1. Department of Physics, Faculty of Science, Tohoku University, Sendai 980-8578
2. Nanostructure and Material Property Group, PRESTO, JST, Kawaguchi, Saitama 332-0012
3. Department of Advanced Materials Science, Faculty of Frontier Sciences, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561
4. Correlated Electron Research Center (CERC), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8564