1. School of Physical Science and Technology, Southwest University, Chongqing 400715, P. R. China
2. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China
3. The Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, United Kingdom
4. Institute for Advanced Study, Shenzhen University, Shenzhen 518060, P. R. China