1. Department of Physics, University of Gothenburg, 412 96 Gothenburg, Sweden
2. Physikalisches Institut (EP3), Universität Würzburg, 97074 Würzburg, Germany
3. NanOsc AB, 164 40 Kista, Sweden
4. Materials Physics, School of ICT, KTH-Royal Institute of Technology, Electrum 229, 164 40 Kista, Sweden