1. Solid
State Physics and NanoLund, Lund University, Box 118, SE-221 00 Lund, Sweden
2. Department
of Mathematics, Physics and Electrical Engineering, Halmstad University, Box 823, SE-301 18 Halmstad, Sweden
3. Department
of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden