1. Institute of Physics, Technische Universität Chemnitz, D-09107 Chemnitz, Germany
2. Institute of Physics, University of Augsburg, D-86159 Augsburg, Germany
3. Helmholtz Zentrum Dresden Rossendorf, P.O. Box 510119, D-01314 Dresden, Germany
4. Technische Universität Dresden, D-01062 Dresden, Germany
5. Institute of Physics, University of Münster, D-48149 Münster, Germany