1. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
2. School of Physical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
3. Physics Department, Faculty of Science, Al-Azhar University, Nasr City E-11884 Cairo, Egypt
4. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China
5. Physics Department E20, Technical University of Munich, D-85748 Garching, Germany