1. Department
of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
2. Institut
für Theoretische Physik, Universität Bremen, P.O. Box 330 440, 28334 Bremen, Germany
3. Department
of Physics and Astronomy, University of California, Riverside, California 92521, United States
4. Bremen
Center for Computational Materials Science, Universität Bremen, 28334 Bremen, Germany
5. Materials
Science and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan
6. Department
of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States