1. Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstr. 400, 01328 Dresden, Germany
2. Empa-Swiss Federal Laboratories for Materials Science and Technology, Ueberlandstr. 129, 8600 Dübendorf, Switzerland
3. Chemnitz University of Technology, Institute of Physics, Reichenhainer Str. 70, 09126 Chemnitz, Germany
4. Institute of Physics of Solids, Dresden University of Technology, 01062 Dresden, Germany