1. Condensed Matter Research Center and Photon Factory, Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
2. Department of Physics, Tohoku University, Sendai 980-8578, Japan
3. Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
4. Department of Applied Physics and Quantum-Phase Electronics Center (QPEC), University of Tokyo, Bunkyo, Tokyo 113-8656, Japan
5. Department of Applied Physics, Waseda University, Shinjuku, Tokyo 169-8555, Japan