1. Department of Physics, Centre for Materials Science and Nanotechnology, University of Oslo, PO Box 1048, Blindern, N-0316 Oslo, Norway
2. Department of Materials Science, National Research Nuclear University MEPhI, 31 Kashirskoe Hwy, 115409 Moscow, Russia
3. Institute of Physics, The Chinese Academy of Sciences, Beijing 100190, China
4. Songshan Lake Materials Laboratory, Dongguan, Guangdong 523808, China