[1] Forschungszentrum Karlsruhe, Inst Nanotechnol, D-76021 Karlsruhe, Germany
[2] Politecn Milan, INFM, CNR, Dept Phys, I-20133 Milan, Italy
[3] Univ Cambridge, Dept Engn, Cambridge CB3 0FA, England
[4] DFG Ctr Funct Nanostruct CFN, D-76131 Karlsruhe, Germany