Author(s): M Dubiel, J Haug, H Kruth, H Hofmeister and W SeifertAffiliation(s): Department of Physics, University of Halle-Wittenberg, Friedemann-Bach-Platz 6, D-06108 Halle, Sachsen-Anhalt, Germany; Max Planck Institute of Microstructure Physics, D-06120 Halle, Germany