[1] Univ Complutense Madrid, Dept Quim Fis 1, Avda Complutense S-N, E-28040 Madrid, Spain
[2] Univ Barcelona, Dept Fis Mat Condensada, E-08028 Barcelona, Spain
[3] Univ Barcelona, UBICS, E-08028 Barcelona, Spain
[4] Univ Barcelona, Inst Nanociencia & Nanotecnol IN2UB, E-08028 Barcelona, Spain