1. Department of Physics, University of Virginia, Charlottesville, VA 22904, U.S.A.
2. Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
3. Department of Advanced Materials, The University of Tokyo, Kashiwa, Chiba 277-8561, Japan
4. Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan
5. The Institute for Solid State Physics, The University of Tokyo, Kashiwa, Chiba 277-8581, Japan
6. Department of Physics, Waseda University, Shinjuku, Tokyo 169-8050, Japan
7. Department of Physics, Korea University, Seoul 136-701, Korea
8. HANARO Center, Korea Atomic Energy Research Institute, Daejeon, Korea
9. Department of Physics and Astronomy, Rutgers University, Piscataway, NJ 08854, U.S.A.
10. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, U.S.A.