1. School of Materials Science and Engineering, Nanyang Technological University, 639798 Singapore
2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
3. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
4. School of Electrical and Electronic Engineering, Nanyang Technological University, 639798 Singapore
5. CINTRA CNRS/NTU/THALES, UMI 3288, 637553 Singapore