1. School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel
2. Institute of Theoretical Solid
State Physics, Karlsruhe Institute of Technology, D-76131 Karlsruhe, Germany
3. Department of Physics, University of Konstanz, D-78457 Konstanz, Germany
4. Departamento de Fı́sica
Teórica de la Materia Condensada and Condensed Matter Physics
Center (IFIMAC), Universidad Autónoma de Madrid, E-28049 Madrid, Spain