1. Key Laboratory of Cluster Science of Ministry of Education, School of Science, Beijing Institute of Technology, Beijing 100081, People's Republic of China
2. Department
of Mathematics and Natural Sciences, Bergische Universität Wuppertal, D-42097 Wuppertal,
Germany
3. Institute
of Solid State Physics, University of Latvia, 8 Kengaraga Strasse, Riga LV1067, Latvia