1. Nano-Bio Spectroscopy group and ETSF Scientific Development Centre, Dpto. Fisica de Materiales, Universidad del Pais Vasco, Centro de Fisica de Materiales CSIC-UPV/EHU-MPC and DIP, E-20018 San Sebastian, Spain
2. Institut Neel, CNRS-UJF, Grenoble, France
3. Institute for Electronics, Microelectronics, and Nanotechnology, CNRS-UMR 8520, Dept. ISEN, B.P. 60069, 59652 Villeneuve d’Ascq Cedex, France
4. IMPMC, Universités Paris 6 et 7, CNRS, IPGP, 140 rue de Lourmel, 75015 Paris, France
5. Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany