1. Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan City 701, Taiwan R.O.C.
2. Physics Department, Technische Universität München, Arcisstraße 21, Munich 85748, Germany
3. Physics Department, Ludwig-Maximilians-Universität München, Geschwister − Scholl - Platz 1, Munich 80799, Germany