1. National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan
2. Department of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
3. Department of Physics, Åbo Akademi, FI-20500 Turku, Finland
4. Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, 01328 Dresden, Germany