1. School of Physics, University of Electronic Science and Technology of China, Cheng Du 610054, China
2. Center for Polymer Studies and Department of Physics, Boston University, Boston, Massachusetts 02215, USA
3. Web Sciences Center, School of Computer Science and Engineering, University of Electronic Science and Technology of China, Chengdu 610073, China
4. School of Mathematical Sciences, University of Electronic Science and Technology of China, Cheng Du 610054, China