1. Department of Chemical and Biomolecular Engineering; University of Akron; Akron OH 44325 USA
2. School of Physics and Electronic Information; Huaibei Normal University; Huaibei Anhui 235000 P. R. China
3. High Magnetic Field Laboratory; Chinese Academy of Sciences; Hefei Anhui 230031 P. R. China
4. Hefei National Laboratory for Physical Science at Microscale; University of Science and Technology of China; Hefei Anhui 230026 P. R. China