This work has been carried out within the framework of the inter-academic Italian-Bulgarian research project (Department of Physics E.R. Caianiello CNR-SPIN unit;
University of Salerno;
and the Institute of Solid State Physics Georgi Nadjakov;
Bulgarian Academy of Sciences). The authors would like to thank Professor M. Gospodinov from ISSP-BAS for the valuable discussions and the support for the sample preparation;