1. Center for Physical Sciences and Technology, Saulėtekio al. 3, Vilnius 10257, Lithuania
2. STU Centre for Nanodiagnostics, Slovak University of Technology, Vazovova 5, Bratislava 811 07, Slovakia
3. Solid State Physics Laboratory, University of Paris-SUD, Bâtiment 510, Orsay 91405, France