1. School of Mathematics and Statistics, Zhengzhou University, Zhengzhou 450001, China
2. Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996, USA
3. State Key Laboratory of Nonlinear Mechanics, Institute of Mechanics, Chinese Academy of Science, Beijing 100190, China
4. B.I. Verkin Institute for Low Temperature Physics and Engineering, National Academy of Sciences of Ukraine, Kharkiv 61103, Ukraine
5. Department of Mathematics, The University of Hong Kong, Pokfulam, Hong Kong
6. Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA