1. I. Physikalisches Institut, Georg-August-Universität, Friedrich-Hund-Platz 1, D-37077 Göttingen, Germany
2. Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China, Hefei 230026, China
3. Department of Materials and Materials Technology, Saarland University, Campus D 2.2, D-66123 Saarbrücken, Germany