[1] Univ Autonoma Madrid, Fac Ciencias, Dept Fis Aplicada, Madrid 28049, Spain
[2] Univ Autonoma Madrid, Inst Nicolas Cabrera, Fac Ciencias, Campus Cantoblanco, Madrid 28049, Spain
[3] Univ Autonoma Madrid, Ctr Microanal Mat, Madrid 28049, Spain
[4] Univ Porto, Fac Ciencias, IFIMUP Inst Phys Adv Mat Nanotechnol & Photon, Dept Fis & Astron, P-4169007 Porto, Portugal
[5] Univ Porto, LEPABE Fac Engn, P-4200465 Porto, Portugal