1. 1. Physikalisches Institut; Universität Stuttgart; Pfaffenwaldring 57 D-70550 Stuttgart Germany
2. Moscow Institute of Physics and Technology (State University); Institutskii Pereulok 9 Dolgoprudny Moscow Region Russia
3. Prokhorov General Physics Institute; Russian Academy of Sciences; Vavilov str. 38 119991 Moscow Russia
4. Max Planck Institut für Festkörperforschung; Heisenbergstrasse 1 D-70569 Stuttgart Germany
5. Institute de Physique de la Matière Condensée; Ecole Polytechnique de Lausanne; 1015 Lausanne Switzerland