[1] Univ Karlsruhe, TH, Inst Angew Phys, D-76131 Karlsruhe, Germany
[2] Univ Karlsruhe, TH, CFN, DFG, D-76131 Karlsruhe, Germany
[3] Iowa State Univ, Ames Lab, Ames, IA 50011 USA
[4] Iowa State Univ, Dept Phys & Astron, Ames, IA 50011 USA
[5] Forschungszentrum Karlsruhe Helmholtz Gemeinschaf, Inst Nanotechnol, D-76021 Karlsruhe, Germany