[1] Bilkent Univ, Dept Phys, TR-06800 Ankara, Turkey
[2] FORTH, IESL, Iraklion, Crete, Greece
[3] US DOE, Ames Lab, Ames, IA 50011 USA
[4] Iowa State Univ, Dept Phys & Astron, Ames, IA 50011 USA
[5] Bilkent Univ, Nanotechnol Res Ctr, TR-06800 Ankara, Turkey