1. CCSE, Japan Atomic Energy Agency, Kashiwa, Chiba 277-8587, Japan
2. Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
3. Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, D-06112 Halle, Germany
4. Department of Computer Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
5. CEMS, RIKEN, Wako, Saitama 351-0198, Japan
6. JST, CREST, Chiyoda, Tokyo 102-0075, Japan