1. Department of Applied Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656
2. CREST, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012
3. Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562