1. a Institute of Physics and Technology of Materials , PO Box MG 7, Bucharest-Magurele, RO-76900, Romania
2. b Institute of Technical Physics and Materials Science , Budapest, Hungary
3. c Center of Advanced Studies in Physics , Romanian Academy , Bucharest, Romania
4. d Laboratoire de Chimie Physique Matière et Rayonnement Unité Mixte , rue Pierre et Marie Curie F, Recherche associée au CNRS 7614 11, Paris Cedex 05, 75231, France