[1] Forschungszentrum Karlsruhe, Inst Nanotechnol, D-76021 Karlsruhe, Germany
[2] Univ Karlsruhe, Inst Angew Phys, D-76131 Karlsruhe, Germany
[3] Iowa State Univ, Dept Elect & Comp Engn, Ames, IA 50011 USA
[4] Iowa State Univ, Microelect Res Ctr, Ames, IA 50011 USA
[5] Iowa State Univ, Ames Lab, Ames, IA 50011 USA
[6] Iowa State Univ, Dept Phys & Astron, Ames, IA 50011 USA
[7] Univ Crete, Fdn Res & Technol Hellas, Inst Elect Struct & Laser, Iraklion 71110, Crete, Greece
[8] Univ Crete, Dept Mat Sci & Technol, Iraklion 71110, Crete, Greece
[9] Zuse Inst Berlin, D-14195 Berlin, Germany
[10] DFG Forschungszentrum Matheon, D-10623 Berlin, Germany
[11] Forschungszentrum Karlsruhe, Inst Nanotechnol, D-76021 Karlsruhe, Germany