1. Coherentia CNR-INFM and Università di Napoli “Federico II”, Dipartimento di Scienze Fisiche, Complesso Universitario Monte Sant’Angelo, Via Cintia, I-80126 Napoli, Italy, Materials Science and Technology Research Division, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea, and Department of Materials Science and Engineering, and Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139