1. Key
Laboratory for Micro-Nano Physics and Technology of Hunan Province,
and State Key Laboratory of Chemo/Biosensing and Chemometrics (CBSC), Hunan University, Changsha 410082, P. R. China
2. Department
of Physics and Key Laboratory of Artificial Micro- and Nano-structures
of Ministry of Education, Wuhan University, Wuhan 430072, P. R. China
3. Key
Laboratory for the Physics and Chemistry of Nanodevices and Department
of Electronics, Peking University, Beijing 100871, P. R. China
4. Division
of Solid State Physics, Lund University, Box 118, S-22100 Lund, Sweden