[1] Univ Karlsruhe, Inst Theoret Festkorperphys, D-76128 Karlsruhe, Germany
[2] Univ Karlsruhe, DFG CFN, D-76128 Karlsruhe, Germany
[3] Univ Karlsruhe, Lichttech Inst, D-76128 Karlsruhe, Germany
[4] Stockholm Univ, AlbaNova Univ Ctr, Dept Phys, SE-10691 Stockholm, Sweden
[5] Univ Innsbruck, Inst Theoret Phys, A-6020 Innsbruck, Austria