1. School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
2. Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA
3. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Source, North China Electric Power University, Beijing 102206, People's Republic of China
4. CAS Key Laboratory of Materials for Energy Conversion, Shanghai institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, People's Republic of China
5. Laboratory of Ion Beam Physics, ETH Zurich, Zurich 8093, Switzerland