1. Materials
Research Center, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
2. Department
of Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China
3. Department
of Engineering, Electrical Engineering Division, University of Cambridge, 9 JJ Thomson Avenue, CB3 0FA, Cambridge, United Kingdom
4. Materials
Research Department, GSI Helmholtz Centre for Heavy Ion Research, 64291 Darmstadt, Germany
5. Materials
Science, Technische Universität Darmstadt, 64287 Darmstadt, Germany