1. The Institute for Advanced Studies Wuhan University Wuhan 430072 P. R. China
2. The Faculty of Electrical Engineering and Computer Science Ningbo University Ningbo 315211 P. R. China
3. Department of Physics The Chinese University of Hong Kong New Territories Hong Kong 999077 P. R. China
4. Frontier Institute of Science and Technology Xi'an Jiaotong University Xi'an 710054 P. R. China