[1] CSIC, Catalan Inst Nanosci & Nanotechnol ICN2, Campus UAB, Barcelona 08193, Spain
[2] BIST, Campus UAB, Barcelona 08193, Spain
[3] Univ Autonoma Barcelona UAB, Dept Phys, Campus UAB, Barcelona 08193, Spain
[4] Univ Chile, Fac Ciencias Fis & Matemat, Dept Fis, Santiago 8370415, Chile
[5] ICREA Inst Catalana Recerca & Estudis Avancats, Barcelona 08010, Spain