[1] Forschungszentrum Karlsruhe, Karlsruhe Inst Technol, Inst Nanotechnol, D-76021 Karlsruhe, Germany
[2] Forschungszentrum Karlsruhe, Karlsruhe Inst Technol, Ctr Funct Nanostruct, D-76021 Karlsruhe, Germany
[3] Univ Karlsruhe, Karlsruhe Inst Technol, Inst Appl Phys, D-76131 Karlsruhe, Germany
[4] Univ Karlsruhe, Karlsruhe Inst Technol, Ctr Funct Nanostruct, D-76131 Karlsruhe, Germany
[5] Univ Karlsruhe, Karlsruhe Inst Technol, Inst Theoret Festkorperphys, D-76131 Karlsruhe, Germany