1. College of Physics and Electronics, Hunan Institute of Science and Technology, Yueyang 414000, P. R. China
2. School of Physics, Huazhong University of Science & Technology, Wuhan 430074, P. R. China
3. Institute of Geophysics and Geomatics, China University of Geosciences, Wuhan 430074, P. R. China
4. Key Laboratory of Pollution Ecology and Environment Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang 110016, P. R. China