1. Dipartimento di Fisica e Geologia, Università di Perugia, Via A. Pascoli, I-06100 Perugia, Italy
2. CEMIN, Centre of Excellence on Nanostructured Innovative Materials, University of Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy
3. Istituto Officina dei Materiali, Italian National Research Council (IOM-CNR), Unit of Perugia, c/o Department of Physics and Geology, University of Perugia, Via A. Pascoli, I-06123 Perugia, Italy