1. Department of Physics; Key Laboratory of Artificial Structures & Quantum Control; Ministry of Education; Shanghai Jiao Tong University; 800 Dongchuan Road; Shanghai; 200240; China
2. Instrumental Analysis Center; Shanghai Jiao Tong University; 800 Dongchuan Road; Shanghai; 200240; China
3. Department of Crystalline Materials Science; Nagoya University; Nagoya; 464-8603; Japan