1. Ningxia Key Laboratory of Photovoltaic Materials, School of Materials and New Energy, Ningxia University, Yinchuan, Ningxia750021, P.R. China
2. Computer Network Information Center, Chinese Academy of Science, Beijing100190, China
3. Max Planck Institute for the Structure and Dynamics of Matter and Center for Free-Electron Laser Science, Luruper Chaussee 149, 22761Hamburg, Germany