1. Web Sciences Center, School of Computer Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China
2. Department of Physics, University of Fribourg, Chemin du Musée 3, Fribourg CH-1700, Switzerland
3. Department of Modern Physics and Nonlinear Science Center, University of Science and Technology of China, Hefei 230026, P. R. China