1. Department of Physics, Case Western Reserve University, 2076 Adelbert Road, Cleveland, Ohio 44106, USA
2. School of Science, Shenyang Ligong University, Shenyang 110159, People's Republic of China
3. School of Physics and Electronics, Shandong Normal University, Jinan 250014, People's Republic of China
4. Institue of Materials and Clean Energy, Shandong Normal University, Jinan 250014, People's Republic of China