1. Max-Planck-Institut für Mikrostrukturphysik D-06120 Halle Germany
2. Institute of PhysicsMartin-Luther-Universität Halle-Wittenberg D-06099 Halle Germany
3. Institute of General and Inorganic ChemistryBulgarian Academy of Sciences 1113 Sofia Bulgaria
4. Institute of PhysicsThe Czech Academy of Sciences 182 21 Prague Czech Republic