1. Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
2. Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 4, Tsukuba 305-8562, Japan.
3. CREST, Japan Science and Technology Corporation (JST)., Japan.
4. Department of Physics, McCullough Building, Stanford University, Stanford CA 94305–4045, USA.