[1] Karlsruhe Inst Technol, Inst Theorie Kondensierten Materie, D-76128 Karlsruhe, Germany
[2] Karlsruhe Inst Technol, Inst Nanotechnol, D-76021 Karlsruhe, Germany
[3] LD Landau Theoret Phys Inst, RU-119334 Moscow, Russia
[4] Yale Univ, Dept Phys, New Haven, CT 06520 USA
[5] Karlsruhe Inst Technol, DFG Ctr Funct Nanostruct CFN, D-76128 Karlsruhe, Germany