1. Key Laboratory of Materials Physics and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, HFIPS, Chinese Academy of Sciences, P.O. Box 1129, Hefei 230031, China
2. University of Science and Technology of China, Hefei 230026, China
3. Institut für Physik & IMN MacroNano@ (ZIK), Technische Universität Imenau, 98693 Ilmenau, Germany