1. School of Physics and Electronics; Hunan University; Changsha 410082 P. R. China
2. Pen-Tung Sah Institute of Micro-Nano Science and Technology; Xiamen University; Xiamen 361005 P. R. China
3. Institute for Superconducting and Electronic Materials; University of Wollongong; Wollongong 2500 Australia
4. Department of Materials Science and Engineering, and Department of Energy and Resources Engineering, College of Engineering; Peking University; Beijing 100871 P. R. China