1. Institut für Theoretische Physik, Universität Bremen, P.O. Box
330 440, 28334 Bremen, Germany
2. Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS), Suwon 440-746, Republic of Korea
3. Bremen Center for Computational Materials
Science, Universität Bremen, 28334 Bremen, Germany
4. Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Republic of Korea