1. Department of Materials Science and Engineering, University of Ioannina, 45 110, Ioannina, Greece, Institute of Materials Science, N.C.S.R. “Demokritos” Agia Paraskevi 15310, Athens, Greece, Department of Physics, University of South Florida, Tampa, Florida 33620, and Department of Physics & Astronomy, University of Delaware, Newark, Delaware 19716