1. a Department of Physics, Applied Physics and Stanford Synchrotron Radiation Laboratory , Stanford University , California , 94305 , USA
2. b Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley , California , 94720 , USA
3. c Department of Applied Physics , University of Tokyo , Bunkyo-ku, Tokyo , 113-8656 , Japan
4. d Correlated Electron Research Center, Agency of Industrial Science and Technology , Tsukuba , 305-0046 , Japan