1. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China
2. INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan
3. Department of Physics, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
4. Anhui Laboratory of Advanced Photon Science and Technology, University of Science and Technology of China, Hefei 230026, P. R. China