[1] Karlsruhe Inst Technol, Inst Nanotechnol, D-76021 Karlsruhe, Germany
[2] Univ Strasbourg, IPCMS, UMR 7504, CNRS, F-67034 Strasbourg, France
[3] Karlsruhe Inst Technol, Inst Theorie Kondensierten Mat, D-76128 Karlsruhe, Germany
[4] Delft Univ Technol, Kavli Inst Nanosci, NL-2600 GA Delft, Netherlands