1. Department of Solid State Sciences, Ghent University, Krijgslaan 281, 9000 Ghent, Belgium
2. Department of Physics, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland
3. Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany