1. Key Laboratory of Materials PhysicsAnhui Key Laboratory of Nanomaterials and NanotechnologyCAS Center for Excellence in NanoscienceInstitute of Solid State PhysicsChinese Academy of Sciences Hefei 230031 P. R. China
2. Institute of Physical Science and Information TechnologyAnhui University Hefei 230031 P. R. China
3. University of Science and Technology of China Hefei 230026 P. R. China