1. University of Regensburg,Department of Physics,Regensburg,Germany,94040
2. University of Michigan,Department of Electrical Engineering and Computer Science,Ann Arbor,MI,USA,48109
3. Helmut Schmidt University,Faculty of Electrical Engineering,Hamburg,Germany,22043
4. Institute of Experimental Physics I, Justus Liebig University Giessen,Giessen,Germany,35392