1. National Laboratory of Solid State Microstructures; School of Physics; Nanjing University; Nanjing 210093 China
2. National Laboratory of Solid State Microstructures; Department of Materials Science and Engineering; Nanjing University; Nanjing 210093 China
3. King Abdullah University of Science & Technology (KAUST), Physical Sci. and Eng.; Thuwal 23955-6900 Kingdom of Saudi Arabia