1. Center for Nanoscale Materials, Argonne National Laboratory, 9700 South Cass Avenue, Building 440, Argonne, Illinois 60439, USA
2. Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, USA
3. Computational Physics Laboratory, Tampere University, P.O. Box 692, FI-33014 Tampere, Finland
4. Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan
5. Department of Physics, Northeastern University, Boston, Massachusetts 02115, USA
6. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
7. Department of Chemistry, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208, USA