1. Department of Physics; Hangzhou Normal University; Hangzhou 310046 China
2. Division of Materials Science; Nanyang Technological University; Singapore 639798 Singapore
3. Department of Physics; Chongqing University; Chongqing 401331 China
4. Key Laboratory of Artificial Structures and Quantum Control (Ministry of Education); School of Physics and Astronomy; Shanghai JiaoTong University; Shanghai 200240 China