1. State Key Laboratory of Material Processing and Die & Mould Technology; School of Materials Science and Engineering; Huazhong University of Science and Technology; Wuhan 430074 P. R. China
2. Department of Physics; Southern University of Science and Technology; Shenzhen 518055 P. R. China
3. Department of Applied Physics; Chongqing University; Chongqing 401331 P. R. China
4. Department of Energy and Environment; Instituto de Ciencia de Materiales de Madrid; Cantoblanco E-28049 Madrid Spain