department of Electrical Engineering and Computer Science Vanderbilt University Nashville TN 37235 USA