1. Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
2. Ioffe Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russian Federation
3. Comptek Solutions Inc., Voimakatu 14, FI-20520 Turku, Finland
4. Department of Materials Science and Engineering, The University of Texas at Dallas, Richardson, Texas 75080, USA