1. Guangxi Key Laboratory for Informational Materials and School of Materials Science and Engineering, Guilin University of Electronic Technology, 1 Jinji Road, Guilin, Guangxi 541004, P. R. China
2. Institut für Materialphysik, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany