1. Fachbereich Physik, Universität Osnabrück, D-49069 Osnabrück, Germany, Department of Mathematics and Natural Sciences, Bergische Universität Wuppertal, D-42097 Wuppertal, Germany, School of Science, Beijing Institute of Technology, 100081 Beijing, People’s Republic of China, and Institute of Solid State Physics, University of Latvia, 8 Kengaraga Str., Riga LV1067, Latvia