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 Anhui 230601 P. R. China
3. State Key Laboratory of Multiphase Complex SystemsInstitute of Process EngineeringChinese Academy of Sciences Beijing 100190 P. R. China
4. Hefei National Laboratory for Physical Sciences at the MicroscaleUniversity of Science and Technology of China Hefei Anhui 230026 P. R. China