1. Department of Physics, University of Calabria, Via P. Bucci, 87036 Rende (CS), Italy
2. CNR NANOTEC—Istituto di Nanotecnologia, UOS Cosenza, 87036, Rende (CS), Italy
3. Department of Physics, Case Western Reserve University, 10600 Euclid Avenue, Cleveland, Ohio 44106, USA