1. Reykjavik University, Menntavegur 1, IS-102,Department of Engineering,Reykjavik,Iceland
2. Chalmers University of Technology, SE-412 96,Department of Physics,Gothenburg,Sweden
3. University of Bucharest,Faculty of Physics,Magurele-Ilfov,Romania,077125
4. National Institute of Materials Physics,Magurele-Ilfov,Romania,077125