1. Department of Physics and Astronomy, University of California, Davis, California 95616, USA
2. Department of Optical Sciences and Engineering, Fudan University, Shanghai 200045, China
3. Center for Correlated Electron Systems, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea
4. Department of Physics and Astronomy, Seoul National University, Seoul 08826, Republic of Korea
5. Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom