[1] CSIC, Catalan Inst Nanosci & Nanotechnol ICN2, Campus UAB, Barcelona 08193, Spain
[2] Barcelona Inst Sci & Technol, Campus UAB, Barcelona 08193, Spain
[3] Univ Autonoma Barcelona, Dept Phys, Campus UAB, E-08193 Barcelona, Spain
[4] ICREA, Barcelona 08010, Spain