1. Institute for Solid State Physics, Jena University, Max-Wien-Platz 1, 07743 Jena, Germany
2. Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden
3. Division of Physics, School of Computer Science, Physics and Mathematics, Linnæus University, 39233 Kalmar-Sweden
4. Department of Mathematics, Physics, and Electrical Engineering, Halmstad University, Box 823, SE-301 18, Halmstad, Sweden