1. Department of Physics, Colorado School of Mines, Golden, Colorado 80401, USA
2. Materials Science Division, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, Illinois 60639, USA
3. Department of Physics, Northern Illinois University, DeKalb, Illinois 60115, USA
4. Theory of Condensed Matter Physics, Karlsruhe Institute of Technology, Karlsruhe D-76131, Germany
5. Heidelberger Akademie der Wissenschaften, Heidelberg 69117, Germany