[1] Univ Santiago de Compostela, Dept Phys Chem, Santiago De Compostela 15782, Spain
[2] Univ Santiago de Compostela, Dept Appl Phys, Santiago De Compostela 15782, Spain
[3] Univ Santiago de Compostela, Inst Invest Tecnol, E-15782 Santiago De Compostela, Spain
[4] Ctr Atom Bariloche, RA-8400 San Carlos De Bariloche, Rio Negro, Argentina
[5] Univ Texas Austin, Texas Mat Inst, ETC 9102, Austin, TX 78712 USA