1. Faculty of Physics and Center
for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, Lotharstr. 1, 47048 Duisburg, Germany
2. Institute of Electrical
Measurement and Fundamental Electrical Engineering, TU Braunschweig, Hans-Sommer-Str.
66, 38106 Braunschweig, Germany