1. Department of Physics; Jinggangshan University; Ji'an 343009 People's Republic of China
2. State Key Laboratory for Powder Metallurgy; Central South University; Changsha Hunan 410083 People's Republic of China
3. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics; Chinese Academy of Sciences; Beijing 100190 People's Republic of China
4. Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Department of Physics; Tongji University; Shanghai 200092 People's Republic of China