1. Department of Physics and Geology, University of Perugia, I-06123 Perugia, Italy
2. Kotelnikov IRE RAS, Saratov Branch, 410019 Saratov, Russia
3. Saratov State University, 410012 Saratov, Russia
4. Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche (IOM-CNR), I-06123 Perugia, Italy
5. University of California-Riverside, Riverside, California 92521, USA