1. Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 446-701, Korea
2. Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany
3. Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, United States
4. Department of Physics, Columbia University, New York, New York 10027, United States
5. Department of Chemistry, Columbia University, New York, New York 10027, United States