1. Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan.
2. Neutron Scattering Laboratory, Institute for Solid State Physics, University of Tokyo, Tokai, Ibaraki 319-1106, Japan.
3. Correlated Electron Research Center (CERC) and Joint Research Center for Atom Technology (JRCAT), Tsukuba 305-0046, Japan.