1. Department
of Physics and Key Laboratory of Artificial Micro- and Nano-structures
of Ministry of Education, Wuhan University, Wuhan 430072, China
2. School
of Mathematics and Information Sciences, North China University of Water Resources and Electric Power, Zhengzhou 450046, China
3. Department
of Physics, Southeast University, Nanjing 211189, China
4. Department
of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
5. Department
of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 804, Taiwan