[1] CIN2 ICN CSIC, Barcelona 08193, Spain
[2] Univ Autonoma Barcelona, Catalan Inst Nanotechnol, E-08193 Barcelona, Spain
[3] ICREA, Barcelona 08070, Spain
[4] Univ Granada, Fac Ciencias, Dpto Elect & Tecnol Computadores, E-18071 Granada, Spain
[5] Univ Granada, Fac Ciencias, CITIC, E-18071 Granada, Spain
[6] Univ Savoie, GrenobleINP, UJF,UMR 5130, CNRS,IMEP LAHC, F-38016 Grenoble, France
[7] MINATEC, LETI, CEA, F-38054 Grenoble, France