1. School of Materials Science and Engineering, State Key Laboratory of Mining Response and Disaster Prevention and Control in Deep Coal Mines, Anhui University of Science and Technology, Huainan, Anhui 232001, PR China
2. Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, Yunnan University, Kunming, Yunnan 650504, PR China
3. Department of Physics, Umeå University, Umeå S-901 87, Sweden