1. Department
of Physics and Astronomy, University of Turku, FIN-20014 Turku, Finland
2. Ioffe
Physical-Technical Institute, Russian Academy of Sciences, St. Petersburg 194021, Russian Federation
3. Applied
Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden
4. Division
for Materials Theory, Department of Physics and Materials Science, Uppsala University, P.O.
Box 516, SE-75120, Uppsala, Sweden
5. Research Institute
for Solid State Physics and Optics, P.O. Box 49, H-1525 Budapest, Hungary