1. Quantum Beam Science Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
2. Laboratoire Léon Brillouin, CEA-CNRS, CEA/Saclay, 91191 Gif sur Yvette, France
3. Department of Physics, Tohoku University, Sendai 980-8578, Japan
4. Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
5. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
6. Graduate School of Integrated Arts and Science, Kochi University, Kochi 780-8520, Japan
7. Department of Quantum Matter, ADSM, Hiroshima University, Higashihiroshima, Hiroshima 739-8530, Japan