[1] Univ Calabria, Dipartimento Fis, Via P Bucci,Cubo 33B, I-87036 Arcavacata Di Rende, CS, Italy
[2] Bulgarian Acad Sci, Georgi Nadjakov Inst Solid State Phys, 72 Tzarigradsko Chaussee, BU-1784 Sofia, Bulgaria
[3] Wuhan Univ, Sch Phys & Technol, Key Lab Artificial Micro Nano Struct, Minist Educ, Wuhan 430072, Peoples R China
[4] Univ Calabria, Dipartimento Fis, CNISM, Unit Ric Cosenza, Via P Bucci,Cubo 31C, I-87036 Arcavacata Di Rende, Italy