1. Key Lab of Materials Physics, Institute of Solid State Physics, Chinese Academy of Science, Hefei, 230031, Anhui, P.R. China, Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong, Yuseong, Daejeon, 305-701, South Korea, and Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Science (CAS), Ningbo city, 315040, Zhejiang, P.R. China